Obstetrics is a specialty that is concerned with the mother and fetus during pregnancy, childbirth, and the immediate postpartum period. Obstetricians study obstetrics and gynecology and are referred to as OB/GYN, Obstetrics and Gynecology.


Media 18.1. Reproductive System, Part 4 – Pregnancy & Development: Crash Course A&P #43 [Online video]. Copyright 2015 by CrashCourse.


Fertilization occurs when sperm and an oocyte combine. Because each of these reproductive cells is a haploid cell containing half of the genetic material needed to form a human being, their combination forms a diploid cell. This new single cell is called a zygote.


Most of the time, a woman releases a single egg during an ovulation cycle.


A full-term pregnancy lasts approximately 270 days (approximately 38.5 weeks) from conception to birth. Because it is easier to remember the first day of the last menstrual period (LMP) than to estimate the date of conception, obstetricians set the due date as 284 days (approximately 40.5 weeks) from the LMP. This assumes that conception occurred on day 14 of the woman’s cycle, which is usually a good approximation. The 40 weeks of an average pregnancy are usually discussed in terms of three trimesters, each approximately 13 weeks. During the second and third trimesters, the pre-pregnancy uterus is about the size of a fist and grows dramatically to contain the fetus, causing a number of anatomical changes in the mother.


The process of childbirth can be divided into three stages (see Figure 18.1):


For vaginal birth to occur, the cervix must dilate fully to 10 cm in diameter, wide enough to deliver the newborn’s head. The dilation stage is the longest stage of labor and typically takes 6 to 12 hours. However, it varies widely and may take minutes, hours, or days, depending in part on whether the mother has given birth before. In each subsequent labor, this stage tends to be shorter.


The five criteria, skin color, heart rate, reflex, muscle tone, and respiration, are assessed and each criterion is assigned a score of 0, 1, or 2. Scores are taken one minute after birth and again five minutes after birth. Each time scores are taken, the five scores are added together. High scores (out of a possible 10) indicate the baby has made the transition from the womb well, whereas lower scores indicate that the baby may be in distress.


The technique for determining an Apgar score is quick and easy, painless for the newborn, and does not require any instruments except for a stethoscope. A convenient way to remember the five scoring criteria is to apply the mnemonic APGAR:


Of the five Apgar criteria, heart rate and respiration are the most critical. Poor scores for either of these measurements may indicate the need for immediate medical attention to resuscitate or stabilize the newborn. In general, any score lower than 7 at the 5-minute mark indicates that medical assistance may be needed. A total score below 5 indicates an emergency. Normally, a newborn will get an intermediate score of 1 for some of the Apgar criteria and will progress to a 2 by the five-minute assessment. Scores of 8 or above are normal.


Preeclampsia and eclampsia refer to hypertensive disorders that occur during pregnancy among women without a history of high blood pressure. Signs and symptoms of preeclampsia include blood pressure at or greater than 140/90 mmHg at or after 20 weeks of pregnancy, edema, and protein in the urine. Women with preeclampsia are at risk of preterm birth. If the condition is severe enough to cause seizures or a coma, it is referred to as eclampsia. If not treated, preeclampsia and eclampsia can be fatal (Office of Communications, n.d.).


An ectopic pregnancy occurs when a fertilized egg implants outside of the uterus, usually in the fallopian tube. Risk factors include older age, smoking, the use of an intrauterine device, in vitro fertilization, and prior pelvic infections, including chlamydia. Signs and symptoms include pain in the abdomen or shoulder, vaginal bleeding, and dizziness. Treatment may involve surgery or the use of medication to stop the growth of ectopic tissue (Mummert & Gnugnoli, 2021; MedlinePlus, 2021a).


Miscarriage, also known as spontaneous abortion, refers to the loss of pregnancy before the 20th week. It is the most common form of pregnancy loss. Risk factors include older maternal age, a history of miscarriages, disorders of the uterus or cervix, and chronic diseases such as polycystic ovary syndrome. Signs and symptoms include vaginal spotting, abdominal pain, and cramping. However, these symptoms can be confused with symptoms of an ectopic or normal pregnancy. Treatment is generally not required in women who miscarry early in their pregnancy. If tissue remains in the uterus after miscarriage, treatment includes the use of medication to aid expulsion or a surgical procedure called dilation and curettage (D&C) (Dugas & Slane, 2021; MedlinePlus, 2021b).


Alpha-fetoprotein (AFP) is a protein that is produced during fetal development. The AFP test is an analysis of the mother’s blood serum to determine whether the level of AFP denotes a risk of a birth defect.


A technique used to assess the general wellbeing of a newborn. The newborn is assessed one minute after birth and again five minutes after birth.


A process where the semen is introduced into the vagina by mechanical means.


The position of the fetus is feet first. Ideally, the position of the fetus should be headfirst for a safer delivery.


A condition where the infant’s head is larger than the mother’s pelvis.


A suture inserted into the cervix to prevent dilation and prevention miscarriage.


A malignant, fast-growing tumor that develops from trophoblastic cells (cells that help an embryo attach to the uterus and help form the placenta).


A procedure in which tissue from the placenta is tested for fetal chromosomal disorders.


A thick, yellowish substance secreted from a mother’s breasts in the first postpartum days.


A procedure to scrape and remove tissue from the inner lining of the uterus. The cervix is dilated (made larger) and a curette (spoon-shaped instrument) is inserted into the uterus to remove tissue. The procedure is used to test tissue for signs of disease, after a miscarriage, or to treat certain conditions.


A very serious condition in pregnant women with hypertension; patients are at high risk of coma, convulsions, and even death.


A pregnancy in which the fertilized ovum is implanted in any tissue other than the uterine wall.


Incision made in the posterior vaginal wall and perineum that facilitates vaginal birth.


The period required for embryonic and fetal development in utero; pregnancy.


Diabetes mellitus that occurs during pregnancy but resolves by the end of pregnancy in women without a history of diabetes.


The number of pregnancies, complete or incomplete, experienced by a woman.


Excessive vomiting during pregnancy. Hyperemesis can occur with any pregnant woman, even a woman who miscarried. Often these women may require hospitalization for fluid and electrolyte intake.


The process of bringing on or starting labor through artificial means.


A process where the ova is fertilized outside the body and then implanted into the uterus.


A substance produced by the body, such as during strenuous exercise, that aids in various chemical processes in the body.


Fetal wastes consisting of ingested amniotic fluid, cellular debris, mucus, and bile.


Pertaining to the newborn’s first thirty days of life outside of the uterus.


A doctor who specializes in caring for women during pregnancy and childbirth.


A branch of medicine that specializes in the care of women during pregnancy and childbirth and in the diagnosis and treatment of diseases of the female reproductive organs; also called OB/GYN.


Occurs when the placenta prematurely becomes detached from the uterine wall, resulting in uterine bleeding, fetal distress, or fetal death; also known as abruptio placenta.


Low placement of fetus within the uterus, which causes the placenta to partially or completely cover the opening of the cervix as it grows.


A condition where there is excessive amniotic fluid in the placenta.


The abnormal condition in pregnancy where the patient experiences hypertension, edema, and proteinuria.


Having to do with the time a female is pregnant, before birth occurs; also called antenatal.


Time directly after childbirth (6 to 8 weeks after giving birth).


A branch of embryology for the study of congenital malformations and developmental abnormalities.


Delivery of an infant through the vagina in a female who has had a prior cesarean section.


A single cell formed by the fusion of an egg and sperm; also called the fertilized egg.


Centers for Disease Control and Prevention. (n.d.-a). Infertility FAQs. https://www.cdc.gov/reproductivehealth/infertility/index.htm


Centers for Disease Control and Prevention. (n.d.-b). Data & statistics on birth defects. https://www.cdc.gov/ncbddd/birthdefects/data.html


Centers for Disease Control and Prevention. (n.d.-c). Diagnosis of birth defects. https://www.cdc.gov/ncbddd/birthdefects/diagnosis.html


CrashCourse. (2019, November 23). Reproductive System, Part 4 – Pregnancy & Development: Crash Course A&P #43 [Video]. YouTube. https://youtu.be/BtsSbZ85yiQ


Dugas, C., & Slane, V. H. (2021). Miscarriage. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK532992/


MedlinePlus. (2021a). Ectopic pregnancy. U.S. National Library of Medicine. https://medlineplus.gov/ectopicpregnancy.html


MedlinePlus. (2021b). Miscarriage. U.S. National Library of Medicine. https://medlineplus.gov/miscarriage.html


Mummert, T., & Gnugnoli, D. M. (2021). Ectopic pregnancy. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK539860/


Office of Communications. (n.d.). About preeclampsia and eclampsia. Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health. https://www.nichd.nih.gov/health/topics/preeclampsia/conditioninfo


The female reproductive system produces gametes and reproductive hormones. In addition, the female reproductive system supports the developing fetus and delivers it to the outside world. The female reproductive system is located primarily inside the pelvic cavity. The female gonads are called ovaries and the gamete they produce is called an oocyte.


Media 17.1. Reproductive System, Part 1 – Female Reproductive System: Crash Course A&P #40 [Online video]. Copyright 2015 by CrashCourse.


The external female reproductive structures are referred to collectively as the vulva and they include:


The vagina is a muscular canal (approximately 10 cm long) that is the entrance to the reproductive tract. It also serves as the exit from the uterus during menses and childbirth. The outer walls of the anterior and posterior vagina are columns with ridges. The superior fornix meets the uterine cervix. The cervix is the opening to the uterus.


The outer walls of the anterior and posterior vagina are columns with ridges.


Together, the middle and inner layers allow the expansion of the vagina to accommodate intercourse and childbirth. The thin, perforated hymen can partially surround the opening to the vaginal orifice. The Bartholin’s glands and the lesser vestibular glands (located near the clitoris) secrete mucus, which keeps the vestibular area moist.


Together, the middle and inner layers allow the expansion of the vagina to accommodate intercourse and childbirth. The thin, perforated hymen can partially surround the opening to the vaginal orifice. The Bartholin’s glands and the lesser vestibular glands (located near the clitoris) secrete mucus, which keeps the vestibular area moist


The vagina has a normal population of microorganisms that help to protect against infection. There are both pathogenic bacteria and yeast in the vagina. In a healthy woman, the most predominant type of vaginal bacteria is from the genus Lactobacillus, which secretes lactic acid. The lactic acid protects the vagina by maintaining an acidic pH (below 4.5).


