A glucose-sparing effect occurs when GH stimulates lipolysis, or the breakdown of adipose tissue, releasing fatty acids into the blood. As a result, many tissues switch from glucose to fatty acids as their main energy source, which means that less glucose is taken up from the bloodstream.


GH also initiates the diabetogenic effect in which GH stimulates the liver to break down glycogen to glucose, which is then deposited into the blood. The name “diabetogenic” is derived from the similarity in elevated blood glucose levels observed between individuals with untreated diabetes mellitus and individuals experiencing GH excess. Blood glucose levels rise as the result of a combination of glucose-sparing and diabetogenic effects.


GH indirectly mediates growth and protein synthesis by triggering the liver and other tissues to produce a group of proteins called insulin-like growth factors (IGFs). These proteins enhance cellular proliferation and inhibit apoptosis, or programmed cell death. IGFs stimulate cells to increase their uptake of amino acids from the blood for protein synthesis. Skeletal muscle and cartilage cells are particularly sensitive to stimulation from IGFs.


Dysfunction of the endocrine system’s control of growth can result in several disorders. For example, gigantism is a disorder in children that is caused by the secretion of abnormally large amounts of GH, resulting in excessive growth. A similar condition in adults is acromegaly, a disorder that results in the growth of bones in the face, hands, and feet in response to excessive levels of GH in individuals who have stopped growing. Abnormally low levels of GH in children can cause growth impairment—a disorder called pituitary dwarfism (also known as growth hormone deficiency).


The posterior pituitary is actually an extension of the neurons of the nuclei of the hypothalamus. The cell bodies of these regions rest in the hypothalamus, but their axons descend as the hypothalamic–hypophyseal tract within the infundibulum and end in axon terminals that comprise the posterior pituitary (see Figure 14.6).


The posterior pituitary gland does not produce hormones, but rather stores and secretes hormones produced by the hypothalamus. The paraventricular nuclei produce the hormone oxytocin, whereas the supraoptic nuclei produce ADH. These hormones travel along the axons into storage sites in the axon terminals of the posterior pituitary. In response to signals from the same hypothalamic neurons, the hormones are released from the axon terminals into the bloodstream.


When fetal development is complete, the peptide-derived hormone oxytocin (tocia- = “childbirth”) stimulates uterine contractions and dilation of the cervix. Throughout most of pregnancy, oxytocin hormone receptors are not expressed at high levels in the uterus. Toward the end of pregnancy, the synthesis of oxytocin receptors in the uterus increases, and the smooth muscle cells of the uterus become more sensitive to its effects. Oxytocin is continually released throughout childbirth through a positive feedback mechanism. As noted earlier, oxytocin prompts the uterine contractions that push the fetal head toward the cervix. In response, cervical stretching stimulates additional oxytocin to be synthesized by the hypothalamus and released from the pituitary. This increases the intensity and effectiveness of uterine contractions and prompts additional dilation of the cervix. The feedback loop continues until birth.


Although the mother’s high blood levels of oxytocin begin to decrease immediately following birth, oxytocin continues to play a role in maternal and newborn health. First, oxytocin is necessary for the milk ejection reflex (commonly referred to as “let-down”) in breastfeeding women. As the newborn begins suckling, sensory receptors in the nipples transmit signals to the hypothalamus. In response, oxytocin is secreted and released into the bloodstream. Within seconds, cells in the mother’s milk ducts contract, ejecting milk into the infant’s mouth. Secondly, in both males and females, oxytocin is thought to contribute to parent–newborn bonding, known as attachment. Oxytocin is also thought to be involved in feelings of love and closeness, as well as in the sexual response.


The solute concentration of the blood, or blood osmolarity, may change in response to the consumption of certain foods and fluids, as well as in response to disease, injury, medications, or other factors. Blood osmolarity is constantly monitored by osmoreceptors—specialized cells within the hypothalamus that are particularly sensitive to the concentration of sodium ions and other solutes. In response to high blood osmolarity, which can occur during dehydration or following a very salty meal, the osmoreceptors signal the posterior pituitary to release antidiuretic hormone (ADH). The target cells of ADH are located in the tubular cells of the kidneys. Its effect is to increase epithelial permeability to water, allowing increased water reabsorption. The more water reabsorbed from the filtrate, the greater the amount of water that is returned to the blood and the less that is excreted in the urine. A greater concentration of water results in a reduced concentration of solutes. ADH is also known as vasopressin because, in very high concentrations, it causes constriction of blood vessels, which increases blood pressure by increasing peripheral resistance.


The release of ADH is controlled by a negative feedback loop. As blood osmolarity decreases, the hypothalamic osmoreceptors sense the change and prompt a corresponding decrease in the secretion of ADH. As a result, less water is reabsorbed from the urine filtrate. Interestingly, drugs can affect the secretion of ADH. For example, alcohol consumption inhibits the release of ADH, resulting in increased urine production that can eventually lead to dehydration and a hangover. A disease called diabetes insipidus is characterized by chronic underproduction of ADH that causes chronic dehydration. Because little ADH is produced and secreted, not enough water is reabsorbed by the kidneys. Although patients feel thirsty and increase their fluid consumption, this doesn’t effectively decrease the solute concentration in their blood because ADH levels are not high enough to trigger water reabsorption in the kidneys. Electrolyte imbalances can occur in severe cases of diabetes insipidus.


The adrenocorticotropic hormone (ACTH), also called corticotropin, stimulates the adrenal cortex (the more superficial “bark” of the adrenal glands) to secrete corticosteroid hormones such as cortisol. ACTH comes from a precursor molecule known as proopiomelanocortin (POMC) which produces several biologically active molecules when cleaved, including ACTH, melanocyte-stimulating hormone, and the brain opioid peptides known as endorphins. The release of ACTH is regulated by the corticotropin-releasing hormone (CRH) from the hypothalamus in response to normal physiologic rhythms. A variety of stressors can also influence its release, and the role of ACTH in the stress response is discussed later in this chapter.


The endocrine glands secrete a variety of hormones that control the development and regulation of the reproductive system (these glands include the anterior pituitary, the adrenal cortex, and the gonads—the testes in males and the ovaries in females). Much of the development of the reproductive system occurs during puberty and is marked by the development of sex-specific characteristics in both male and female adolescents. Puberty is initiated by gonadotropin-releasing hormone (GnRH), a hormone produced and secreted by the hypothalamus. GnRH stimulates the anterior pituitary to secrete gonadotropins—hormones that regulate the function of the gonads. The levels of GnRH are regulated through a negative feedback loop; high levels of reproductive hormones inhibit the release of GnRH. Throughout life, gonadotropins regulate reproductive function and, in the case of women, the onset and cessation of reproductive capacity.


The gonadotropins include two glycoprotein hormones: follicle-stimulating hormone (FSH) stimulates the production and maturation of sex cells, or gametes, including ova in women and sperm in men. FSH also promotes follicular growth; these follicles then release estrogens in the female ovaries. Luteinizing hormone (LH) triggers ovulation in women, as well as the production of estrogens and progesterone by the ovaries. LH stimulates production of testosterone by the male testes.


As its name implies, prolactin (PRL) promotes lactation (milk production) in women. During pregnancy, it contributes to the development of the mammary glands, and after birth, it stimulates the mammary glands to produce breast milk. However, the effects of prolactin depend heavily upon the permissive effects of estrogens, progesterone, and other hormones. And as noted earlier, the let-down of milk occurs in response to stimulation from oxytocin.


In a non-pregnant woman, prolactin secretion is inhibited by prolactin-inhibiting hormone (PIH), which is actually the neurotransmitter dopamine, and is released from neurons in the hypothalamus. Only during pregnancy do prolactin levels rise in response to prolactin-releasing hormone (PRH) from the hypothalamus.


The cells in the zone between the pituitary lobes secrete a hormone known as melanocyte-stimulating hormone (MSH) that is formed by cleavage of the proopiomelanocortin (POMC) precursor protein. Local production of MSH in the skin is responsible for melanin production in response to UV light exposure. The role of MSH made by the pituitary is more complicated. For instance, people with lighter skin generally have the same amount of MSH as people with darker skin. Nevertheless, this hormone is capable of darkening the skin by inducing melanin production in the skin’s melanocytes. Women also show increased MSH production during pregnancy; in combination with estrogens, it can lead to darker skin pigmentation, especially the skin of the areolas and labia minora. Table 14.4 is a summary of the pituitary hormones and their principal effects.


The release of T3 and T4 from the thyroid gland is regulated by thyroid-stimulating hormone (TSH). Low blood levels of T3 and T4 stimulate the release of thyrotropin-releasing hormone (TRH) from the hypothalamus, which triggers the secretion of TSH from the anterior pituitary. In turn, TSH stimulates the thyroid gland to secrete T3 and T4. The levels of TRH, TSH, T3, and T4 are regulated by a negative feedback system in which increasing levels of T3 and T4 decrease the production and secretion of TSH. The thyroid hormones, T3 and T4, are often referred to as metabolic hormones because their levels influence the body’s basal metabolic rate, the amount of energy used by the body at rest.


The thyroid gland also secretes a hormone called calcitonin that is produced by the parafollicular cells (also called C cells) that stud the tissue between distinct follicles. Calcitonin is released in response to a rise in blood calcium levels.


The parathyroid glands are tiny, round structures usually found embedded in the posterior surface of the thyroid gland. A thick connective tissue capsule separates the glands from the thyroid tissue. Most people have four parathyroid glands, but occasionally there are more in tissues of the neck or chest. The function of one type of parathyroid cells, the oxyphil cells, is not clear. The primary functional cells of the parathyroid glands are the chief cells. These epithelial cells produce and secrete the parathyroid hormone (PTH), the major hormone involved in the regulation of blood calcium levels.


The adrenal glands are wedges of glandular and neuroendocrine tissue adhering to the top of the kidneys by a fibrous capsule (see Figure 14.8). The adrenal glands have a rich blood supply and experience one of the highest rates of blood flow in the body. They are served by several arteries branching off the aorta, including the suprarenal and renal arteries. Blood flows to each adrenal gland at the adrenal cortex and then drains into the adrenal medulla. Adrenal hormones are released into the circulation via the left and right suprarenal veins.


The adrenal cortex consists of multiple layers of lipid-storing cells that occur in three structurally distinct regions, each of which produces different hormones. As a component of the hypothalamic-pituitary-adrenal (HPA) axis, it secretes steroid hormones important for the regulation of the long-term stress response, blood pressure and blood volume, nutrient uptake and storage, fluid and electrolyte balance, and inflammation. The HPA axis involves the stimulation of hormone release of adrenocorticotropic hormone (ACTH) from the pituitary by the hypothalamus. ACTH then stimulates the adrenal cortex to produce the hormone cortisol. This pathway will be discussed in more detail below.


The adrenal medulla is neuroendocrine tissue composed of postganglionic sympathetic nervous system (SNS) neurons. It is really an extension of the autonomic nervous system, which regulates homeostasis in the body. The sympathomedullary (SAM) pathway involves the stimulation of the medulla by impulses from the hypothalamus via neurons from the thoracic spinal cord. The medulla is stimulated to secrete the amine hormones epinephrine and norepinephrine.


One of the major functions of the adrenal gland is to respond to stress. Stress can be either physical or psychological or both. Physical stresses include exposing the body to injury, walking outside in cold and wet conditions without a coat on, or malnutrition. Psychological stresses include the perception of a physical threat, a fight with a loved one, or just a bad day at school.


The body responds in different ways to short-term stress and long-term stress following a pattern known as the general adaptation syndrome (GAS). Stage one of GAS is called the alarm reaction. This is short-term stress, the fight-or-flight response, mediated by the hormones epinephrine and norepinephrine from the adrenal medulla via the SAM pathway. Their function is to prepare the body for extreme physical exertion. Once this stress is relieved, the body quickly returns to normal. The section on the adrenal medulla covers this response in more detail.


If the stress is not soon relieved, the body adapts to the stress in the second stage called the stage of resistance. If a person is starving, for example, the body may send signals to the gastrointestinal tract to maximize the absorption of nutrients from food.


If the stress continues for a longer term, however, the body responds with symptoms quite different than the fight-or-flight response. During the stage of exhaustion, individuals may begin to suffer depression, the suppression of their immune response, severe fatigue, or even a fatal heart attack. These symptoms are mediated by the hormones of the adrenal cortex, especially cortisol, released as a result of signals from the HPA axis.


Adrenal hormones also have several non–stress-related functions, including the increase of blood sodium and glucose levels, which will be described in detail below.


Media 14.2 Endocrine System, Part 2 – Hormone Cascades: Crash Course A&P #24 [Online video]. Copyright 2015 by CrashCourse.


The most superficial region of the adrenal cortex is the zona glomerulosa, which produces a group of hormones collectively referred to as mineralocorticoids because of their effect on body minerals, especially sodium and potassium. These hormones are essential for fluid and electrolyte balance.


Aldosterone is the major mineralocorticoid. It is important in the regulation of the concentration of sodium and potassium ions in urine, sweat, and saliva. For example, it is released in response to elevated blood K+, low blood Na+, low blood pressure, or low blood volume. In response, aldosterone increases the excretion of K+ and the retention of Na+, which in turn increases blood volume and blood pressure. Its secretion is prompted when CRH from the hypothalamus triggers ACTH release from the anterior pituitary.


Aldosterone is also a key component of the renin-angiotensin-aldosterone system (RAAS) in which specialized cells of the kidneys secrete the enzyme renin in response to low blood volume or low blood pressure. Renin then catalyzes the conversion of the blood protein angiotensinogen, produced by the liver, to the hormone angiotensin I. Angiotensin I is converted in the lungs to angiotensin II by angiotensin-converting enzyme (ACE). Angiotensin II has three major functions:


For individuals with hypertension, or high blood pressure, drugs are available that block the production of angiotensin II. These drugs, known as ACE inhibitors, block the ACE enzyme from converting angiotensin I to angiotensin II, thus mitigating the latter’s ability to increase blood pressure.


The intermediate region of the adrenal cortex is the zona fasciculata, named as such because the cells form small fascicles (bundles) separated by tiny blood vessels. The cells of the zona fasciculata produce hormones called glucocorticoids because of their role in glucose metabolism. The most important of these is cortisol, some of which the liver converts to cortisone. A glucocorticoid produced in much smaller amounts is corticosterone. In response to long-term stressors, the hypothalamus secretes CRH, which in turn triggers the release of ACTH by the anterior pituitary. ACTH triggers the release of glucocorticoids. Their overall effect is to inhibit tissue building while stimulating the breakdown of stored nutrients to maintain adequate fuel supplies. In conditions of long-term stress, for example, cortisol promotes the catabolism of glycogen to glucose, the catabolism of stored triglycerides into fatty acids and glycerol, and the catabolism of muscle proteins into amino acids. These raw materials can then be used to synthesize additional glucose and ketones for use as body fuels. The hippocampus, which is part of the temporal lobe of the cerebral cortices and important in memory formation, is highly sensitive to stress levels because of its many glucocorticoid receptors.


You are probably familiar with prescription and over-the-counter medications containing glucocorticoids, such as cortisone injections into inflamed joints, prednisone tablets, and steroid-based inhalers used to manage severe asthma, and hydrocortisone creams applied to relieve itchy skin rashes. These drugs reflect another role of cortisol—the downregulation of the immune system, which inhibits the inflammatory response.


The deepest region of the adrenal cortex is the zona reticularis, which produces small amounts of a class of steroid sex hormones called androgens. During puberty and most of adulthood, androgens are produced in the gonads. The androgens produced in the zona reticularis supplement the gonadal androgens. They are produced in response to ACTH from the anterior pituitary and are converted in the tissues to testosterone or estrogens. In adult women, they may contribute to the sex drive, but their function in adult men is not well understood. In post-menopausal women, as the functions of the ovaries decline, the main source of estrogens becomes the androgens produced by the zona reticularis.


The medullary tissue is composed of unique postganglionic SNS neurons called chromaffin cells, which are large and irregularly shaped, and produce the neurotransmitters epinephrine (also called adrenaline) and norepinephrine (or noradrenaline). Epinephrine is produced in greater quantities—approximately a 4 to 1 ratio with norepinephrine—and is the more powerful hormone. Because the chromaffin cells release epinephrine and norepinephrine into the systemic circulation, where they travel widely and exert effects on distant cells, they are considered hormones. Derived from the amino acid tyrosine, they are chemically classified as catecholamines.


The secretion of medullary epinephrine and norepinephrine is controlled by a neural pathway that originates from the hypothalamus in response to danger or stress (the SAM pathway). Both epinephrine and norepinephrine signal the liver and skeletal muscle cells to convert glycogen into glucose, resulting in increased blood glucose levels. These hormones increase the heart rate, pulse, and blood pressure to prepare the body to fight the perceived threat or flee from it. In addition, the pathway dilates the airways, raising blood oxygen levels. It also prompts vasodilation, further increasing the oxygenation of important organs such as the lungs, brain, heart, and skeletal muscle. At the same time, it triggers vasoconstriction to blood vessels serving less essential organs such as the gastrointestinal tract, kidneys, and skin, and downregulates some components of the immune system. Other effects include a dry mouth, loss of appetite, pupil dilation, and a loss of peripheral vision.


The pancreas is a long, slender organ, most of which is located posterior to the bottom half of the stomach (see Figure 14.9). Although it is primarily an exocrine gland, secreting a variety of digestive enzymes, the pancreas has an endocrine function. Its pancreatic islets—clusters of cells formerly known as the islets of Langerhans—secrete the hormones glucagon, insulin, somatostatin, and pancreatic polypeptide (PP).


Glucose is required for cellular respiration and is the preferred fuel for all body cells. The body derives glucose from the breakdown of the carbohydrate-containing foods and drinks we consume. Glucose not immediately taken up by cells for fuel can be stored by the liver and muscles as glycogen, or converted to triglycerides and stored in the adipose tissue. Hormones regulate both the storage and the utilization of glucose as required. Receptors located in the pancreas sense blood glucose levels, and subsequently the pancreatic cells secrete glucagon or insulin to maintain normal levels.


The male testes and female ovaries—which produce the sex cells (sperm and ova) and secrete the gonadal hormones. The roles of the gonadotropins released from the anterior pituitary (FSH and LH) were discussed earlier.


The primary hormone produced by the male testes is testosterone, a steroid hormone important in the development of the male reproductive system, the maturation of sperm cells, and the development of male secondary sex characteristics such as a deepened voice, body hair, and increased muscle mass. Interestingly, testosterone is also produced in the female ovaries but at a much reduced level. In addition, the testes produce the peptide hormone inhibin, which inhibits the secretion of FSH from the anterior pituitary gland. FSH stimulates spermatogenesis.