Lactic acid, in combination with other vaginal secretions, makes the vagina a self-cleansing organ. However, douching can disrupt the normal balance of healthy microorganisms, and increase a woman’s risk for infections and irritation. It is recommended that women do not douche and that they allow the vagina to maintain its normal healthy population of protective microbial flora.


The ovaries are the female gonads. There are two, one at each entrance to the fallopian tube. They are each about 2 to 3 cm in length, about the size of an almond. The ovaries are located within the pelvic cavity. The ovary itself is attached to the uterus via the ovarian ligament. The ovarian stroma forms the bulk of the adult ovary. Oocytes develop within the outer layer of this stroma, each surrounded by supporting cells. This grouping of an oocyte and its supporting cells is called a follicle.


The fallopian tubes, also known as uterine tubes, are the conduit of the oocyte from the ovary to the uterus. Each of the two fallopian tubes is close to, but not directly connected to, the ovary.


The uterus is the muscular organ that nourishes and supports the growing embryo. Its average size is approximately 5 cm wide by 7 cm long, and it has three sections.


The wall of the uterus is made up of three layers:


Following ovulation, the Fallopian tube receives the oocyte. Oocytes lack flagella, and therefore cannot move on their own.


Media 17.2. The ovarian cycle | Reproductive system physiology | NCLEX-RN | Khan Academy [Online video]. Copyright 2014 by Khan Academy Medicine.


Menopause is the cessation of the menstrual cycle that occurs as a result of the loss of ovarian follicles and the hormones that they produce, namely estrogen. The earliest changes occur during the menopausal transition, often referred to as perimenopause, when a woman’s cycle becomes irregular but does not stop entirely. As estrogen levels change, other signs and symptoms that occur are hot flashes and night sweats, trouble sleeping, vaginal dryness, mood swings, difficulty focusing, and thinning of hair on the head along with the growth of more hair on the face. Depending on the individual, these symptoms can be entirely absent, moderate, or severe.


A woman is considered to have completed menopause if she has not menstruated in a full year. After that point, she is considered postmenopausal. The average age for this change is consistent worldwide at between 50 and 52 years of age, but it can normally occur in a woman’s forties or later in her fifties. Poor health, including smoking, can lead to earlier loss of fertility and earlier menopause. After menopause, low levels of estrogen increase women’s risks of high cholesterol, cardiovascular disease, and osteoporosis.


Many terms and phrases related to the female reproductive system are abbreviated. Learn these common abbreviations by expanding the list below.


Breast cancer is the second most common cancer among women in the United States. It starts in the cells that line the ducts or the lobule of the breast. Some warning signs include a new lump in the breast or axilla, thickening or swelling, dimpling of the breast skin, redness or flaky skin, discharge, and change in breast size. Risk factors include a familial or personal history of breast cancer, obesity, hormonal treatment, and mutations in breast cancer-related genes (BRCA1 or BRCA2) (Centers for Disease Control and Prevention, n.d.-a; National Cancer Institute, 2021a).


(Centers for Disease Control and Prevention, n.d.-a; National Cancer Institute, 2021a).


Treatment options include chemotherapy, hormone therapy, immunotherapy, radiation, and surgical interventions such as mastectomy, and biopsy (National Cancer Institute, 2021a). To learn more about breast cancer, view the National Cancer Institute’s web page on breast cancer.


Cervical cancer is typically slow-growing cancer and is highly curable when found and treated early. Advanced cervical cancer may cause abnormal bleeding or discharge from the vagina such as bleeding after sex. It is diagnosed during a Papanicolaou test (or Pap smear), which looks for precancerous lesions on the cervix. The Pap test can find cervical cancer early when treatment is most effective. Almost all cervical cancers are caused by human papillomavirus (HPV). The HPV test looks for HPV strains which is the virus that can cause precancerous cell changes.  In the United States, a vaccine called Gardasil® 9 is available, which prevents infection with HPV and significantly reduces the risk of cervical cancer (National Cancer Institute, 2021b, 2021c). To learn more, visit the National Cancer Institute’s web page on cervical cancer.


Endometriosis is an abnormal condition of the endometrium. Endometriosis occurs when this tissue grows and implants outside the uterus. The female hormone estrogen causes these implants to grow, bleed, and become inflamed. The inflammation causes scar tissue around nearby organs, which can interfere with their normal functioning and cause pain (Office on Women’s Health, 2019a).


Endometriosis generally appears in women in their 30s and 40s. Signs and symptoms may include dysmenorrhea, dyspareunia, menstrual irregularity, and infertility. Diagnosis may include laparoscopy and endometrial biopsy. There is no cure for endometriosis. Treatment may include hormonal birth control, surgical interventions such as hysterectomy and oophorectomy. The cause of endometriosis is unknown (MedlinePlus, 2021; Office on Women’s Health, 2019a). For more information, visit the Office on Women’s Health web page on endometriosis.


Polycystic ovary syndrome (PCOS) has no known etiology, but researchers have linked it to excessive production of insulin and androgens. Excessive insulin in the body can cause insulin resistance and lead to type 2 diabetes. High levels of androgens can prevent the ovaries from releasing an egg during the menstrual cycle. The most common signs and symptoms of PCOS include oligomenorrhea, amenorrhea, polymenorrhea, one or both ovaries with multiple small painless cysts, acrochordons, acanthosis nigricans, hirsutism, thinning hair, acne, weight gain, anxiety, depression, hyperglycemia, and infertility. Treatments like medications such as birth control pills or antiandrogens  can help balance the hormones in your body and relieve some of the symptoms (Office on Women’s Health, 2019b). To learn more, visit the Office on Women’s Health’s web page on endometriosis.


Although the terms sexually transmitted infections (STI) and sexually transmitted diseases (STD) are often used interchangeably, they have distinct meanings. STIs refer to infections caused by a virus, bacteria, fungus, or parasite via sexual contact. STDs refer to the disease state that develops as a result of infection (U.S. Department of Health and Human Services, 2020).


Chlamydia is one of the most common sexually transmitted diseases. It is caused by the bacterium Chlamydia trachomatis, which infects the cervix and other organs of the reproductive tract in women. Chlamydia spreads through unprotected oral, anal, or vaginal sex with an infected person. Chlamydia can also spread from mother to child during childbirth. Many people with chlamydia do not have any symptoms and unknowingly pass the infection to their sexual partner(s). If symptoms develop, they may not appear for several weeks after sexual contact with an infected person. Women may have abnormal vaginal discharge, polyuria, and painful intercourse. Untreated chlamydia in women can cause pelvic inflammatory disease, which can cause permanent damage to the reproductive tract and infertility. Chlamydia is easy to treat with antibiotics and can be cured. However, until a patient finishes their treatment, they continue to have the infection and can continue to pass it to others. Patients should be re-tested three months after treatment, as re-infection is common (Centers for Disease Control and Prevention, n.d.-b).


Gonorrhea is a sexually transmitted disease caused by the bacterium Neisseria gonorrhoeae. It infects the mucous membranes of the reproductive tract, including the cervix, uterus, and fallopian tubes in women. Infections can also infect the mouth, throat, eyes, and anus. Gonorrhea is spread through unprotected oral, vaginal or anal sex with an infected person. It can also spread from mother to child during childbirth. Many people infected with gonorrhea have no symptoms and can unknowingly pass the infection on to their sexual partner(s). Signs and symptoms vary depending on which part of the body is infected. Women may have dysuria, increased vaginal discharge, or vaginal bleeding not related to their menstrual cycle. Gonorrhea infection from oral sex may lead to a sore throat, whereas infection from anal sex may cause itchiness and discharge from the anus. Untreated chlamydia can cause pelvic inflammatory disease. Gonorrhea can be treated and cured with antibiotics in combination with an intramuscular (IM) injection. However, until the patient finishes their treatment, they continue to have the infection and can pass it to others. Patients should be re-tested three months after treatment, as re-infection is common (Centers for Disease Control and Prevention, n.d.-c).


In every state in the United States, chlamydia and gonorrhea are notifiable and reportable diseases. This means that when a person tests positive for either of the STIs, public health departments and the Centers for Disease Control and Prevention (CDC) are informed so that they may monitor trends, identify outbreaks, and take the necessary steps to prevent further spread of the disease (Centers for Disease Control and Prevention, n.d.-d, n.d.-e).


Human papillomavirus (HPV) is another common sexually transmitted infection (STI). Both males and females can be infected with HPV. There are over 200 strains of HPV. Some strains can cause visible genital warts, while others cause genital, anal, throat, and cervical cancers. HPV spreads through sexual activity and skin-to-skin contact in the genital area with an infected person. Since some people are asymptomatic, they don’t know they have the virus and consequently pass the virus to their sexual partners. Treatments are available for genital warts but there is no cure for HPV (MedlinePlus, 2021). For more information, visit the CDC’s web page on HPV.


Genital herpes is a sexually transmitted disease that is caused by a virus called the herpes simplex virus (HSV). There are two types of herpes simplex viruses:


Signs and symptoms might include dysuria, enlarged glands, myalgia, and fever. Once a patient is infected with HSV, the virus remains in their body even after the symptoms are gone and can cause recurring outbreaks. When the virus becomes active again, the symptoms return but are usually less painful and heal faster (Centers for Disease Control and Prevention, n.d.-f).


Herpes is spread through direct contact with the sores or blisters of an infected person. Contact (and transfer of the virus) can occur from genitals-to-genitals, mouth-to-genitals, or mouth-to-mouth. Herpes can also be passed to the anal area. Herpes spreads easily during sexual contact while symptoms are present or just before an outbreak of symptoms. An infected person may spread herpes even when they have no symptoms; this is called asymptomatic shedding. One can spread the herpes virus to other parts of their body after touching the sores. The fingers, eyes, and other body areas can accidentally become infected in this way. Hand washing after touching sores and blisters is recommended to prevent spreading the virus (Centers for Disease Control and Prevention, n.d.-f).


There is no cure for herpes. Antiviral pills help to reduce symptoms and speed the healing of blisters or sores and are prescribed by a doctor. All sexual partner(s) should be informed. The only way to reduce the risk of transmission of herpes is to avoid direct contact with the sores and to use condoms. Condoms will reduce but not eliminate risk, as the virus can be present and shed from the skin in the genital area (Centers for Disease Control and Prevention, n.d.-f).