The primary hormones produced by the ovaries are estrogens, which include estradiol, estriol, and estrone. Estrogens play an important role in a larger number of physiological processes, including the development of the female reproductive system, regulation of the menstrual cycle, the development of female secondary sex characteristics such as increased adipose tissue, and the development of breast tissue, and the maintenance of pregnancy. Another significant ovarian hormone is progesterone, which contributes to regulation of the menstrual cycle and is important in preparing the body for pregnancy as well as maintaining pregnancy. In addition, the granulosa cells of the ovarian follicles produce inhibin, which—as in males—inhibits the secretion of FSH. During the initial stages of pregnancy, an organ called the placenta develops within the uterus. The placenta supplies oxygen and nutrients to the fetus, excretes waste products, and produces and secretes estrogens and progesterone. The placenta produces human chorionic gonadotropin (hCG) as well. The hCG hormone promotes progesterone synthesis and reduces the mother’s immune function to protect the fetus from immune rejection. It also secretes human placental lactogen (hPL), which plays a role in preparing the breasts for lactation and relaxin, which is thought to help soften and widen the pubic symphysis in preparation for childbirth.


Addison’s disease is a rare disorder that causes low blood glucose levels and low blood sodium levels. The signs and symptoms of Addison’s disease are vague and are typical of other disorders as well, making diagnosis difficult. They may include general weakness, abdominal pain, weight loss, nausea, vomiting, sweating, and cravings for salty food. For more information, visit the National Institute of Diabetes and Digestive and Kidney Diseases’ web page on adrenal insufficiency and Addison’s disease.


Cushing’s syndrome is characterized by high blood glucose levels and the accumulation of lipid deposits on the face and neck. It is caused by the hypersecretion of cortisol. The most common source of Cushing’s disease is a pituitary tumor that secretes cortisol or ACTH in abnormally high amounts. For more information, visit the National Institute of Diabetes and Digestive and Kidney Diseases’ web page on Cushing’s syndrome.


Hirsutism is a symptom of excessive production of androgens, causing hair growth in women where they typically do not have hair growth. It is usually caused by polycystic ovary syndrome (PCOS), although certain medications and tumors of the ovary or adrenal gland can cause hirsutism (Hafsi & Badri, 2021). For more information, visit the Mayo Clinic’s web page on hirsutism.


Hyperthyroidism is a condition marked by excessively high levels of thyroid hormones. Signs and symptoms include weight loss, heat sensitivity, and an increased or irregular heart rate. It may be caused by Graves’ disease (an autoimmune disorder), thyroiditis, or a diet high in iodine (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 hyperthyroidism.


Hypothyroidism is a condition marked by low levels of thyroid hormones. Signs and symptoms include weight gain, cold sensitivity, and a slowed heart rate. It may be caused by Hashimoto’s disease (an autoimmune disorder), thyroiditis, or surgical removal of some or all of the thyroid. Although rare in the United States, a diet low in iodine can also lead to hypothyroidism (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 hypothyroidism.


Graves’ disease is a disorder of the thyroid gland, resulting in hyperthyroidism. The immune system attacks the thyroid, resulting in excessive production of thyroid hormone. Researchers continue to investigate why some people develop Graves’ disease (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 Graves’ disease.


Diabetes insipidus is a condition caused by a lack of or hyposecretion of the antidiuretic hormone (ADH). The condition can also be caused by the failure of the kidneys to respond to ADH. Signs and symptoms include polyuria and excessive thirst (National Institute of Diabetes and Digestive and Kidney Diseases, n.d.-d). For more information, visit the National Institute of Diabetes and Digestive and Kidney Diseases’ web page on diabetes insipidus.


Diabetes mellitus is a condition marked by a disorder of the pancreas, resulting in high levels of glucose in the blood. There are three common types of diabetes mellitus:


Endocrinology is a specialization in the field of medicine that focuses on the treatment of endocrine system disorders. Endocrinologists—medical doctors who specialize in this field—are experts in treating diseases associated with hormonal systems, ranging from thyroid disease to diabetes. Endocrine surgeons treat endocrine disease through the removal of the affected endocrine gland or tissue. Some patients experience health problems as a result of the normal decline in hormones that can accompany aging. These patients can consult with an endocrinologist to weigh the risks and benefits of hormone replacement therapy intended to boost their natural levels of reproductive hormones. In addition to treating patients, endocrinologists may be involved in research to improve the understanding of endocrine system disorders and develop new treatments for these diseases.


The thyroid scan is a procedure used to check the size, shape, and position of the thyroid. Before the procedure, a small amount of radioactive iodine is administered intravenously or by oral capsule. A special camera then takes images of the thyroid, allowing the technician to determine the cause of hyperthyroidism or identify thyroid nodules (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 thyroid tests.


Radioactive iodine uptake is a procedure used to check the function of the thyroid. Before the procedure, a small amount of radioactive iodine is administered in liquid or capsule form. The technician then uses a gamma probe to measure how much radioactive iodine the thyroid takes up from the blood, which can be an indication of Graves’ disease or one or more thyroid nodules. The radioactive iodine uptake test can be performed at the same time as a thyroid scan (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 thyroid tests.


Blood serum tests are used to determine the concentration and presence of various endocrine hormones in the blood, including TSH, T4, and T3. Blood serum tests can also detect the presence of thyroid antibodies which can indicate an autoimmune thyroid disorder (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 thyroid tests.


Most of the surgeries and procedures performed with the endocrine system involve the removal of a gland or an incision into the gland. Once an endocrine gland is surgically removed, due to a tumor or enlargement, hormone replacement treatment is required. Medication is required to artificially or synthetically replace the hormone produced by the gland and the function it regulates.


A disorder that results in the growth of bones in the face, hands, and feet in response to excessive levels of growth hormone in individuals who have stopped growing.


A chemical that elicits a response in the same cell that secreted it.


A ductless gland that releases secretions directly into surrounding tissues and fluids.


Cells, tissues, and organs that secrete hormones as a primary or secondary function and play an integral role in normal bodily processes.


A doctor who has special training in diagnosing and treating disorders of the endocrine system.


A hormone that causes the breakdown of glycogen into glucose; also known as adrenaline.


Cells, tissues, and organs that secrete substances directly to target tissues via glandular ducts.


Secretion of an endocrine organ that travels via the bloodstream or lymphatics to induce a response in target cells or tissues in another part of the body.


The disease state caused by excessive production of hormones by the thyroid.


The disease state caused by insufficient production of thyroid hormone by the thyroid gland.


Chemicals that are made by nerve cells and used to communicate with other cells, including other nerve cells and muscle cells.


A chemical in the body that can act as a neurotransmitter and a hormone. It is released from the adrenal gland in response to stress and low blood pressure and is also known as noradrenaline.


A rare condition in which the pituitary gland stops making most or all hormones.


Cellular signaling in which a factor secreted by a cell affects other cells in the local environment.


Property of membranes and other structures to permit passage of light, heat, gases, liquids, metabolites, and mineral ions.


A set of symptoms or conditions that occur together and suggest the presence of a certain disease or an increased chance of developing the disease.


A chemical reaction that results in the synthesis (joining) of components that were formerly separate.


Centers for Disease Control and Prevention. (n.d.). What is diabetes? https://www.cdc.gov/diabetes/basics/diabetes.html


CrashCourse. (2015, June 22). Endocrine system, part 1 – glands & hormones: Crash course A&P #23 [Video]. YouTube. https://www.youtube.com/watch?v=eWHH9je2zG4


CrashCourse. (2015, June 29). Endocrine system, part 2 – hormone cascade: Crash course A&P #24 [Video]. YouTube. https://www.youtube.com/watch?v=eWHH9je2zG4


Genetic and Rare Diseases Information Center. (2017). Gigantism. National Institutes of Health, U.S. Department of Health and Human Services. https://rarediseases.info.nih.gov/diseases/6506/gigantism


Hafsi, W., & Badri, T. (2021). Hirsutism. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK470417/


MedlinePlus. (2020). Radioactive iodine uptake. US National Library of Medicine. https://medlineplus.gov/ency/article/003689.htm


National Institute of Diabetes and Digestive and Kidney Diseases. (n.d.-a). Acromegaly. National Institutes of Health, U.S. Department of Health and Human Services. https://www.niddk.nih.gov/health-information/endocrine-diseases/acromegaly


National Institute of Diabetes and Digestive and Kidney Diseases. (n.d.-b). Hyperthyroidism. National Institutes of Health, U.S. Department of Health and Human Services. https://www.niddk.nih.gov/health-information/endocrine-diseases/hyperthyroidism


National Institute of Diabetes and Digestive and Kidney Diseases. (n.d.-c). Graves’ disease. National Institutes of Health, U.S. Department of Health and Human Services. https://www.niddk.nih.gov/health-information/endocrine-diseases/graves-disease


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


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


Wright Jr, K. P., McHill, A. W., Birks, B. R., Griffin, B. R., Rusterholz, T., & Chinoy, E. D. (2013). Entrainment of the human circadian clock to the natural light-dark cycle. Current Biology, 23(16), 1554-1558. https://doi.org/10.1016/j.cub.2013.06.039


The digestive system is continually at work, yet people seldom appreciate the complex tasks it performs in a choreographed biologic symphony. Consider what happens when you eat an apple. Of course, you enjoy the apple’s taste as you chew it, but in the hours that follow, unless something goes amiss and you get a stomachache, you don’t notice that your digestive system is working. You may be taking a walk or studying or sleeping, having forgotten all about the apple, but your stomach and intestines are busy digesting it and absorbing its vitamins and other nutrients. By the time any waste material is excreted, the body has appropriated all it can use from the apple. In short, whether you pay attention or not, the organs of the digestive system perform their specific functions, allowing you to use the food you eat to keep you going.


This chapter examines the structure and functions of these organs and explores the mechanics and chemistry of the digestive processes. The function of the digestive system is to break down the foods you eat, release their nutrients, and absorb those nutrients into the body. Although the small intestine is the workhorse of the system, where the majority of digestion occurs, and where most of the released nutrients are absorbed into the blood or lymph, each of the digestive system organs makes a vital contribution to this process (see Figure 13.1).


Media 13.1 Digestive System, Part 1: Crash Course A&P #33. Copyright 2015 by CrashCourse.


At the entrance to the mouth are the lips, or labia (singular = labium). Their outer covering is skin, which transitions to a mucous membrane in the mouth proper. Lips are very vascular with a thin layer of keratin; hence, the reason they are red.


The pocket-like part of the mouth that is framed on the inside by the gums and teeth, and on the outside by the cheeks and lips is called the oral vestibule. Moving farther into the mouth, the opening between the oral cavity and throat (oropharynx) is called the fauces (like the kitchen “faucet”). The main open area of the mouth, or oral cavity proper, runs from the gums and teeth to the fauces.


When you are chewing, you do not find it difficult to breathe simultaneously. The next time you have food in your mouth, notice how the arched shape of the roof of your mouth allows you to handle both digestion and respiration at the same time. This arch is called the palate. The anterior region of the palate serves as a wall (or septum) between the oral and nasal cavities as well as a rigid shelf against which the tongue can push food. It is created by the maxillary and palatine bones of the skull and, given its bony structure, is known as the hard palate. If you run your tongue along the roof of your mouth, you’ll notice that the hard palate ends in the posterior oral cavity, and the tissue becomes fleshier. This part of the palate, known as the soft palate, is composed mainly of skeletal muscle. You can therefore manipulate, subconsciously, the soft palate—for instance, to yawn, swallow, or sing (see Figure 13.2).


A fleshy bead of tissue called the uvula drops down from the center of the posterior edge of the soft palate. Although some have suggested that the uvula is a vestigial organ, it serves an important purpose. When you swallow, the soft palate and uvula move upward, helping to keep foods and liquid from entering the nasal cavity. Unfortunately, it can also contribute to the sound produced by snoring. Two muscular folds extend downward from the soft palate, on either side of the uvula. Toward the front, the palatoglossal arch lies next to the base of the tongue; behind it, the palatopharyngeal arch forms the superior and lateral margins of the fauces. Between these two arches are the palatine tonsils, clusters of lymphoid tissue that protect the pharynx. The lingual tonsils are located at the base of the tongue.


Perhaps you have heard it said that the tongue is the strongest muscle in the body. Those who stake this claim cite its strength proportional to its size. Although it is difficult to quantify the relative strength of different muscles, it remains indisputable that the tongue is a workhorse, facilitating ingestion, mechanical digestion, chemical digestion (lingual lipase), sensation (of taste, texture, and temperature of food), swallowing, and vocalization.


The tongue is attached to the mandible, the styloid processes of the temporal bones, and the hyoid bone. The hyoid is unique in that it only distantly/indirectly articulates with other bones. The tongue is positioned over the floor of the oral cavity. A medial septum extends the entire length of the tongue, dividing it into symmetrical halves.


The top and sides of the tongue are studded with papillae, extensions of lamina propria of the mucosa, which are covered in stratified squamous epithelium (see Figure 13.3).


Many small salivary glands are housed within the mucous membranes of the mouth and tongue. These minor exocrine glands are constantly secreting saliva, either directly into the oral cavity or indirectly through ducts, even while you sleep. In fact, an average of 1 to 1.5 liters of saliva is secreted each day. Usually, just enough saliva is present to moisten the mouth and teeth. Secretion increases when you eat because saliva is essential to moisten food and initiate the chemical breakdown of carbohydrates. Small amounts of saliva are also secreted by the labial glands in the lips. In addition, the buccal glands in the cheeks, palatal glands in the palate, and lingual glands in the tongue help ensure that all areas of the mouth are supplied with adequate saliva.


The pharynx (throat) is involved in both digestion and respiration. It receives food and air from the mouth, and air from the nasal cavities. When food enters the pharynx, involuntary muscle contractions close off the air passageways. A short tube of skeletal muscle lined with a mucous membrane, the pharynx runs from the posterior oral and nasal cavities to the opening of the esophagus and larynx. It has three subdivisions. The most superior, the nasopharynx, is involved only in breathing and speech. The other two subdivisions, the oropharynx and the laryngopharynx, are used for both breathing and digestion. The oropharynx begins inferior to the nasopharynx and is continuous below with the laryngopharynx. The inferior border of the laryngopharynx connects to the esophagus, whereas the anterior portion connects to the larynx, allowing air to flow into the bronchial tree.


The esophagus is a muscular tube that connects the pharynx to the stomach. It is approximately 25.4 cm (10 in) in length, located posterior to the trachea, and remains in a collapsed form when not engaged in swallowing. As you can see in Figure 13.4, the esophagus runs a mainly straight route through the mediastinum of the thorax. To enter the abdomen, the esophagus penetrates the diaphragm through an opening called the esophageal hiatus.


The upper esophageal sphincter, which is continuous with the inferior pharyngeal constrictor, controls the movement of food from the pharynx into the esophagus. The upper two-thirds of the esophagus consists of both smooth and skeletal muscle fibers, with the latter fading out in the bottom third of the esophagus. Rhythmic waves of peristalsis, which begin in the upper esophagus, propel the bolus of food toward the stomach. Meanwhile, secretions from the esophageal mucosa lubricate the esophagus and food. Food passes from the esophagus into the stomach at the lower esophageal sphincter (also called the gastroesophageal or cardiac sphincter). Recall that sphincters are muscles that surround tubes and serve as valves, closing the tube when the sphincters contract and opening it when they relax.


There are four main regions in the stomach: the cardia, fundus, body, and pylorus (see Figure 13.5). The cardia (or cardiac region) is the point where the esophagus connects to the stomach and through which food passes into the stomach. Located inferior to the diaphragm, above and to the left of the cardia, is the dome-shaped fundus. Below the fundus is the body, the main part of the stomach. The funnel-shaped pylorus connects the stomach to the duodenum. The wider end of the funnel, the pyloric antrum, connects to the body of the stomach. The narrower end is called the pyloric canal, which connects to the duodenum. The smooth muscle pyloric sphincter is located at this latter point of connection and controls stomach emptying. In the absence of food, the stomach deflates inward, and its mucosa and submucosa fall into a large fold called a ruga.


The convex lateral surface of the stomach is called the greater curvature; the concave medial border is the lesser curvature. The stomach is held in place by the lesser omentum, which extends from the liver to the lesser curvature, and the greater omentum, which runs from the greater curvature to the posterior abdominal wall.


Chyme released from the stomach enters the small intestine, which is the primary digestive organ in the body. Not only is this where most digestion occurs, but it is also where practically all absorption occurs. The longest part of the alimentary canal, the small intestine, is about 3.05 meters (10 feet) long in a living person (but about twice as long in a cadaver due to the loss of muscle tone). Since this makes it about five times longer than the large intestine, you might wonder why it is called “small.” In fact, its name derives from its relatively smaller diameter of only about 2.54 cm (1 in), compared with 7.62 cm (3 in) for the large intestine. As you will see shortly, in addition to its length, the folds and projections of the lining of the small intestine work to give it an enormous surface area, which is approximately 200 m2, more than 100 times the surface area of your skin. This large surface area is necessary for complex processes of digestion and absorption that occur within it.


The coiled tube of the small intestine is subdivided into three regions. From proximal  to distal, these are the duodenum, jejunum, and ileum (see Figure 13.6).


The first part of the large intestine is the cecum, a sac-like structure that is suspended inferior to the ileocecal valve. It is about 6 cm (2.4 in) long, receives the contents of the ileum, and continues the absorption of water and salts. The appendix (or vermiform appendix) is a winding tube that attaches to the cecum. Although the 7.6-cm (3-in) long appendix contains lymphoid tissue, suggesting an immunologic function, this organ is generally considered vestigial. However, at least one recent report assumes a survival advantage conferred by the appendix: in diarrheal illness, the appendix may serve as a bacterial reservoir to repopulate the enteric bacteria for those surviving the initial phases of the illness. Moreover, its twisted anatomy provides a haven for the accumulation and multiplication of enteric bacteria. The mesoappendix, the mesentery of the appendix, tethers it to the mesentery of the ileum.


The cecum blends seamlessly with the colon. Upon entering the colon, the food residue first travels up the ascending colon on the right side of the abdomen. At the inferior surface of the liver, the colon bends to form the right colic flexure (hepatic flexure) and becomes the transverse colon. The region defined as the hindgut begins with the last third of the transverse colon and continues. Food residue passing through the transverse colon travels across to the left side of the abdomen, where the colon angles sharply immediately inferior to the spleen, at the left colic flexure (splenic flexure). From there, food residue passes through the descending colon, which runs down the left side of the posterior abdominal wall. After entering the pelvis inferiorly, it becomes the s-shaped sigmoid colon, which extends medially to the midline (see Figure 13.7). The ascending and descending colon, and the rectum (discussed next) are located in the retroperitoneum. The transverse and sigmoid colon are tethered to the posterior abdominal wall by the mesocolon.


Chemical digestion in the small intestine relies on the activities of three accessory digestive organs: the liver, pancreas, and gallbladder (see Figure 13.8). The digestive role of the liver is to produce bile and export it to the duodenum. The gallbladder primarily stores, concentrates, and releases bile. The pancreas produces pancreatic juice, which contains digestive enzymes and bicarbonate ions, and delivers it to the duodenum.


The liver is the largest gland in the body, weighing about three pounds in an adult. It is also one of the most important organs. In addition to being an accessory digestive organ, it plays a number of roles in metabolism and regulation. The liver lies inferior to the diaphragm in the right upper quadrant of the abdominal cavity and receives protection from the surrounding ribs.


The liver is divided into two primary lobes: a large right lobe and a much smaller left lobe. In the right lobe, some anatomists also identify an inferior quadrate lobe and a posterior caudate lobe, which are defined by internal features. The liver is connected to the abdominal wall and diaphragm by five peritoneal folds referred to as ligaments.