Obstetrics and gynecology, also known as OB/GYN, is a branch of medicine focusing on the diagnosis, treatment, management, and prevention of diseases and disorders of the female reproductive system, as well as the care of women during pregnancy and childbirth. Subspecialties in women’s health include contraception, reproductive endocrinology, and infertility (National Cancer Institute, n.d.-a). To learn more, visit the American College of Surgeons’ web page on obstetrics and gynecology.


A hysterectomy is performed to stage or treat endometriosis, cancers, and precancers of the female reproductive tract, and some non-cancerous conditions that have not responded to other forms of treatment. There are three types of hysterectomy:


Sometimes the ovaries and fallopian tubes are removed at the same time that a hysterectomy is done. A bilateral salpingo-oophorectomy (BSO) removes both ovaries and fallopian tubes. A unilateral salpingo-oophorectomy removes one ovary and one fallopian tube (National Cancer Institute, n.d.-b, n.d.-c). For more information, visit the Cleveland Clinic’s web page on hysterectomy.


A disorder that causes darkening and thickening of the armpit and other body folds.


Common benign skin growths that appear as small, raised, brown or skin-colored bumps; also called skin tags.


Substances that keep androgens (male sex hormones) from binding to proteins called androgen receptors. Preventing this binding blocks the effects of these hormones in the body.


A drug used to treat infections caused by bacteria and other microorganisms.


Drugs that inhibit the ability of the human immunodeficiency virus (HIV) or other types of retroviruses to multiply in the body.


Glands that produce a thick mucus that maintains moisture in the vulva area; also referred to as the greater vestibular glands.


A lighted magnifying instrument used to check the cervix, vagina, and vulva for signs of disease.


A procedure in which a lighted, magnifying instrument called a colposcope is used to examine the cervix, vagina, and vulva.


A specialty in the field of medicine that focuses on the treatment of endocrine system disorders.


A disease characterized by the presence of endometrial-like tissue found outside the uterus.


The innermost layer of the uterus. It provides the site of implantation for a fertilized egg and sheds during menstruation if no egg is fertilized.


A procedure that uses an endoscope to examine the inside of the body.


An abnormal opening or passage between two organs or between an organ and the surface of the body.


A doctor who has special training in diagnosing and treating diseases of the female reproductive organs.


A medical-surgical specialty concerned with the physiology and disorders primarily of the female genital tract, as well as female endocrinology and reproductive physiology.


A condition in which women and children have excess coarse body hair of an adult male distribution pattern as a result of elevated androgen levels.


A position below or lower than another part of the body proper.


A procedure that uses a laparoscope, inserted through the abdominal wall, to examine the inside of the abdomen.


The use of film or a computer to create a picture of the breast.


The surgical procedure to remove all or part of a breast.


The cessation of the menstrual cycle; is considered complete when a woman has not menstruated in a full year.


Release of a secondary oocyte and associated granulosa cells from an ovary.


A procedure in which a small brush is used to gently remove cells from the surface of the cervix and the area around it so they can be checked under a microscope for cervical cancer or cell changes that may lead to cervical cancer.


The transitional period before and after menopause wherein the menstrual cycle is irregular and hormone levels widely fluctuate.


The protrusion of an organ or part of an organ into a natural or artificial orifice.


An instrument used to widen an opening of the body to make it easier to look inside.


A position above or higher than another part of the body proper.


Centers for Disease Control and Prevention. (n.d.-a). Breast cancer statistics. https://www.cdc.gov/cancer/breast/statistics/index.htm


CrashCourse. (2015, October 2015). Reproductive system, part 1 – female reproductive system: Crash course A&P #40 [Video]. YouTube. https://www.youtube.com/watch?v=RFDatCchpus


MedlinePlus. (2021). Endometriosis. U.S. National Library of Medicine. https://medlineplus.gov/endometriosis.html


National Cancer Institute. (n.d.-a). Definition of obstetrics and gynecology. National Institutes of Health, U.S. Department of Health and Human Services. https://www.cancer.gov/publications/dictionaries/cancer-terms/def/obstetrics-and-gynecology


National Cancer Institute. (n.d.-b). Definition of bilateral salpingo-oophorectomy. National Institutes of Health, U.S. Department of Health and Human Services. https://www.cancer.gov/publications/dictionaries/cancer-terms/def/bilateral-salpingo-oophorectomy


National Cancer Institute. (n.d.-c). Definition of unilateral salpingo-oophorectomy. National Institutes of Health, U.S. Department of Health and Human Services. https://www.cancer.gov/publications/dictionaries/cancer-terms/def/unilateral-salpingo-oophorectomy


National Cancer Institute. (2021a). Breast cancer treatment (adult) (PDQ®)–Patient version. National Institutes of Health, U.S. Department of Health and Human Services.

 https://www.cancer.gov/types/breast/patient/breast-treatment-pdq


National Cancer Institute. (2021b). Cervical cancer treatment (adult) (PDQ®)–Patient version. National Institutes of Health, U.S. Department of Health and Human Services.

https://www.cancer.gov/types/cervical/patient/cervical-treatment-pdq


National Cancer Institute. (2021c). Human papillomavirus (HPV) vaccines. National Institutes of Health, U.S. Department of Health and Human Services. https://www.cancer.gov/about-cancer/causes-prevention/risk/infectious-agents/hpv-vaccine-fact-sheet


Office on Women’s Health. (2019a). Endometriosis. U.S. Department of Health and Human Services. https://www.womenshealth.gov/a-z-topics/endometriosis


Office on Women’s Health. (2019b). Hysterectomy. U.S. Department of Health and Human Services. https://www.womenshealth.gov/a-z-topics/hysterectomy


Click on prefixes, combining forms, and suffixes to reveal a list of word parts to memorize for the Male Reproductive System.


Gametes are the reproductive cells that combine to form a fetus. Organs called gonads produce the gametes, along with the hormones that regulate human reproduction. The male gametes are called sperm. Spermatogenesis occurs within the seminiferous tubules that make up most of the testis. The scrotum is a sac that holds the testes outside of the body cavity.


Media 16.1. Reproductive System, Part 2 – Male Reproductive System: Crash Course A&P 41 [Online video]. Copyright 2015 by CrashCourse.


The structures of the male reproductive system include the testes, the epididymis, the penis, and the ducts and glands that produce and carry semen. Sperm exit the scrotum through the vas deferens. The spermatic cord is an enclosed sheath which includes the vas deferens, arteries, veins and nerves. The seminal vesicles and prostate add fluids to the sperm to create semen.


Spermatogenesis occurs in the seminiferous tubules that form the bulk of each testis. The process begins at puberty, after which time sperm is produced constantly throughout a man’s life. One production cycle takes approximately 64 days. One production cycle is considered from spermatogonia through to formed sperm. A new cycle starts approximately every 16 days, although this timing is not synchronous across the seminiferous tubules.


Sperm is smaller than most cells in the body; in fact, the volume of a sperm cell is 85,000 times less than that of the female gamete. Approximately 100 to 300 million sperm are produced each day, whereas women typically ovulate only one oocyte per month as is true for most cells in the body, the structure of sperm cells speaks to their function. Sperm have a distinctive head, mid-piece, and tail region (see Figure 16.2).


To fertilize an egg, sperm must be moved from the seminiferous tubules in the testes, through the epididymis, and—later during ejaculation—along the length of the penis and out into the female reproductive tract. It takes an average of 12 days for sperm to move through the coils of the epididymis, with the shortest recorded transit time in humans being one day.


It takes an average of 12 days for sperm to move through the coils of the epididymis, with the shortest recorded transit time in humans being one day.


Sperm enter the head of the epididymis and are moved by the contraction of smooth muscles lining the epididymal tubes. As the sperm mature they acquire the ability to move under their own power. Once inside the female reproductive tract, they will use this ability to move independently toward the unfertilized egg. The more mature sperm are then stored in the tail of the epididymis until ejaculation occurs.


As the sperm mature they acquire the ability to move under their own power.


Once inside the female reproductive tract, they will use this ability to move independently toward the unfertilized egg.


During ejaculation, sperm exit the tail of the epididymis and are pushed by smooth muscle contraction to the vas deferens (also called the ductus deferens). The vas deferens is a thick, muscular tube that is bundled together inside the scrotum with connective tissue, blood vessels, and nerves into a structure called the spermatic cord. From each epididymis, each vas deferens extends through the inguinal canal in the abdominal wall and continues to a region called the ampulla. The sperm is mixed with fluid from the paired seminal vesicles and moves into its associated ejaculatory duct. The ejaculatory ducts transport the seminal fluid to the prostate gland.


The vas deferens is a thick, muscular tube that is bundled together inside the scrotum with connective tissue, blood vessels, and nerves into a structure called the


From each epididymis, each vas deferens extends through the inguinal canal in the abdominal wall and continues to a region called the ampulla.


The sperm is mixed with fluid from the paired seminal vesicles and moves into its associated ejaculatory duct.


The ejaculatory ducts transport the seminal fluid to the prostate gland.


The prostate gland secretes an alkaline, milky fluid to the passing seminal fluid (referred to as semen) to first coagulate and then decoagulate the semen following ejaculation. The temporary thickening of semen helps retain it within the female reproductive tract. Once decoagulated, the sperm can pass farther into the female reproductive tract.


The temporary thickening of semen helps retain it within the female reproductive tract. Once decoagulated, the sperm can pass farther into the female reproductive tract


Bulbourethral glands release a thick, salty fluid that lubricates the end of the urethra and vagina, and helps to clean urine residues from the penile urethra.


Many terms and phrases related to the male reproductive system are abbreviated. Learn these common abbreviations by expanding the list below.


Erectile dysfunction (ED) is a condition in which a male has difficulty either initiating or maintaining an erection. The combined prevalence of minimal, moderate, and complete ED is approximately 40% in men at age 40 and reaches nearly 70% by 70 years of age. In addition to aging, ED is associated with diabetes, vascular disease, psychiatric disorders, prostate disorders, the use of some drugs such as certain antidepressants, and problems with the testes resulting in low testosterone concentrations. These physical and emotional conditions can lead to disruptions in the vasodilation pathway and result in an inability to achieve an erection.


According to the Centers for Disease Control and Prevention (CDC), prostate cancer is the second most common cancer occurring in men. However, some forms of prostate cancer grow very slowly and may not require treatment. Aggressive forms of prostate cancer, in contrast, involve metastasis to organs like the lungs and brain. There is no link between Benign Prostatic Hyperplasia (BPH) and prostate cancer, but the symptoms are similar. Prostate cancer is detected by medical history, a blood test, and a digital rectal exam that allows physicians to palpate the prostate and check for unusual masses. If a mass is detected, the cancer diagnosis is confirmed by biopsy of the cells (Betts et al., 2013; Centers for Disease Control, n.d-a.).