The porta hepatis (“gate to the liver”) is where the hepatic artery and hepatic portal vein enter the liver. These two vessels, along with the common hepatic duct, run behind the lateral border of the lesser omentum on the way to their destinations. The hepatic portal vein delivers partially deoxygenated blood containing nutrients absorbed from the small intestine and actually supplies more oxygen to the liver than do the much smaller hepatic arteries. In addition to nutrients, drugs and toxins are also absorbed. After processing the bloodborne nutrients and toxins, the liver releases nutrients needed by other cells back into the blood, which drains into the central vein and then through the hepatic vein to the inferior vena cava. With this hepatic portal circulation, all blood from the alimentary canal passes through the liver. This largely explains why the liver is the most common site for the metastasis of cancers that originate in the alimentary canal.


Bile produced by the liver is a mixture secreted by the liver to accomplish the emulsification of lipids in the small intestine.


Bilirubin, the main bile pigment, is a waste product produced when the spleen removes old or damaged red blood cells from the circulation. These breakdown products, including proteins, iron, and toxic bilirubin, are transported to the liver via the splenic vein of the hepatic portal system. In the liver, proteins and iron are recycled, whereas bilirubin is excreted in the bile. It accounts for the green color of bile. Bilirubin is eventually transformed by intestinal bacteria into stercobilin, a brown pigment that gives your stool its characteristic color. In some disease states, bile does not enter the intestine, resulting in white (‘acholic’) stool with a high fat content, since virtually no fats are broken down or absorbed.


Between meals, bile is produced but conserved. The valve-like hepatopancreatic ampulla closes, allowing bile to divert to the gallbladder, where it is concentrated and stored until the next meal.


The soft, oblong, glandular pancreas lies transversely in the retroperitoneum behind the stomach. Its head is nestled into the “c-shaped” curvature of the duodenum with the body extending to the left about 15.2 cm (6 in) and ending as a tapering tail in the hilum of the spleen. It is a curious mix of exocrine (secreting digestive enzymes) and endocrine (releasing hormones into the blood) functions (Figure 13.9).


The exocrine part of the pancreas arises as little grape-like cell clusters, each called an acinus (plural = acini), located at the terminal ends of pancreatic ducts. These acinar cells secrete enzyme-rich pancreatic juice into tiny merging ducts that form two dominant ducts. The larger duct fuses with the common bile duct (carrying bile from the liver and gallbladder) just before entering the duodenum via a common opening (the hepatopancreatic ampulla). The smooth muscle sphincter of the hepatopancreatic ampulla controls the release of pancreatic juice and bile into the small intestine. The second and smaller pancreatic duct, the accessory duct (duct of Santorini), runs from the pancreas directly into the duodenum, approximately 1 inch above the hepatopancreatic ampulla. When present, it is a persistent remnant of pancreatic development.


Scattered through the sea of exocrine acini are small islands of endocrine cells, the islets of Langerhans. These vital cells produce the hormones pancreatic polypeptide, insulin, glucagon, and somatostatin.


The gallbladder is 8 to 10 cm (~3 to 4 in) long and is nested in a shallow area on the posterior aspect of the right lobe of the liver. This muscular sac stores, concentrates, and, when stimulated, propels the bile into the duodenum via the common bile duct. It is divided into three regions. The fundus is the widest portion and tapers medially into the body, which in turn narrows to become the neck. The neck angles slightly superiorly as it approaches the hepatic duct. The cystic duct is 1 to 2 cm (less than 1 in) long and turns inferiorly as it bridges the neck and hepatic duct.


The simple columnar epithelium of the gallbladder mucosa is organized in rugae, similar to those of the stomach. There is no submucosa in the gallbladder wall. The wall’s middle, muscular coat is made of smooth muscle fibers. When these fibers contract, the gallbladder’s contents are ejected through the cystic duct and into the bile duct (Figure 13.10). The visceral peritoneum reflected from the liver capsule holds the gallbladder against the liver and forms the outer coat of the gallbladder. The gallbladder’s mucosa absorbs water and ions from bile, concentrating it by up to 10-fold.


Media 13.2 What does the liver do? – Emma Bryce. Copyright 2014 by TED-Ed.


The processes of digestion include six activities: ingestion, propulsion, mechanical or physical digestion, chemical digestion, absorption, and defecation.


The first of these processes, ingestion, refers to the entry of food into the alimentary canal through the mouth. There, the food is chewed and mixed with saliva, which contains enzymes that begin breaking down the carbohydrates in the food plus some lipid digestion via lingual lipase. Chewing increases the surface area of the food and allows an appropriately sized bolus to be produced.


Food leaves the mouth when the tongue and pharyngeal muscles propel it into the esophagus. This act of swallowing, the last voluntary act until defecation, is an example of propulsion, which refers to the movement of food through the digestive tract. It includes both the voluntary process of swallowing and the involuntary process of peristalsis. Peristalsis consists of sequential, alternating waves of contraction and relaxation of alimentary wall smooth muscles, which act to propel food along (see Figure 13.11). These waves also play a role in mixing food with digestive juices. Peristalsis is so powerful that foods and liquids you swallow enter your stomach even if you are standing on your head.


Digestion includes both mechanical and chemical processes. Mechanical digestion is a purely physical process that does not change the chemical nature of the food. Instead, it makes the food smaller to increase both surface area and mobility. It includes mastication, or chewing, as well as tongue movements that help break food into smaller bits and mix food with saliva. Although there may be a tendency to think that mechanical digestion is limited to the first steps of the digestive process, it occurs after the food leaves the mouth, as well. The mechanical churning of food in the stomach serves to further break it apart and expose more of its surface area to digestive juices, creating an acidic “soup” called chyme.


Segmentation, which occurs mainly in the small intestine, consists of localized contractions of circular muscle of the muscularis layer of the alimentary canal. These contractions isolate small sections of the intestine, moving their contents back and forth while continuously subdividing, breaking up, and mixing the contents. By moving food back and forth in the intestinal lumen, segmentation mixes food with digestive juices and facilitates absorption.


In chemical digestion, starting in the mouth, digestive secretions break down complex food molecules into their chemical building blocks (for example, proteins into separate amino acids). These secretions vary in composition but typically contain water, various enzymes, acids, and salts. The process is completed in the small intestine.


Food that has been broken down is of no value to the body unless it enters the bloodstream and its nutrients are put to work. This occurs through the process of absorption, which takes place primarily within the small intestine. There, most nutrients are absorbed from the lumen of the alimentary canal into the bloodstream through the epithelial cells that make up the mucosa. Lipids are absorbed into lacteals and are transported via the lymphatic vessels to the bloodstream.


In defecation, the final step in digestion, undigested materials are removed from the body as feces.


Age-related changes in the digestive system begin in the mouth and can affect virtually every aspect of the digestive system. Taste buds become less sensitive, so food isn’t as appetizing as it once was. A slice of pizza is a challenge, not a treat, when you have lost teeth, your gums are diseased, and your salivary glands aren’t producing enough saliva. Swallowing can be difficult, and ingested food moves slowly through the alimentary canal because of reduced strength and tone of muscular tissue. Neurosensory feedback is also dampened, slowing the transmission of messages that stimulate the release of enzymes and hormones.


Pathologies that affect the digestive organs—such as hiatal hernia, gastritis, and peptic ulcer disease—can occur at greater frequencies as you age. Problems in the small intestine may include duodenal ulcers, maldigestion, and malabsorption. Problems in the large intestine include hemorrhoids, diverticular disease, and constipation. Conditions that affect the function of accessory organs—and their abilities to deliver pancreatic enzymes and bile to the small intestine—include jaundice, acute pancreatitis, cirrhosis, and gallstones.


In some cases, a single organ is in charge of a digestive process. For example, ingestion occurs only in the mouth and defecation only in the anus. However, most digestive processes involve the interaction of several organs and occur gradually as food moves through the alimentary canal (see Figure 13.12). Some chemical digestion occurs in the mouth. Some absorption can occur in the mouth and stomach; for example, alcohol and aspirin.


Neural and endocrine regulatory mechanisms work to maintain the optimal conditions in the lumen needed for digestion and absorption. These regulatory mechanisms, which stimulate digestive activity through mechanical and chemical activity, are controlled both extrinsically and intrinsically.


Media 13.3 Digestive System, Part 3: Crash Course A&P #35. Copyright 2015 by CrashCourse.


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


Gastroesophageal reflux disease (GERD) is caused by stomach acid flowing upwards into the esophagus. Those suffering from the condition will often feel a burning sensation in the chest or throat (MedlinePlus, 2021a). To learn more about GERD, visit the National Institute of Diabetes and Digestive and Kidney Diseases’ web page.


Cholecystitis is known as inflammation of the gallbladder. Gall stone development can block the gallbladder’s release of bile, leading to an inflammatory response. Cholecystitis can be characterized as acute (the sudden onset of irritation of the gallbladder) or chronic (prolonged irritation of the gallbladder usually caused by repeated bouts of acute cholecystitis). Treatment may require a cholecystectomy (Jones, Genova, et al., 2021; Jones, Gnanapandithan, et al., 2021). To learn more, visit the Mayo Clinic’s web page on cholecystitis.


Cirrhosis is a condition whereby the liver scars. Potentially fatal complications can arise, including liver failure. It generally cannot be reversed, although many of the underlying causes of cirrhosis can be treated and may help lower the likelihood of cirrhosis-related complications. Cirrhosis is caused by different forms of liver disease and chronic alcoholism (National Institute of Diabetes and Digestive and Kidney Diseases, n.d.-a).


Cirrhosis often has no signs or symptoms until liver damage is extensive and may include:


To learn more, visit the National Institute of Diabetes and Digestive and Kidney Diseases’ web page on cirrhosis.


Esophageal cancer begins in the innermost layer of the esophagus and spreads to the other layers. Risk factors include tobacco use, heavy alcohol use, and older age (National Cancer Institute, 2021a). To learn more, visit the Mayo Clinic’s web page on esophageal cancer.


Esophageal cancer begins in the innermost layer of the esophagus and spreads to the other layers. Risk factors include tobacco use, heavy alcohol use, and older age


Inflammation of the liver is referred to as hepatitis. It can be caused by heavy alcohol use, toxins, drugs, or as a result of an autoimmune response, but it is most often caused by a virus. Viral hepatitis is caused by one of several viruses: hepatitis A, B, C, D, or E. In the United States, 90% of hepatitis cases are caused by the A, B, and C viruses. Many cases are asymptomatic (Mehta & Reddivari, 2021).


For more information, visit the National Institute of Diabetes and Digestive and Kidney Diseases’ web page on viral hepatitis.


Individuals who possess celiac disease have an immune sensitivity reaction occurring in the small intestines when they consume gluten. Typically people with this condition are genetically predisposed to the condition. Damage to the small intestine will occur if continued consumption of gluten occurs. Following a gluten-free diet generally resolves most symptoms and complications of celiac disease (National Institute of Diabetes and Digestive and Kidney Diseases, n.d.-b). For more information, visit the Mayo Clinic’s web page on celiac disease.


Crohn’s disease and ulcerative colitis are chronic inflammatory bowel diseases (IBD) whereby a section or segments of the digestive tract experience inflammation. Crohn’s disease can occur anywhere along the digestive tract from the mouth to the anus, although it is most often found in the small intestines. This often leads to the malabsorption of nutrients from food. Ulcerative colitis is localized inflammation and ulcers in the colon (National Institute of Diabetes and Digestive and Kidney Diseases, n.d.-c, n.d.-d). To learn more, visit the Mayo Clinic’s web pages on Crohn’s disease and ulcerative colitis.


Colon cancer is cancer formation in the colon portion of the digestive tract. Familial or personal history of rectal or colon cancer, heavy use of tobacco or alcohol, and older age are risk factors for colon cancer. It is often diagnosed through a colonoscopy (National Cancer Institute, 2021b). To learn more, visit the National Cancer Institute’s web page on colon cancer.


A hernia occurs when part of an organ or tissue squeezes through a weak spot in a surrounding muscle. A hernia can happen in the groin (inguinal hernia), around the belly button (umbilical hernia), through a scar (incisional hernia), through the diaphragm (hiatal hernia), or as a result of a birth defect (congenital diaphragmatic hernia) (MedlinePlus, 2021b). For more information, visit the Cleveland Clinic’s web page on hernias.


Irritable bowel syndrome (IBS) is a common disorder affecting the large intestines. It often involves abdominal pain, bloating, and changes in bowel movements, although individuals with IBS may experience remission and flare-ups. Diet and lifestyle modifications often help in the management of the condition. (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 IBS.


A polyp is a small growth of tissue protruding from the intestinal wall. Most are harmless but can transition over time into a cancerous growth. Typically, they are found in men and adults. over the age of 45 (National Institute of Diabetes and Digestive and Kidney Diseases, n.d.-f). To learn more, review the National Institute of Diabetes and Digestive and Kidney Diseases’ web page on polyps.


Gastroenterology is a branch of internal medicine that focuses on the diagnosis and treatment of conditions afflicting the digestive system. A physician who specializes in this area is known as a gastroenterologist. (Bureau of Labor Statistics, 2021; National Cancer Institute, n.d.). To learn more about gastroenterology, visit the American College of Gastroenterology’s web page.


Upper and lower gastrointestinal series are procedures that involve the introduction of a contrast medium known as barium. Barium can be introduced by ingesting or by enema. After induction of the barium, x-rays are taken of the upper and lower gastrointestinal system structures (National Institute of Diabetes and Digestive and Kidney Diseases, n.d.-g, n.d.-h ). To learn more, visit the National Institute of Diabetes and Digestive and Kidney Diseases’ web pages on upper GI series and lower GI series.


The fecal occult blood test is a test for hidden blood in a fecal sample. It can be performed at home and involves the patient putting a small segment of fecal output on a test card that is then mailed to their healthcare provider or a laboratory. Blood detection can be an indicator of bleeding along the digestive tract or an abnormal growth, such as colorectal cancer (MedlinePlus, 2020a).


A stool culture procedure involves the collection of a small sample of feces. The sample is analyzed for abnormal bacterial growth and parasites through a culture check (MedlinePlus, 2020b).


An EGD (upper endoscopy) is a procedure whereby a physician examines the upper gastrointestinal tract (esophagus, stomach, duodenum) using a special instrument called an endoscope. The physician examines the tissues and is able to take a biopsy, if needed (Ahlawat et al., 2021)


Abnormal buildup of fluid in the abdomen that may cause swelling.


Molecules composed of carbon, hydrogen, and oxygen. Carbohydrates are found in plant-based foods and dairy products and are an important fuel source.


A type of chronic, progressive liver disease in which liver cells are replaced by scar tissue.


A thin, tube-like instrument used to examine the inside of the colon.


Examination of the inside of the colon using a colonoscope, inserted into the rectum.


An opening into the colon from the outside of the body.


A method to examine the inside of the colon by taking a series of x-rays.


A position in a limb that is farther from the point of attachment or the trunk of the body.


Inflammation of one or more pouches or sacs that bulge out from the wall of a hollow organ, such as the colon.


A condition marked by small sacs or pouches in the walls of a hollow organ, such as the colon.


Acute inflammation of the intestine presenting with abdominal pain and bloody diarrhea.


The process of breaking down the fat into smaller blood cells, which makes it easy for enzymes to function and digest food.


A thin, tube-like instrument used to look at tissues inside the body.


A gland whose secretions leave through a duct that opens directly, or indirectly, to the external environment.


The part of a hollow organ that is across from, or farthest away from, the organ’s opening.


A doctor who has special training in diagnosing and treating disorders of the digestive system.


A subspecialty of internal medicine concerned with the study of the physiology and diseases of the digestive system and related structures.


A surgical procedure that connects part of the stomach to the jejunum.


A thin, tube-like instrument used to examine the inside of the stomach.


Examination of the inside of the stomach using a gastroscope passed through the mouth and esophagus.


An enlarged or swollen blood vessel, usually located near the anus or the rectum.


A procedure in which the ileum is brought through the abdominal wall.


The lymphatic vessels of the small intestine which absorb digested fats.


A thin, tube-like instrument used to look at tissues and organs inside the abdomen.


A feeling of sickness or discomfort in the stomach that may come with an urge to vomit.


A common, chronic disease marked by an abnormally high, unhealthy amount of body fat.


A fold of the peritoneum (the thin tissue that lines the abdomen) that surrounds the stomach and other organs in the abdomen.


A pair of soft tissue masses located at the rear of the throat (pharynx).


Examination by pressing on the surface of the body to feel the organs or tissues underneath.


Having to do with the parietal peritoneum (the tissue that lines the abdominal wall and pelvic cavity) and visceral peritoneum (the tissue that covers most of the organs in the abdomen, including the intestines).


A thin, tube-like instrument used to look inside the anus and rectum.


A procedure that uses a proctoscope to look inside the anus and rectum.


A position in a limb that is nearer to the point of attachment or the trunk of the body.


A band of smooth muscle at the junction between the pylorus of the stomach and the duodenum of the small intestine.


The backward flow of liquid from the stomach into the esophagus.


Examination of the lower colon using a sigmoidoscope, inserted into the rectum.


A surgically-created opening from an area inside the body to the outside.


Ahlawat, R., Hoilat, G. J., & Ross, A. B. (2021). Esophagogastroduodenoscopy. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK532268/


Bureau of Labor Statistics. (2021). Physicians and surgeons. In Occupational outlook handbook. U.S. Department of Labor. https://www.bls.gov/ooh/healthcare/physicians-and-surgeons.htm


Centers for Disease Control and Prevention. (n.d.-a). What is viral hepatitis? https://www.cdc.gov/hepatitis/abc/index.htm


Centers for Disease Control and Prevention. (n.d.-b). Hepatitis D questions and answers for health professionals. https://www.cdc.gov/hepatitis/hdv/hdvfaq.htm


Centers for Disease Control and Prevention. (n.d.-c). Hepatitis E questions and answers for health professionals. https://www.cdc.gov/hepatitis/hev/hevfaq.htm


CrashCourse. (2005, September 7). Digestive System, Part 1: Crash Course A&P #33 [Video]. YouTube. https://www.youtube.com/watch?v=yIoTRGfcMqM


CrashCourse. (2005, September 22). Digestive System, Part 3: Crash Course A&P #35 [Video]. YouTube. https://www.youtube.com/watch?v=jGme7BRkpuQ


Jones, M. W., Genova, R., & O’Rourke, M. C. (2021). Acute cholecystitis. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK459171/


Jones, M. W., Gnanapandithan, K., Panneerselvam, D., & Ferguson, T. (2021). Chronic cholecystitis. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK470236/


MedlinePlus. (2021a). Gerd. U.S. National Library of Medicine. https://medlineplus.gov/gerd.html


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


MedlinePlus. (2020a). Fecal occult blood test (FOBT). U.S. National Library of Medicine. https://medlineplus.gov/lab-tests/fecal-occult-blood-test-fobt/


MedlinePlus. (2020b). Bacteria culture test. U.S. National Library of Medicine. https://medlineplus.gov/lab-tests/bacteria-culture-test/


Mehta, P. & Reddivari, A. K. R. (2021). Hepatitis. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK554549/