According to the Centers for Disease Control and Prevention (CDC), prostate cancer is the second most common cancer occurring in men. However, some forms of prostate cancer grow very slowly and may not require treatment. Aggressive forms of prostate cancer, in contrast, involve metastasis to organs like the lungs and brain. There is no link between Benign Prostatic Hyperplasia (BPH) and prostate cancer, but the symptoms are similar. Prostate cancer is detected by medical history, a blood test, and a digital rectal exam that allows physicians to palpate the prostate and check for unusual masses. If a mass is detected, the cancer diagnosis is confirmed by biopsy of the cells


Testicular cancer begins in the testicle or testis. It is most often found in men aged 20 to 35 years old, although it can occur at any age. Common signs and symptoms include a painless lump in the testicle, swelling, and a build-up of fluid in the scrotum. Testicular cancer is treatable when diagnosed early. An orchiectomy may be required for diagnosing and treating testicular cancer (National Cancer Institute, 2021a). For more information, visit the Cleveland Clinic’s web page on testicular cancer.


scrotum. Testicular cancer is treatable when diagnosed early. An orchiectomy may be required for diagnosing and treating testicular cancer (National Cancer Institute, 2021a). For more information, visit the Cleveland Clinic’s web page on testicular cancer.


Chlamydia is one of the most common sexually transmitted diseases. It is caused by the bacteria Chlamydia trachomatis, which infects the urethra and prostate in men. Chlamydia spreads through unprotected oral, anal, or vaginal sex with an infected person. Many people with chlamydia do not have any symptoms and unknowingly pass the infection to their sexual partner(s). If symptoms develop, they may not appear for several weeks after sexual contact with an infected person. Males may have penile discharge, itching around the urethra, and pain in or swelling of the testicles. Chlamydia is easy to treat with antibiotics and can be cured. However, until a patient finishes their treatment, they continue to have the infection and can continue to pass it to others (Centers for Disease Control and Prevention, n.d.-b).


Gonorrhea is a sexually transmitted disease caused by the bacterium Neisseria gonorrhoeae. It infects the mucous membranes of the reproductive tract, including the urethra in men. Infections can also infect the mouth, throat, eyes, and anus. Gonorrhea is spread through unprotected oral, vaginal or anal sex with an infected person. Many people infected with gonorrhea have no symptoms and can unknowingly pass the infection on to their sexual partner(s). Symptoms vary depending on which part of the body is infected. Males may have yellowish-white discharge from the penis or dysuria. Gonorrhea infection from oral sex may lead to a sore throat, whereas infection from anal sex may cause itchiness and discharge from the anus. Gonorrhea can be treated and cured with antibiotics in combination with an intramuscular (IM) injection. However, until the patient finishes their treatment, they continue to have the infection and can pass it to others (Centers for Disease Control and Prevention, n.d.-c).


In every state in the United States, chlamydia and gonorrhea are notifiable and reportable diseases. This means that when a person tests positive for either of the STIs, public health departments and the CDC are informed so that they may monitor trends, identify outbreaks, and take the necessary steps to prevent further spread of the disease (Centers for Disease Control and Prevention, n.d.-d, n.d.-e).


Human papillomavirus (HPV) is another common STI. Both males and females can be infected with HPV. There are over 200 strains of HPV. Some strains can cause visible genital warts, while others cause genital, anal, throat, and cervical cancers. HPV spreads through sexual activity and skin-to-skin contact in the genital area with an infected person. Since some people are asymptomatic, they don’t know they have the virus and consequently pass the virus to their sexual partners. Treatments are available for genital warts, but there is no cure for HPV. However, in the United States, a vaccine called Gardasil® 9 is available, which prevents infection with HPV (MedlinePlus, 2021; National Cancer Institute, 2021b). For more information, visit the Centers for Disease Control and Prevention’s web page on HPV.


Urology is a specialty that focuses on the diagnosis, treatment, and surgical repair of the urinary tract. To learn more about urology as a specialty, visit the American College of Surgeons’ web page.


A vasectomy is a procedure in which a small section of the ductus (vas) deferens is removed from the scrotum. This cuts off the path taken by sperm through the ductus deferens. Although it can be reversed, clinicians consider it permanent and advise men to undergo it only if they are certain that they no longer wish to father children. For more information, view this video from MedlinePlus on vasectomies.


The removal or destruction of a body part or tissue or its function. Ablation may be performed by surgery, hormones, drugs, radiofrequency, heat, or other methods.


A benign condition in which an overgrowth of prostate tissue pushes against the urethra and the bladder, blocking the flow of urine.


Glands that secrete a lubricating mucus that cleans and lubricates the urethra prior to and during ejaculation; also called Cowper’s glands.


A sheath that is worn over the penis during sexual behavior in order to prevent pregnancy or spread of sexually transmitted disease.


Either of two columns of erectile tissue in the penis that fill with blood during an erection.


Column of erectile tissue in the penis that fills with blood during an erection and surrounds the penile urethra on the ventral portion of the penis.


The failure of one or both testes to descend into the scrotum prior to birth.


Duct that transports sperm from the epididymis through the spermatic cord and into the ejaculatory duct; also referred to as the vas deferens.


Duct that connects the ampulla of the ductus deferens with the duct of the seminal vesicle at the prostatic urethra.


Excision of a whole organ or mass without cutting into it.


A coiled tubular structure in which sperm start to mature and are stored until ejaculation.


Bulbous end of the penis that contains a large number of nerve endings.


Hormone released by the hypothalamus that regulates the production of follicle-stimulating hormone and luteinizing hormone from the pituitary gland.


Reproductive organs (testes in men and ovaries in women) that produce gametes and reproductive hormones.


Accumulation of serous fluid between the layers of membrane covering the testis.


Opening in the abdominal wall that connects the testes to the abdominal cavity.


Cells between the seminiferous tubules of the testes that produce testosterone; a type of interstitial cell.


Condition of a suboptimal concentration of spermatozoa in the ejaculated semen to ensure successful fertilization of an ovum.


Flap of skin that forms a collar around, and thus protects and lubricates, the glans penis; also referred to as the foreskin.


A gland at the base of the bladder surrounding the urethra that contributes fluid to semen during ejaculation.


An external pouch of skin and muscle that houses the testes.


Ejaculatory fluid composed of sperm and secretions from the seminal vesicles, prostate, and bulbourethral glands.


Cells that support germ cells through the process of spermatogenesis; a type of sustentacular cell.


Bundle of nerves and blood vessels that supplies the testes; contains ductus deferens.


A procedure in which a small section of the ductus deferens is cut and sealed to interrupt sperm delivery. It is an effective form of male birth control.


Creation of an artificial opening between ducts to restore fertility to males who have had a vasectomy.


Centers for Disease Control and Prevention. (n.d.-a). Prostate cancer statistics. https://www.cdc.gov/cancer/prostate/statistics/index.htm


Centers for Disease Control and Prevention. (n.d.-b). Chlamydia – CDC fact sheet (Detailed).

 https://www.cdc.gov/std/chlamydia/stdfact-chlamydia-detailed.htm


Centers for Disease Control and Prevention. (n.d.-c). Gonorrhea – CDC fact sheet (Detailed). https://www.cdc.gov/std/gonorrhea/stdfact-gonorrhea-detailed.htm


Centers for Disease Control and Prevention. (n.d.-d). Reporting and confidentiality. https://www.cdc.gov/std/treatment-guidelines/clinical-reporting.htm


Centers for Disease Control and Prevention. (n.d.-e). What is case surveillance? https://www.cdc.gov/nndss/about/index.html


Centers for Disease Control and Prevention. (n.d.-f). Genital herpes fact sheet. https://www.cdc.gov/std/herpes/stdfact-herpes.htm


CrashCourse. (2015, November 9). Reproductive system, part 2 – Male reproductive system: Crash course A&P 41. YouTube. https://youtu.be/-XQcnO4iX_U


MedlinePlus. (2021). HPV. U.S. National Library of Medicine. https://medlineplus.gov/hpv.html


National Cancer Institute. (2021a). Testicular cancer treatment (PDQ®)-Patient version. National Institutes of Health, U.S. Department of Health and Human Services. https://www.cancer.gov/types/testicular/patient/testicular-treatment-pdq


National Cancer Institute. (2021b). Human papillomavirus (HPV) vaccines. National Institutes of Health, U.S. Department of Health and Human Services. https://www.cancer.gov/about-cancer/causes-prevention/risk/infectious-agents/hpv-vaccine-fact-sheet


U.S. Department of Health and Human Services. (2020). Sexually transmitted infections National Strategic Plan for the United States: 2021-2025. https://www.hhs.gov/sites/default/files/STI-National-Strategic-Plan-2021-2025.pdf


The urinary system has roles you may be well aware of on a daily basis. Cleansing the blood and ridding the body of wastes probably come to mind. However, there are additional, equally important functions, played by the system. Take, for example, regulation of pH, a function shared with the lungs and the buffers in the blood. Additionally, the regulation of blood pressure is a role shared with the heart and blood vessels. What about regulating the concentration of solutes in the blood? Did you know that the kidney is important in determining the concentration of red blood cells? Eighty-five percent of the erythropoietin (EPO) produced to stimulate red blood cell production is produced in the kidneys. The kidneys also perform the final synthesis step of vitamin D production, converting calcidiol to calcitriol, the active form of vitamin D. If the kidneys fail, these functions are compromised or lost altogether, with devastating effects on homeostasis.


Media 15.1. Urinary System, Part 1: Crash Course A&P #38 [Online video]. Copyright 2015 by CrashCourse.


The kidneys lie on either side of the spine in the retroperitoneal space between the parietal peritoneum and the posterior abdominal wall, well protected by muscle, fat, and ribs. They are roughly the size of your fist. The male kidney is typically a bit larger than the female kidney. The kidneys are well vascularized, receiving about 25% of the cardiac output at rest. Figure 15.1 displays the location of the kidneys.


A frontal section through the kidney reveals an outer region called the renal cortex and an inner region called the medulla (see Figure 15.2). The renal columns are connective tissue extensions that radiate downward from the cortex through the medulla to separate the most characteristic features of the medulla, the renal pyramids and renal papillae. The papillae are bundles of collecting ducts that transport urine made by nephrons to the calyces of the kidney for excretion. The renal columns also serve to divide the kidney into 6 to 8 lobes and provide a supportive framework for vessels that enter and exit the cortex. The pyramids and renal columns taken together constitute the kidney lobes.


A frontal section through the kidney reveals an outer region called the


). The


are connective tissue extensions that radiate downward from the cortex through the medulla to separate the most characteristic features of the medulla, the


. The papillae are bundles of collecting ducts that transport urine made by nephrons to the


of the kidney for excretion. The renal columns also serve to divide the kidney into 6 to 8 lobes and provide a supportive framework for vessels that enter and exit the cortex. The pyramids and renal columns taken together constitute the kidney lobes.


The renal hilum is the entry and exit site for structures servicing the kidneys: vessels, nerves, lymphatics, and ureters. The medial-facing hila are tucked into the sweeping convex outline of the cortex. Emerging from the hilum is the renal pelvis, which is formed from the major and minor calyces in the kidney. The smooth muscle in the renal pelvis funnels urine via peristalsis into the ureter. The renal arteries form directly from the descending aorta, whereas the renal veins return cleansed blood directly to the inferior vena cava. The artery, vein, and renal pelvis are arranged in an anterior-to-posterior order.


The renal artery first divides into segmental arteries, followed by further branching to form interlobar arteries that pass through the renal columns to reach the cortex (see Figure 15.3). The interlobar arteries, in turn, branch into arcuate arteries, cortical radiate arteries, and then into afferent arterioles. The afferent arterioles service about 1.3 million nephrons in each kidney.


Nephrons are the “functional units” of the kidney; they cleanse the blood and balance the constituents of the circulation. The afferent arterioles form a tuft of high-pressure capillaries about 200 µm in diameter, the glomerulus. The rest of the nephron consists of a continuous sophisticated tubule whose proximal end surrounds the glomerulus in an intimate embrace—this is Bowman’s capsule. The glomerulus and Bowman’s capsule together form the renal corpuscle. As mentioned earlier, these glomerular capillaries filter the blood based on particle size. After passing through the renal corpuscle, the capillaries form a second arteriole, the efferent arteriole (see Figure 15.4). These will next form a capillary network around the more distal portions of the nephron tubule, the peritubular capillaries and vasa recta, before returning to the venous system. As the glomerular filtrate progresses through the nephron, these capillary networks recover most of the solutes and water, and return them to the circulation. Since a capillary bed (the glomerulus) drains into a vessel that in turn forms a second capillary bed, the definition of a portal system is met. This is the only portal system in which an arteriole is found between the first and second capillary beds. Portal systems also link the hypothalamus to the anterior pituitary, and the blood vessels of the digestive viscera to the liver.


The kidneys and ureters are completely retroperitoneal, and the bladder has a peritoneal covering only over the dome. As urine is formed, it drains into the calyces of the kidney, which merge to form the funnel-shaped renal pelvis in the hilum of each kidney. The hilum narrows to become the ureter of each kidney. As urine passes through the ureter, it does not passively drain into the bladder but rather is propelled by waves of peristalsis. The ureters are approximately 30 cm long. The inner mucosa is lined with transitional epithelium and scattered goblet cells that secrete protective mucus. The muscular layer of the ureter consists of longitudinal and circular smooth muscles that create the peristaltic contractions to move the urine into the bladder without the aid of gravity. Finally, a loose adventitial layer composed of collagen and fat anchors the ureters between the parietal peritoneum and the posterior abdominal wall.


The urinary bladder collects urine from both ureters ( see Figure 15.5). The bladder lies anterior to the uterus in females, posterior to the pubic bone and anterior to the rectum. During late pregnancy, its capacity is reduced due to compression by the enlarging uterus, resulting in increased frequency of urination. In males, the anatomy is similar, minus the uterus, and with the addition of the prostate inferior to the bladder. The bladder is partially retroperitoneal (outside the peritoneal cavity) with its peritoneal-covered “dome” projecting into the abdomen when the bladder is distended with urine.


The urethra transports urine from the bladder to the outside of the body for disposal. The urethra is the only urologic organ that shows any significant anatomic difference between males and females; all other urine transport structures are identical (see Figure 15.6).


The urethra in both males and females begins inferior and central to the two ureteral openings forming the three points of a triangular-shaped area at the base of the bladder called the trigone (Greek tri- = “triangle” and the root of the word “trigonometry”). The urethra tracks posterior and inferior to the pubic symphysis (see Figure 15.6). In both males and females, the proximal urethra is lined by transitional epithelium, whereas the terminal portion is a nonkeratinized, stratified squamous epithelium. In the male, pseudostratified columnar epithelium lines the urethra between these two cell types. Voiding is regulated by an involuntary autonomic nervous system-controlled internal urinary sphincter, consisting of smooth muscle and voluntary skeletal muscle that forms the external urinary sphincter below it.


Micturition is a less-often used, but proper term for urination or voiding. It results from an interplay of involuntary and voluntary actions by the internal and external urethral sphincters. When bladder volume reaches about 150 mL, an urge to void is sensed but is easily overridden. Voluntary control of urination relies on consciously preventing relaxation of the external urethral sphincter to maintain urinary continence. As the bladder fills, subsequent urges become harder to ignore. Ultimately, voluntary constraint fails with resulting incontinence, which will occur as bladder volume approaches 300 to 400 ml.


Nerves involved in the control of urination include the hypogastric, pelvic, and pudendal. Voluntary micturition requires an intact spinal cord and functional pudendal nerve arising from the sacral micturition center. Since the external urinary sphincter is voluntary skeletal muscle, actions by cholinergic neurons maintain contraction (and thereby continence) during filling of the bladder. At the same time, sympathetic nervous activity via the hypogastric nerves suppresses contraction of the detrusor muscle. With further bladder stretch, afferent signals traveling over sacral pelvic nerves activate parasympathetic neurons. This activates efferent neurons to release acetylcholine at the neuromuscular junctions, producing detrusor contraction and bladder emptying.


Having reviewed the anatomy of the urinary system now is the time to focus on physiology. You will discover that different parts of the nephron utilize specific processes to produce urine: filtration, reabsorption, and secretion. You will learn how each of these processes works and where they occur along the nephron and collecting ducts. The physiologic goal is to modify the composition of the plasma and, in doing so, produce the waste product urine.


Nephrons take a simple filtrate of the blood and modify it into urine. Many changes take place in the different parts of the nephron before urine is created for disposal. The term “forming urine” will be used hereafter to describe the filtrate as it is modified into true urine. The principal task of the nephron population is to balance the plasma to homeostatic set points and excrete potential toxins in the urine. They do this by accomplishing three principle functions—filtration, reabsorption, and secretion. They also have additional secondary functions that exert control in three areas: blood pressure (via the production of renin), red blood cell production (via the hormone EPO), and calcium absorption (via the conversion of calcidiol to calcitriol, the active form of vitamin D).


The descending and ascending portions of the loop of Henle (sometimes referred to as the nephron loop) are, of course, just continuations of the same tubule. They run adjacent and parallel to each other after having made a hairpin turn at the deepest point of their descent. The descending loop of Henle consists of an initial short, thick portion and long, thin portion, whereas the ascending loop consists of an initial short, thin portion followed by a long, thick portion. The descending and ascending thin portions consist of simple squamous epithelium. Different portions of the loop have different permeabilities for solutes and water.


The collecting ducts are continuous with the nephron but are not technically part of it. In fact, each duct collects filtrate from several nephrons for final modification. Collecting ducts merge as they descend deeper in the medulla to form about 30 terminal ducts, which empty at a papilla.


The volume of filtrate formed by both kidneys per minute is termed the glomerular filtration rate (GFR). The heart pumps about 5 L blood per min under resting conditions. Approximately 20% or one liter enters the kidneys to be filtered. On average, this liter results in the production of about 125 mL/min filtrate produced in men (range of 90 to 140 mL/min) and 105 mL/min filtrate produced in women (range of 80 to 125 mL/min). This amount equates to a volume of about 180 L/day in men and 150 L/day in women. Ninety-nine percent of this filtrate is returned to the circulation by reabsorption so that only about 1 to 2 liters of urine are produced per day.


GFR is influenced by the hydrostatic pressure and colloid osmotic pressure on either side of the capillary membrane of the glomerulus. Recall that filtration occurs as pressure forces fluid and solutes through a semipermeable barrier with the solute movement constrained by particle size. Hydrostatic pressure is the pressure produced by a fluid against a surface. If you have fluid on both sides of a barrier, both fluids exert pressure in opposing directions. The net fluid movement will be in the direction of the lower pressure. Osmosis is the movement of solvent (water) across a membrane that is impermeable to a solute in the solution. This creates osmotic pressure which will exist until the solute concentration is the same on both sides of a semipermeable membrane. As long as the concentration differs, water will move. Glomerular filtration occurs when glomerular hydrostatic pressure exceeds the luminal hydrostatic pressure of Bowman’s capsule. There is also an opposing force, the osmotic pressure, which is typically higher in the glomerular capillary. To understand why this is so, look more closely at the microenvironment on either side of the filtration membrane.


You will find osmotic pressure exerted by the solutes inside the lumen of the capillary as well as inside of Bowman’s capsule. Since the filtration membrane limits the size of particles crossing the membrane, the osmotic pressure inside the glomerular capillary is higher than the osmotic pressure in Bowman’s capsule. Recall that cells and the medium-to-large proteins cannot pass between the podocyte processes or through the fenestrations of the capillary endothelial cells. This means that red and white blood cells, platelets, albumins, and other proteins too large to pass through the filter remain in the capillary, creating an average colloid osmotic pressure of 30 mm Hg within the capillary. The absence of proteins in Bowman’s space (the lumen within Bowman’s capsule) results in an osmotic pressure near zero. Thus, the only pressure moving fluid across the capillary wall into the lumen of Bowman’s space is hydrostatic pressure. Hydrostatic (fluid) pressure is sufficient to push water through the membrane despite the osmotic pressure working against it. The sum of all of the influences, both osmotic and hydrostatic, results in a net filtration pressure (NFP) of about 10 mm Hg (see Figure 15.7).