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


National Cancer Institute. (2021a). Esophageal cancer treatment (adult) (PDQ®)–Patient Version. National Institutes of Health, U.S. Department of Health. https://www.cancer.gov/types/esophageal/patient/esophageal-treatment-pdq


National Cancer Institute. (2021b). Colon cancer treatment (PDDQ®)–Patient Version. National Institutes of Health, U.S. Department of Health. https://www.cancer.gov/types/colorectal/patient/colon-treatment-pdq


National Institute of Diabetes and Digestive and Kidney Diseases. (n.d.-a). Cirrhosis. National Institutes of Health, U.S. Department of Health. https://www.niddk.nih.gov/health-information/liver-disease/cirrhosis/all-content


National Institute of Diabetes and Digestive and Kidney Diseases. (n.d.-b). Celiac disease. National Institutes of Health, U.S. Department of Health. https://www.niddk.nih.gov/health-information/digestive-diseases/celiac-disease/all-content


National Institute of Diabetes and Digestive and Kidney Diseases. (n.d.-c). Crohn’s disease. National Institutes of Health, U.S. Department of Health. https://www.niddk.nih.gov/health-information/digestive-diseases/crohns-disease/all-content


National Institute of Diabetes and Digestive and Kidney Diseases. (n.d.-d). Ulcerative colitis. National Institutes of Health, U.S. Department of Health. https://www.niddk.nih.gov/health-information/digestive-diseases/ulcerative-colitis/all-content


National Institute of Diabetes and Digestive and Kidney Diseases. (n.d.-e). Irritable bowel syndrome. National Institutes of Health, U.S. Department of Health. https://www.niddk.nih.gov/health-information/digestive-diseases/irritable-bowel-syndrome/all-content


National Institute of Diabetes and Digestive and Kidney Diseases. (n.d.-f). Colon polyps. National Institutes of Health, U.S. Department of Health. https://www.niddk.nih.gov/health-information/digestive-diseases/colon-polyps/definition-facts


National Institute of Diabetes and Digestive and Kidney Diseases. (n.d.-g). Upper GI series. National Institutes of Health, U.S. Department of Health. https://www.niddk.nih.gov/health-information/diagnostic-tests/upper-gi-series


National Institute of Diabetes and Digestive and Kidney Diseases. (n.d.-h). Lower GI series. National Institutes of Health, U.S. Department of Health. https://www.niddk.nih.gov/health-information/diagnostic-tests/lower-gi-series


TED-Ed. (2014, November 25). What does the liver do? – Emma Bryce [Video]. YouTube. https://www.youtube.com/watch?v=wbh3SjzydnQ


The major structures of the respiratory system function primarily to provide oxygen to body tissues for cellular respiration, remove the waste product carbon dioxide, and help to maintain acid-base balance. Portions of the respiratory system are also used for non-vital functions, such as sensing odors, speech production, and for straining, such as coughing.


Media 12.1. Respiratory System, Part 1: Crash Course A&P #31 [Online video]. Copyright 2015 by CrashCourse.


The major entrance and exit for the respiratory system is through the nose. When discussing the nose, it is helpful to divide it into two major sections:


The nares open into the nasal cavity, which is separated into left and right sections by the nasal septum (Figure 12.2). The nasal septum is formed anteriorly by a portion of the septal cartilage and posteriorly by the perpendicular plate of the ethmoid bone and the thin vomer bones.


Each lateral wall of the nasal cavity has three bony projections: the inferior conchae are separate bones, and the superior and middle conchae are portions of the ethmoid bone. Conchae increase the surface area of the nasal cavity, disrupting the flow of air as it enters the nose and causing air to bounce along the epithelium, where it is cleaned and warmed. The conchae and meatuses trap water during exhalation preventing dehydration.


The floor of the nasal cavity is composed of the hard palate and the soft palate. Air exits the nasal cavities via the internal nares and moves into the pharynx.


Paranasal sinuses serve to warm and humidify incoming air and are lined with a mucosa which produces mucus. Paranasal sinuses are named for their associated bone:


The nares and anterior portion of the nasal cavities are lined with mucous membranes, containing sebaceous glands and hair follicles that serve to prevent the passage of large debris, such as dirt, through the nasal cavity. An olfactory epithelium used to detect odors is found deeper in the nasal cavity.


The conchae, meatuses, and paranasal sinuses are lined by respiratory epithelium composed of pseudostratified ciliated columnar epithelium (Figure 12.3). The epithelium contains specialized epithelial cells that produce mucus to trap debris. The cilia of the respiratory epithelium help to remove mucus and debris with a constant beating motion, sweeping materials towards the throat to be swallowed.


This moist epithelium functions to warm and humidify incoming air. Capillaries located just beneath the nasal epithelium warm the air by convection. Serous and mucus-producing cells also secrete defensins, or immune cells that patrol the connective tissue providing additional protection.


The pharynx is divided into three major regions: the nasopharynx, the oropharynx, and the laryngopharynx (see Figure 12.4).


At the top of the nasopharynx are the pharyngeal tonsils. The function of the pharyngeal tonsil is not well understood, but it contains a rich supply of lymphocytes and is covered with ciliated epithelium that traps and destroys invading pathogens that enter during inhalation. The pharyngeal tonsils are large in children but tend to regress with age and may even disappear. The uvula and soft palate move like a pendulum during swallowing, swinging upward to close off the nasopharynx to prevent ingested materials from entering the nasal cavity. Auditory (Eustachian) tubes that connect to each middle ear cavity open into the nasopharynx. This connection is why colds often lead to ear infections.


The oropharynx is bordered superiorly by the nasopharynx and anteriorly by the oral cavity. The oropharynx contains two distinct sets of tonsils:


Similar to the pharyngeal tonsil, the palatine and lingual tonsils are composed of lymphoid tissue, and trap and destroy pathogens entering the body through the oral or nasal cavities.


The laryngopharynx is inferior to the oropharynx and posterior to the larynx. It continues the route for ingested material and air until its inferior end, where the digestive and respiratory systems diverge. The stratified squamous epithelium of the oropharynx is continuous with the laryngopharynx. Anteriorly, the laryngopharynx opens into the larynx, whereas posteriorly, it enters the esophagus.


The structure of the larynx is formed by several pieces of cartilage. Three large cartilage pieces form the major structure of the larynx.


When the epiglottis is in the “closed” position, the unattached end of the epiglottis rests on the glottis. A vestibular fold, or false vocal cord, is one of a pair of folded sections of mucous membrane. A true vocal cord is one of the white, membranous folds attached by muscle to the thyroid and arytenoid cartilages of the larynx on their outer edges. The inner edges of the true vocal cords are free, allowing oscillation to produce sound.


The act of swallowing causes the pharynx and larynx to lift upward, allowing the pharynx to expand and the epiglottis of the larynx to swing downward, closing the opening to the trachea. These movements produce a larger area for food to pass through, while preventing food and beverages from entering the trachea.


Similar to the nasal cavity and nasopharynx, this specialized epithelium produces mucus to trap debris and pathogens as they enter the trachea. The cilia beat the mucus upward towards the laryngopharynx, where it can be swallowed down the esophagus.


The trachea is formed by 16 to 20 stacked, C-shaped pieces of hyaline cartilage that are connected by dense connective tissue. The trachealis muscle and elastic connective tissue together form the fibroelastic membrane. The fibroelastic membrane allows the trachea to stretch and expand slightly during inhalation and exhalation, whereas the rings of cartilage provide structural support and prevent the trachea from collapsing. The trachealis muscle can be contracted to force air through the trachea during exhalation. The trachea is lined with pseudostratified ciliated columnar epithelium, which is continuous with the larynx. The esophagus borders the trachea posteriorly.


The trachea branches into the right and left primary bronchi at the carina. These bronchi are also lined by pseudostratified ciliated columnar epithelium containing mucus-producing goblet cells (Figure 12.7b). The carina is a raised structure that contains specialized nervous tissue that induces violent coughing if a foreign body, such as food, is present. Rings of cartilage, similar to those of the trachea, support the structure of the bronchi and prevent their collapse. The primary bronchi enter the lungs at the hilum. The bronchi continue to branch into a bronchial tree. A bronchial tree (or respiratory tree) is the collective term used for these multiple-branched bronchi. The main function of the bronchi, like other conducting zone structures, is to provide a passageway for air to move into and out of each lung. The mucous membrane traps debris and pathogens.


A bronchiole branches from the tertiary bronchi. Bronchioles, which are about 1 mm in diameter, further branch until they become the tiny terminal bronchioles, which lead to the structures of gas exchange. There are more than 1,000 terminal bronchioles in each lung. The muscular walls of the bronchioles do not contain cartilage like those of the bronchi. This muscular wall can change the size of the tubing to increase or decrease airflow through the tube.


In contrast to the conducting zone, the respiratory zone includes structures that are directly involved in gas exchange. The respiratory zone begins where the terminal bronchioles join a respiratory bronchiole, the smallest type of bronchiole (see Figure 12.8), which then leads to an alveolar duct, opening into a cluster of alveoli.


An alveolar duct opens into a cluster of alveoli. An alveolus is one of the many small, grape-like sacs that are attached to the alveolar ducts. An alveolar sac is a cluster of many individual alveoli that are responsible for gas exchange. An alveolus is approximately 200 μm in diameter with elastic walls that allow the alveolus to stretch during air intake, which greatly increases the surface area available for gas exchange. Alveoli are connected to their neighbors by alveolar pores, which help maintain equal air pressure throughout the alveoli and lung (see Figure 12.9).


The lungs are pyramid-shaped, paired organs that are connected to the trachea by the right and left bronchi; on the inferior surface, the lungs are bordered by the diaphragm. The lungs are enclosed by the pleurae, which are attached to the mediastinum. The right lung is shorter and wider than the left lung, and the left lung occupies a smaller volume than the right. The cardiac notch allows space for the heart (see Figure 12.10). The apex of the lung is the superior region, whereas the base is the opposite region near the diaphragm. The costal surface of the lung borders the ribs. The mediastinal surface faces the midline.