A proper concentration of solutes in the blood is important in maintaining osmotic pressure both in the glomerulus and systemically. There are disorders in which too much protein passes through the filtration slits into the kidney filtrate. This excess protein in the filtrate leads to a deficiency of circulating plasma proteins. In turn, the presence of protein in the urine increases its osmolarity; this holds more water in the filtrate and results in an increase in urine volume. Because there is less circulating protein, principally albumin, the osmotic pressure of the blood falls. Less osmotic pressure pulling water into the capillaries tips the balance towards hydrostatic pressure, which tends to push it out of the capillaries. The net effect is that water is lost from the circulation to interstitial tissues and cells. This “plumps up” the tissues and cells, a condition termed systemic edema.


The renal corpuscle filters the blood to create a filtrate that differs from blood mainly in the absence of cells and large proteins. From this point to the ends of the collecting ducts, the filtrate or forming urine is undergoing modification through secretion and reabsorption before true urine is produced. Here, some substances are reabsorbed, whereas others are secreted. Note the use of the term “reabsorbed.” All of these substances were “absorbed” in the digestive tract—99% of the water and most of the solutes filtered by the nephron must be reabsorbed. Water and substances that are reabsorbed are returned to the circulation by the peritubular and vasa recta capillaries.


It is vital that the flow of blood through the kidney is at a suitable rate to allow for filtration. This rate determines how much solute is retained or discarded, how much water is retained or discarded, and ultimately, the osmolarity of blood and the blood pressure of the body.


Urinalysis (urine analysis) often provides clues to renal disease. Normally, only traces of protein are found in urine, and when higher amounts are found, damage to the glomeruli is the likely basis. Unusually large quantities of urine may point to diseases like diabetes mellitus or hypothalamic tumors that cause diabetes insipidus. The color of urine is determined mostly by the breakdown products of red blood cell destruction (see Figure 15.8). The “heme” of hemoglobin is converted by the liver into water-soluble forms that can be excreted into the bile and indirectly into the urine. This yellow pigment is urochrome. Urine color may also be affected by certain foods like beets, berries, and fava beans. A kidney stone or a cancer of the urinary system may produce sufficient bleeding to manifest as pink or even bright red urine. Diseases of the liver or obstructions of bile drainage from the liver impart a dark “tea” or “cola” hue to the urine. Dehydration produces darker, concentrated urine that may also possess the slight odor of ammonia. Most of the ammonia produced from protein breakdown is converted into urea by the liver, so ammonia is rarely detected in fresh urine. The strong ammonia odor you may detect in bathrooms or alleys is due to the breakdown of urea into ammonia by bacteria in the environment. About one in five people detect a distinctive odor in their urine after consuming asparagus; other foods such as onions, garlic, and fish can impart their own aromas. These food-caused odors are harmless.


Urine volume varies considerably. The normal range is one to two liters per day. The kidneys must produce a minimum urine volume of about 500 mL/day to rid the body of wastes. Output below this level may be caused by severe dehydration or renal disease and is termed oliguria. The virtual absence of urine production is termed anuria. Excessive urine production is polyuria, which may be due to diabetes mellitus or diabetes insipidus. In diabetes mellitus, blood glucose levels exceed the number of available sodium-glucose transporters in the kidney, and glucose appears in the urine. The osmotic nature of glucose attracts water, leading to its loss in the urine. In the case of diabetes insipidus, insufficient pituitary antidiuretic hormone (ADH) release or insufficient numbers of ADH receptors in the collecting ducts means that too few water channels are inserted into the cell membranes that line the collecting ducts of the kidney. Insufficient numbers of water channels (aquaporins) reduce water absorption, resulting in high volumes of very dilute urine.


Several hormones have specific, important roles in regulating kidney function. They act to stimulate or inhibit blood flow. Some of these are endocrine, acting from a distance, whereas others are paracrine, acting locally.


Renin is an enzyme that is produced by the granular cells of the afferent arteriole. It enzymatically converts angiotensinogen (made by the liver, freely circulating) into angiotensin I. Its release is stimulated by prostaglandins to decrease extracellular fluid volume.


Angiotensin II is a potent vasoconstrictor that plays an immediate role in the regulation of blood pressure. It acts systemically to cause vasoconstriction as well as constriction of both the afferent and efferent arterioles of the glomerulus. In instances of blood loss or dehydration, it reduces both GFR and renal blood flow, thereby limiting fluid loss and preserving blood volume. Its release is usually stimulated by decreases in blood pressure, and so the preservation of adequate blood pressure is its primary role.


Aldosterone, often called the “salt-retaining hormone,” is released from the adrenal cortex in response to angiotensin II or directly in response to increased plasma potassium. It promotes sodium reabsorption by the nephron, promoting the retention of water.


Diuretics are drugs that can increase water loss by interfering with the recapture of solutes and water from the forming urine. They are often prescribed to lower blood pressure. Coffee, tea, and alcoholic beverages are familiar diuretics. ADH, released by the posterior pituitary, works to do the exact opposite. It promotes the recovery of water, decreases urine volume, and maintains plasma osmolarity and blood pressure. It does so by stimulating the movement of aquaporin proteins into the apical cell membrane of principal cells of the collecting ducts to form water channels, allowing the transcellular movement of water from the lumen of the collecting duct into the interstitial space in the medulla of the kidney by osmosis. From there, it enters the vasa recta capillaries to return to the circulation. Water is attracted by the high osmotic environment of the deep kidney medulla.


Parathyroid hormone (PTH) is produced by the parathyroid glands in response to decreased circulating calcium levels.


Homeostasis requires that volume and osmolarity be preserved. Blood volume is important in maintaining sufficient blood pressure, and there are nonrenal mechanisms involved in its preservation, including vasoconstriction, which can act within seconds of a drop in pressure. Thirst mechanisms are also activated to promote the consumption of water lost through respiration, evaporation, or urination. Hormonal mechanisms are activated to recover volume while maintaining a normal osmotic environment. These mechanisms act principally on the kidney.


A diuretic is a compound that increases urine volume. Three familiar drinks contain diuretic compounds: coffee, tea, and alcohol. The caffeine in coffee and tea works by promoting vasodilation in the nephron, which increases GFR. Alcohol increases GFR by inhibiting ADH release from the posterior pituitary, resulting in less water recovery by the collecting duct. In cases of high blood pressure, diuretics may be prescribed to reduce blood volume and, thereby, reduce blood pressure. The most frequently prescribed anti-hypertensive diuretic is hydrochlorothiazide.


Nitrogen wastes are produced by the breakdown of proteins during normal metabolism. Proteins are broken down into amino acids, which in turn are deaminated by having their nitrogen groups removed. Deamination converts the amino (NH2) groups into ammonia (NH3), ammonium ion (NH4+), urea, or uric acid (Figure 15.9). Ammonia is extremely toxic, so most of it is very rapidly converted into urea in the liver. Human urinary wastes typically contain primarily urea with small amounts of ammonium and very little uric acid.


Water-soluble drugs may be excreted in the urine and are influenced by one or all of the following processes: glomerular filtration, tubular secretion, or tubular reabsorption. Drugs that are structurally small can be filtered by the glomerulus with the filtrate. Large drug molecules such as heparin or those that are bound to plasma proteins cannot be filtered and are not readily eliminated. Some drugs can be eliminated by carrier proteins that enable secretion of the drug into the tubule lumen. There are specific carriers that eliminate basic (such as dopamine or histamine) or acidic drugs (such as penicillin or indomethacin). As is the case with other substances, drugs may be both filtered and reabsorbed passively along a concentration gradient.


Media 15.2. Urinary System, Part 2: Crash Course A&P #39 [Online video]. Copyright 2015 by CrashCourse.


Many terms and phrases related to the urinary system are abbreviated. Learn these common abbreviations by expanding the list below.


Diabetic nephropathy impacts the kidneys as a result of having diabetes mellitus type 1 or 2. Higher levels of blood sugar can lead to high blood pressure and this additional pressure exerted on the kidneys causes destruction of the small filtering structures within the kidney (Varghese & Jialal, 2021). To learn more, visit the Mayo Clinic’s web page on diabetic nephropathy.


Glomerulonephritis refers to acute or chronic nephritis that involves inflammation of the capillaries of the renal glomeruli. It is characterized by blood or protein in the urine and edema. If untreated, it could lead to kidney failure. Glomerulonephritis may be caused by a variety of infections and immune conditions, as a result of genetic defects, or by unknown causes (Genetic and Rare Diseases Information Center, 2012). For more information, visit the Genetic and Rare Diseases Information Center’s web page on glomerulonephritis.


Hydronephrosis is a condition whereby the kidneys begin to swell because of the retention of urine. Several conditions can cause hydronephrosis, such as a kidney stone or pregnancy. Treatment will vary, depending on the cause (Thotakura & Anjum, 2021). To learn more, visit the Cleveland Clinic’s web page on hydronephrosis.


Polycystic kidney disease (PKD) is a genetic disorder whereby cysts grow inside the kidneys. The kidneys enlarge from the cystic collections, and damage to the filtering structures of the kidneys can occur. As the disease progresses, it may lead to kidney failure (National Institute of Diabetes and Digestive and Kidney Diseases, n.d.-a). To learn more, visit the National Institute of Diabetes and Digestive and Kidney Diseases’ web page on polycystic kidney disease.


Renal cell carcinoma is a cancer occurring in the kidney tubes where urine is produced or collected. Signs and symptoms may include blood in the urine and the presence of a lump in the abdomen. Risk factors include smoking and the long-term misuse of pain medicine (National Cancer Institute, 2020). For more information, visit the National Cancer Institute’s web page on renal cell carcinoma.


Renal failure occurs when kidneys become unable to filter waste products from blood. When kidneys stop filtering, high levels of waste may build. Renal failure may be acute (sudden onset) or chronic (gradual onset) (Bindroo et al., 2021). For more information, visit the Cleveland Clinic’s web page on renal failure.


Cystitis is inflammation of the urinary bladder, often caused by an infection. A chronic form of this condition is known as interstitial cystitis. Signs and symptoms of cystitis include bladder pressure, voiding frequently, and abdominal pain (Centers for Disease Control and Prevention, n.d.; Li & Leslie, 2021). To learn more, visit the Mayo Clinic’s web page on cystitis.


A urinary tract infection (UTI) is an infection caused by bacteria. The exact type of bacterial growth is determined by conducting a urine culture. Signs and symptoms include a burning sensation during urination and voiding frequently (National Institute of Diabetes and Digestive and Kidney Diseases, n.d.-b). For more information, visit the National Institute of Diabetes and Digestive and Kidney Diseases’ web page on urinary tract infections.


Urinary incontinence is a loss of bladder control. Those afflicted with the condition will experience urine leakage from the bladder. Urinary incontinence may be related to another health issue, such as prostate problems, or it may be the result of weakened urinary tract muscles (National Institute of Diabetes and Digestive and Kidney Diseases, n.d.-c). For more information, visit the National Institute of Diabetes and Digestive and Kidney Diseases’ web page on bladder incontinence.


A urologist is a medical specialist involved in the diagnosis and treatment of urinary and male genitourinary system conditions, disorders, and diseases (National Cancer Institute, n.d.-a). To learn more about what urologists do and how to become one, visit the Cleveland Clinic’s web page about the specialty.


A urinalysis is the physical, chemical, and microscopic examination of urine. This test detects and measures several substances in the urine such as products of normal and abnormal metabolism and bacteria (Queremel Milani & Jialal, 2021). To learn more, visit the Mayo Clinic’s web page on urinalysis.


A urine culture is a test that can detect and identify bacteria in the urine, which may be causing a urinary tract infection (UTI). If harmful bacteria is found, a sensitivity report is generated. A sensitivity test indicates which antibiotics will be effective in treating the infection (Sinawe & Casadesus, 2021; MedlinePlus, 2021). For more information, visit MedlinePlus’ web page on urine cultures.


A 24-hour urinalysis is a test whereby all urinary output is collected over a 24-hour period of time. The analysis of urinary output over this extended period of time provides a greater indication of normal or abnormal kidney function (Corder et al., 2021). For more information, visit Johns Hopkins Medicine’s web page on 24-hour urine collection.


Computed tomography is a diagnostic imaging procedure that uses a combination of x-rays and computer technology to produce a variety of images (National Cancer Institute, n.d.-b). It provides detailed images of the kidney so that healthcare professionals can diagnose disease, obstructions, and other kidney conditions. To learn more, visit Johns Hopkins Medicine’s page on computed tomography scan of the kidney.


A cystoscopy is a procedure allowing a physician to view the bladder and urethra. The procedure involves the use of a cystoscope, a long tube-like instrument that has a camera and light at the end. The camera’s images are transmitted to a computer, where the urologist can determine whether the patient has bladder stones, cancer, or other urinary tract problems (National Institute of Diabetes and Digestive and Kidney Diseases, n.d.-d). For more information, view the National Institute of Diabetes and Digestive and Kidney Diseases’ web page on cystoscopy.


Dialysis is a treatment that removes waste products from the blood when the kidneys are not fully functioning. This type of therapy is available at home, in a hospital, or clinic. There are two main types: peritoneal dialysis and hemodialysis (National Institute of Diabetes and Digestive and Kidney Diseases, n.d.-e). To learn more, visit the National Institute of Diabetes and Digestive and Kidney Diseases’ web page on kidney failure treatment options.


An intravenous pyelogram (IVP) is a specialized x-ray designed to produce views of the entire urinary tract. Contrast dye is used to produce clear x-ray images. The x-rays can show how well the urinary tract is functioning and identify any blockages (MedlinePlus, 2020). For more information, visit MedlinePlus’ web page on intravenous pyelogram.


A kidney scan is an imaging test which views the kidneys. It is considered a nuclear imaging test as it uses radioactive tracers to evaluate kidney function. A kidney scan may be performed if the patient is allergic to the contrast material used with CT or MRI tests (Banker et al., 2021). For more information, visit MedlinePlus’ web page on renal scans.


When a patient experiences kidney failure, a healthy kidney can be surgically transplanted from a donor to the patient. The donor may be living or recently deceased. Patients may have to wait for many years before a kidney is available and may have to go on dialysis in the meantime (National Institute of Diabetes and Digestive and Kidney Diseases, n.d.-f). For more information, visit the National Institute of Diabetes and Digestive and Kidney Diseases’ web page on kidney transplant.


The outermost layer of organs, blood vessels, and other structures in the body.


Excision of all or part of the bladder to remove a cyst.


A condition in which weakened pelvic muscles cause the bladder to drop from its normal position.


A thin, tube-like instrument used to look inside the bladder and urethra.


Examination of the bladder and urethra using a cystoscope, inserted into the urethra.


A muscle which forms a layer of the wall of the bladder.


To get rid of waste material from the blood, tissues, or organs by a normal discharge (such as sweat, urine, or stool).


A condition in which the tissues in the kidney become inflamed and have problems filtering waste from the blood.


Abnormal enlargement of a kidney, which may be caused by blockage of the ureter (such as by a kidney stone) or chronic kidney disease that prevents urine from draining into the bladder.


Relating to the equilibrium of liquids and the pressure exerted by liquid at rest.


A region of the forebrain below the thalamus; has function in both the autonomic and endocrine systems and regulates homeostasis.


A condition marked by drowsiness and an unusual lack of energy and mental alertness.


The destruction of a calculus (stone) of the kidney, ureter, bladder, or gallbladder by physical forces.


A membranous, bean-shaped organelle that is the “energy transformer” of the cell.


A doctor who has special training in diagnosing and treating kidney disease.


A subspecialty of internal medicine concerned with the anatomy, physiology, and pathology of the kidney.


Surgery to make an opening from the outside of the body to the renal pelvis.


A process by which molecules of a solvent tend to pass through a membrane from a less concentrated solution into a more concentrated one.


A measure of how acidic or alkaline a substance is, as determined by the number of free hydrogen ions in the substance.


Tissue with a single layer of irregularly shaped cells that give the appearance of more than one layer.


Examination of the inside of the kidney and ureter, using a ureteroscope.


Pertaining to urine or the organs of the body that produce and get rid of urine.


A doctor who has special training in diagnosing and treating diseases of the urinary organs in females and the urinary and reproductive organs in males.


A surgical specialty concerned with the study, diagnosis, and treatment of diseases of the urinary tract in both sexes, and the genital tract in the male.


Banker, H., Sheffield, E. G., & Cohen, H. L. (2021). Nuclear renal scan. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK562236/


Bindroo, S., Quintanilla Rodriguez, B.S., & Challa, H. J. (2021). Renal failure. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK519012/


Centers for Disease Control and Prevention. (n.d.). Interstitial cystitis. https://www.cdc.gov/ic/index.html


Corder, C. J., Rathi, B. M., Sharif, S., & Leslie, S. W. (2021). 24-hour urine collection. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK482482/


[CrashCourse]. (2015, October 12). Urinary system, part 1: Crash course A&P #38 [Video]. YouTube. https://www.youtube.com/watch?v=l128tW1H5a8


CrashCourse. (2015, June 22). Urinary system, part 2: Crash course A&P #39 [Video]. YouTube. https://youtu.be/DlqyyyvTI3k


Genetic and Rare Diseases Information Center. (2012). Glomerulonephritis. National Institutes of Health, U.S. Department of Health and Human Services. https://rarediseases.info.nih.gov/diseases/6516/glomerulonephritis


Li, R., & Leslie, S. W. (2021). Cystitis. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK482435/


MedlinePlus. (2021). Antibiotic sensitivity test. U.S. National Library of Medicine. https://medlineplus.gov/lab-tests/antibiotic-sensitivity-test/


MedlinePlus. (2020). Intravenous pyelogram. U.S. National Library of Medicine. https://medlineplus.gov/lab-tests/intravenous-pyelogram-ivp/


National Cancer Institute. (n.d.-a). Definition of urologist. National Institutes of Health, U.S. Department of Health and Human Services. https://www.cancer.gov/publications/dictionaries/cancer-terms/def/urologist


National Cancer Institute. (n.d.-b). Computed tomography (CT) scans and cancer. National Institutes of Health, U.S. Department of Health and Human Services. https://www.cancer.gov/about-cancer/diagnosis-staging/ct-scans-fact-sheet


National Cancer Institute. (2020). Renal cell cancer treatment (PDQ®) – Patient version. National Institutes of Health, U.S. Department of Health and Human Services. https://www.cancer.gov/types/kidney/patient/kidney-treatment-pdq


National Institute of Diabetes and Digestive and Kidney Diseases. (n.d.-a). Polycystic kidney disease? National Institutes of Health, U.S. Department of Health and Human Services. https://www.niddk.nih.gov/health-information/kidney-disease/polycystic-kidney-disease/all-content


National Institute of Diabetes and Digestive and Kidney Diseases. (n.d.-b). Bladder infection (urinary tract infection) in adults. National Institutes of Health, U.S. Department of Health and Human Services. https://www.niddk.nih.gov/health-information/urologic-diseases/bladder-infection-uti-in-adults/all-content


National Institute of Diabetes and Digestive and Kidney Diseases. (n.d.-c). Urinary incontinence. National Institutes of Health, U.S. Department of Health and Human Services. https://www.niddk.nih.gov/health-information/urologic-diseases/bladder-control-problems/all-content


National Institute of Diabetes and Digestive and Kidney Diseases. (n.d.-d). Cystoscopy & ureteroscopy. National Institutes of Health, U.S. Department of Health and Human Services. https://www.niddk.nih.gov/health-information/diagnostic-tests/cystoscopy-ureteroscopy


National Institute of Diabetes and Digestive and Kidney Diseases. (n.d.-e). Choosing a treatment for kidney failure. National Institutes of Health, U.S. Department of Health and Human Services. https://www.niddk.nih.gov/health-information/kidney-disease/kidney-failure/choosing-treatment


National Institute of Diabetes and Digestive and Kidney Diseases. (n.d.-f). Kidney transplant. National Institutes of Health, U.S. Department of Health and Human Services. https://www.niddk.nih.gov/health-information/kidney-disease/kidney-failure/kidney-transplant


Queremel Milani, D. A., & Jialal, I. (2021). Urinalysis. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK557685/


Sinawe, H., Casadesus, D. (2021). Urine culture. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK557569/


Thotakura, R., & Anjum, F. (2021). Hydronephrosis and hydroureter. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK563217/


Varghese, R. T., & Jialal, I. (2021). Diabetic nephropathy. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK534200/


You may never have thought of it this way, but when you send a text message to two friends to meet you at the dining hall at six, you’re sending digital signals that (you hope) will affect their behavior—even though they are some distance away. Similarly, certain cells send chemical signals to other cells in the body that influence their behavior. This long-distance intercellular communication, coordination, and control are critical to maintaining equilibrium (homeostasis). This intercellular activity is the fundamental function of the endocrine system.


Media 14.1 Endocrine System, Part 1 – Glands & Hormones: Crash Course A&P #23 [Online video]. Copyright 2015 by CrashCourse.


The endocrine system consists of cells, tissues, and organs that secrete hormones as a primary or secondary function. The endocrine gland is the major player in this system. The primary function of the endocrine gland is to secrete hormones directly into the surrounding fluid. The surrounding fluid (interstitial fluid) and the blood vessels then transport the hormones throughout the body. The endocrine system includes the pituitary, thyroid, parathyroid, adrenal, and pineal glands (see Figure 14.2). Some of these glands have both endocrine and nonendocrine functions. For example, the pancreas contains cells that function in digestion as well as cells that secrete the endocrine hormones like insulin and glucagon, which regulate blood glucose levels. The hypothalamus, thymus, heart, kidneys, stomach, small intestine, liver, skin, female ovaries, and male testes are other organs that contain cells with endocrine function. Moreover, fat (adipose) tissue has long been known to produce hormones, and recent research has revealed that even bone tissue has endocrine functions.


The ductless endocrine glands are not to be confused with the body’s exocrine system, whose glands release their secretions through ducts. Examples of exocrine glands include the sebaceous and sweat glands of the skin. As just noted, the pancreas also has an exocrine function: most of its cells secrete pancreatic juice through the pancreatic and accessory ducts to the lumen of the small intestine.


The endocrine system uses one method of communication called chemical signaling. These chemical signals are sent by the endocrine organs. The endocrine organs secrete chemicals—called hormones—into the fluid outside of the tissue cells (extracellular fluid). Hormones are then transported primarily via the bloodstream throughout the body, where they bind to receptors on target cells, creating a particular response. For example, when you are presented with a dangerous or a frightening situation, the fight-or-flight response prompts the release of hormones from the adrenal gland—epinephrine and norepinephrine—within seconds. In contrast, it may take up to 48 hours for target cells to respond to certain reproductive hormones.


In addition, endocrine signaling is typically less specific than neural (nerve) signaling. The same hormone may also play a role in a variety of different physiological processes depending on the target cells involved. For example, the hormone oxytocin generates uterine contractions in women who are in labor. This hormone is also important in generating the milk release reflex during breastfeeding and may be involved in the sexual response and feelings of emotional attachment in both males and females.


Generally, the nervous system involves quick responses to rapid changes in the external environment, and the endocrine system is usually slower acting—taking care of the internal environment of the body, maintaining equilibrium (homeostasis), and controlling reproduction (see Table 14.1). So how does the fight-or-flight response, that was mentioned earlier, happen so quickly if hormones are usually slower acting? It is because the two systems are connected. It is the fast action of the nervous system in response to the danger in the environment that stimulates the adrenal glands to secrete their hormones, epinephrine, and norepinephrine. As a result, the nervous system can cause rapid endocrine responses to keep up with sudden changes in both the external and internal environments, when necessary.


There are four different types of chemical signaling occurring in multicellular organisms: endocrine signaling, autocrine signaling, paracrine signaling, and direct signaling.


In endocrine signaling, hormones secreted into the extracellular fluid spreads into the blood or lymphatic system, and can, therefore, travel great distances throughout the body.


In contrast, autocrine signaling occurs within the same cell. An autocrine (auto- = “self”) is a chemical that triggers a response in the same cell that secreted the chemical. For example, Interleukin-1 (or IL-1), is a chemical signaling molecule that plays a role in inflammation. The cells that release IL-1 also have receptors on their surface that bind IL-1, resulting in autocrine signaling.


Paracrine signaling occurs amongst neighboring cells. A paracrine (para- = “near”) is a chemical that triggers a response in neighboring cells. Although paracrines may enter the bloodstream, their concentration is generally too low to elicit a response from distant tissues. A familiar example for those with asthma is histamine, a paracrine that is released by immune cells. Histamine causes the smooth muscle cells of the lungs to constrict, narrowing the airways.


Direct signaling occurs between neighboring cells across gap junctions. Gap junctions are channels that connect neighboring cells, that allow small molecules to move between the neighboring cells.


Although a given hormone may travel throughout the body in the bloodstream, it will affect the activity only of its target cells; that is, cells with receptors for that particular hormone. Once the hormone binds to the receptor, a chain of events is initiated that leads to the target cell’s response. Hormones play a critical role in the regulation of physiological processes because of the target cell responses they regulate. These responses contribute to human reproduction, growth and development of body tissues, metabolism, fluid, and electrolyte balance, sleep, and many other body functions. The major hormones of the human body and their effects are identified in Table 14.2.


The hormones of the human body can be divided into two major groups on the basis of their chemical structure. Hormones derived from amino acids include amines, peptides, and proteins. Those derived from lipids include steroids (see Table 14.3). These chemical groups affect a hormone’s distribution, the type of receptors it binds to, and other aspects of its function.


Hormones derived from the modification of amino acids are referred to as amine hormones. Amine hormones are synthesized from the amino acids tryptophan or tyrosine. An example of a hormone derived from tryptophan is melatonin, which is secreted by the pineal gland and helps regulate circadian rhythm.


Whereas the amine hormones are derived from a single amino acid, peptide and protein hormones consist of multiple amino acids that link to form an amino acid chain. Examples of peptide hormones include antidiuretic hormone (ADH), a pituitary hormone important in fluid balance. Some examples of protein hormones include growth hormone, which is produced by the pituitary gland, and follicle-stimulating hormone (FSH). FSH helps stimulate the maturation of eggs in the ovaries and sperm in the testes.


The primary hormones derived from lipids are steroids. Steroid hormones are derived from lipid cholesterol. For example, the reproductive hormones testosterone and estrogens—which are produced by the gonads (testes and ovaries)—are steroid hormones. The adrenal glands produce the steroid hormone aldosterone, which is involved in osmoregulation, and cortisol, which plays a role in metabolism.


Like cholesterol, steroid hormones are not soluble in water (they are hydrophobic). Because blood is water-based, lipid-derived hormones must travel to their target cell bound to a transport protein.


The message a hormone sends is received by a hormone receptor, a protein located either inside the cell or within the cell membrane. The receptor will process the message by initiating other signaling events or cellular mechanisms that result in the target cell’s response. Hormone receptors recognize molecules with specific shapes and side groups and respond only to those hormones that are recognized. The same type of receptor may be located on cells in different body tissues, and trigger somewhat different responses. Thus, the response triggered by a hormone depends not only on the hormone but also on the target cell.


Once the target cell receives the hormone signal, it can respond in a variety of ways. The response may include the stimulation of protein synthesis, activation or deactivation of enzymes, alteration in the permeability of the cell membrane, altered rates of mitosis and cell growth, and stimulation of the secretion of products. Moreover, a single hormone may be capable of inducing different responses in a given cell.


You will recall that target cells must have receptors specific to a given hormone if that hormone is to trigger a response, but several other factors influence the target cell response. For example, the presence of a significant level of a hormone circulating in the bloodstream can cause its target cells to decrease their number of receptors for that hormone. This process is called downregulation, and it allows cells to become less reactive to excessive hormone levels. When the level of a hormone is chronically reduced, target cells engage in upregulation to increase their number of receptors. This process allows cells to be more sensitive to the hormone that is present. Cells can also alter the sensitivity of the receptors themselves to various hormones.


Two or more hormones can interact to affect the response of cells in a variety of ways. The three most common types of interaction are as follows:


To prevent abnormal hormone levels and a potential disease state, hormone levels must be tightly controlled. The body maintains this control by balancing hormone production and degradation. Feedback loops govern the initiation and maintenance of most hormone secretion in response to various stimuli.


The contribution of feedback loops to homeostasis will only be briefly reviewed here. Positive feedback loops are characterized by the release of additional hormones in response to an original hormone release. The release of oxytocin during childbirth is a positive feedback loop. The initial release of oxytocin begins to signal the uterine muscles to contract, which pushes the fetus toward the cervix, causing it to stretch. This, in turn, signals the pituitary gland to release more oxytocin, causing labor contractions to intensify. The release of oxytocin decreases after the birth of the child.


The more common method of hormone regulation is the negative feedback loop. Negative feedback is characterized by the inhibition of further secretion of a hormone in response to adequate levels of that hormone. This allows blood levels of the hormone to be regulated within a narrow range. An example of a negative feedback loop is the release of glucocorticoid hormones from the adrenal glands, as directed by the hypothalamus and pituitary gland. As glucocorticoid concentrations in the blood rise, the hypothalamus and pituitary gland reduce their signaling to the adrenal glands to prevent additional glucocorticoid secretion (see Figure 14.3).


The anterior pituitary originates from the digestive tract in the embryo and migrates toward the brain during fetal development. There are three regions: the pars distalis is the most anterior, the pars intermedia is adjacent to the posterior pituitary, and the pars tuberalis is a slender “tube” that wraps the infundibulum.


Recall that the posterior pituitary does not synthesize hormones, but merely stores them. In contrast, the anterior pituitary does manufacture hormones. However, the secretion of hormones from the anterior pituitary is regulated by two classes of hormones. These hormones—secreted by the hypothalamus—are the releasing hormones that stimulate the secretion of hormones from the anterior pituitary and the inhibiting hormones that inhibit secretion.


Hypothalamic hormones are secreted by neurons but enter the anterior pituitary through blood vessels. Within the infundibulum is a bridge of capillaries that connects the hypothalamus to the anterior pituitary. This network, called the hypophyseal portal system, allows hypothalamic hormones to be transported to the anterior pituitary without first entering the systemic circulation. The system originates from the superior hypophyseal artery, which branches off the carotid arteries and transports blood to the hypothalamus. The branches of the superior hypophyseal artery form the hypophyseal portal system (see Figure 14.4). Hypothalamic releasing and inhibiting hormones travel through a primary capillary plexus to the portal veins, which carry them into the anterior pituitary. Hormones produced by the anterior pituitary (in response to releasing hormones) enter a secondary capillary plexus, and from there drain into the circulation.


The anterior pituitary produces seven hormones. These are the growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), beta-endorphin, and prolactin. Of the hormones of the anterior pituitary, TSH, ACTH, FSH, and LH are collectively referred to as tropic hormones (trope- = “turning”) because they turn on or off the function of other endocrine glands.


The endocrine system regulates the growth of the human body, protein synthesis, and cellular replication. A major hormone involved in this process is growth hormone (GH), also called somatotropin—a protein hormone produced and secreted by the anterior pituitary gland. Its primary function is anabolic; it promotes protein synthesis and tissue building through direct and indirect mechanisms (see Figure 14.5). GH levels are controlled by the release of GHRH and GHIH (also known as somatostatin) from the hypothalamus.