Each lung is composed of smaller units called lobes. Fissures separate these lobes from each other. The right lung consists of three lobes: the superior, middle, and inferior lobes. The left lung consists of two lobes: the superior and inferior lobes. A pulmonary lobule is a subdivision formed as the bronchi branch into bronchioles. Each lobule receives its own large bronchiole that has multiple branches. An interlobular septum is a wall, composed of connective tissue, which separates lobules from one another.


The major function of the lungs is to perform gas exchange, which requires blood from the pulmonary circulation.


The blood supply of the lungs plays an important role in gas exchange and serves as a transport system for gases throughout the body. Innervation by both the parasympathetic and sympathetic nervous systems provides an important level of control through dilation and constriction of the airway.


Reflexes such as coughing, and the ability of the lungs to regulate oxygen and carbon dioxide levels, also result from autonomic nervous system control. Sensory nerve fibers arise from the vagus nerve, and from the second to fifth thoracic ganglia. The pulmonary plexus is a region on the lung root formed by the entrance of the nerves at the hilum. The nerves then follow the bronchi in the lungs and branch to innervate muscle fibers, glands, and blood vessels.


Each lung is enclosed within a cavity that is surrounded by the pleura. The pleura (plural = pleurae) is a serous membrane that surrounds the lung. The right and left pleurae, which enclose the right and left lungs, respectively, are separated by the mediastinum.


The visceral and parietal pleurae connect to each other at the hilum. The pleural cavity is the space between the visceral and parietal layers.


The difference in pressures drives pulmonary ventilation because air flows down a pressure gradient, that is, air flows from an area of higher pressure to an area of lower pressure.


Pulmonary ventilation comprises two major steps: inspiration and expiration. Inspiration is the process of having air enter the lungs and expiration is the process of expelling air from the lungs (Figure 12.12). A respiratory cycle is one sequence of inspiration and expiration.


Two muscle groups are used during normal inspiration: the diaphragm and the external intercostal muscles. Additional muscles can be used if a bigger breath is required.


Due to the adhesive force of the pleural fluid, the expansion of the thoracic cavity forces the lungs to stretch and expand as well. This increase in volume leads to a decrease in intra-alveolar pressure, creating a pressure lower than atmospheric pressure. As a result, a pressure gradient is created that drives air into the lungs.


The process of normal expiration is passive, meaning that energy is not required to push air out of the lungs.


There are different types, or modes, of breathing that require a slightly different process to allow inspiration and expiration:


Breathing usually occurs without thought, although at times you can consciously control it, such as when you swim under water, sing a song, or blow bubbles. The respiratory rate is the total number of breaths that occur each minute. Respiratory rate can be an important indicator of disease, as the rate may increase or decrease during an illness or in a disease condition. The respiratory rate is controlled by the respiratory center located within the medulla oblongata in the brain, which responds primarily to changes in carbon dioxide, oxygen, and pH levels in the blood.


The normal respiratory rate of a child decreases from birth to adolescence:


Media 12.1. Respiratory System, Part 2: Crash Course A&P #32 [Online video]. Copyright 2015 by CrashCourse.


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


A variety of diseases can affect the respiratory system, such as asthma, emphysema, chronic obstructive pulmonary disorder (COPD), and lung cancer. All of these conditions affect the gas exchange process and result in labored breathing and other difficulties.


COPD is a term used to represent a number of respiratory diseases, including chronic bronchitis and emphysema. COPD is a chronic condition with most symptoms appearing in middle-aged or older adults. Signs and symptoms include shortness of breath, cough, and sputum production. There is no cure for COPD. Shortness of breath may be controlled with bronchodilators. The best plan is to avoid triggers and getting sick. Clients with COPD are advised to avoid people who are sick, get vaccinated against influenza and pneumococcal pneumonia, and reduce their exposure to pollution and cigarette smoke. While there are several risk factors, as many as 75% of cases are associated with cigarette smoking (National Heart, Lung, and Blood Institute, n.d.). To learn more about COPD, visit the National Heart, Lung, and Blood Institute’s web page.


Asthma is a chronic disease characterized by inflammation, edema of the airway, and bronchospasms which can inhibit air from entering the lungs. Bronchospasms can lead to an “asthma attack.” An attack may be triggered by environmental factors such as dust, pollen, pet hair, or dander, changes in the weather, mold, tobacco smoke, and respiratory infections, or by exercise and stress.


Signs and symptoms of an asthma attack involve coughing, shortness of breath, wheezing, and tightness of the chest. Symptoms of a severe asthma attack require immediate medical attention and may include dyspnea that results in cyanotic lips or face, confusion, drowsiness, a rapid pulse, sweating, and severe anxiety. The severity of the condition, frequency of attacks, and identified triggers influence the type of medication that an individual may require. Longer-term treatments are used for those with more severe asthma. Short-term, fast-acting drugs that are used to treat an asthma attack are typically administered via an inhaler. For young children or individuals who have difficulty using an inhaler, asthma medications can be administered via a nebulizer.


Lung cancer is a leading cause of cancer death among men and women. Smoking is the most significant risk factor for lung cancer, with 90% of cases in men and 80% of cases in women attributed to tobacco smoking. Signs and symptoms may include shortness of breath, wheezing, blood in the mucus, hoarseness, and trouble swallowing (MedlinePlus, n.d.).


There are two types of lung cancer, small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC). Both cancers occur when malignant cells form in the tissues of the lung. If metastasis occurs, lung cancer cells spread to other parts of the body. Treatment will depend on the type of lung cancer and the stage at diagnosis. Treatments may include surgery, chemotherapy, targeted therapy, immunotherapy, and radiation therapy (National Cancer Institute, 2021a, 2021b).


Respiratory therapists (RTs) are healthcare professionals that monitor, assess, and treat people who are having problems breathing. RTs must have at least a two-year degree. RTs measure lung capacity, test oxygen and carbon dioxide levels, perform chest physiotherapy to remove mucus from patients’ lungs, and operate ventilator equipment (Bureau of Labor Statistics, 2021). For more information, visit the American Association for Respiratory Care web page.


A thoracic surgeon refers to a surgeon who has specialized in either thoracic (chest) surgery or cardiothoracic (heart and chest) surgery (National Cancer Institute, n.d.). To learn about the career path, read this PDF from The Society of Thoracic Surgeons.


Spirometry testing is used to find out how well lungs are working by measuring air volume.


Both respiratory rate and depth are controlled by the respiratory centers of the brain, which are stimulated by factors such as chemical and pH changes in the blood. These changes are sensed by central chemoreceptors, which are located in the brain, and peripheral chemoreceptors, which are located in the aortic arch and carotid arteries. A rise in carbon dioxide or a decline in oxygen levels in the blood stimulates an increase in respiratory rate and depth.


Media 12.3. Peak Flow and Spirometry – Lung Function Tests [Online video]. Copyright 2012 by Oxford Medical Education.


Small tube that leads from the terminal bronchiole to the respiratory bronchiole and is the point of attachment for alveoli.


Condition caused by a lack of oxygen that leads to impending or actual death.


To withdraw fluid, tissue, or other substances from a body cavity, cyst, or tumor.


A type of drug that causes small airways in the lungs to open up.


Cancer that begins in the tissue that lines or covers the airways of the lungs, including small cell and non-small cell lung cancer.


Inflammation of the lung, particularly the bronchioles and alveoli, that is associated with bronchitis.


A thin, tube-like instrument used to examine the inside of the trachea, bronchi, and lungs.


A procedure involving a bronchoscope to examine the inside of the trachea, bronchi, and lungs.


A ridge at the base of the trachea (windpipe) that separates the openings of the right and left main bronchi (the large air passages that lead from the trachea to the lungs).


A condition that lasts a long time with periods of remission and exacerbation.


A noninvasive imaging technique that uses computers to analyze several cross-sectional X-rays in order to reveal minute details about structures in the body.


The major functions of the conducting zone are to provide a route for incoming and outgoing air, remove debris and pathogens from the incoming air, and warm and humidify the incoming air.


Pertaining to abnormal color of blue (bluish color, lips and nail beds) caused by deoxygenation.


A sheet of skeletal muscle separating the thoracic and abdominal cavities that has to contract and relax for you to breathe.


Condition of difficult speaking, including hoarseness and change in pitch or quality of the voice.


Leaf-shaped piece of elastic cartilage that is a portion of the larynx that swings to close the trachea during swallowing.


A mode of breathing that occurs at rest and does not require the cognitive thought of the individual; also known as quiet breathing.


Exhalation, or the process of causing air to leave the lungs.


The surface and skeletal structures that result in the outward appearance of the nose and contribute to its numerous functions.


A flexible membrane that closes the posterior surface of the trachea, connecting the C-shaped cartilages.


Composed of the vestibular folds, the true vocal cords, and the space between these folds.


Located at the anterior region of the nasal cavity and is composed of bone.


A doctor who has special training in diagnosing and treating blood disorders.


A concave region where blood vessels, lymphatic vessels, and nerves also enter the lungs.


One of the three regions of the pharynx; inferior to the oropharynx and posterior to the larynx.


A cartilaginous structure inferior to the laryngopharynx that connects the pharynx to the trachea and helps regulate the volume of air that enters and leaves the lungs; also known as the voice box.


The second most common type of leukocyte and are essential for the immune response.


A thick, slippery fluid made by the membranes that line certain organs of the body.


The upper part of the throat behind the nose. An opening on each side of the nasopharynx leads into the ear.


A passageway for both air and food; borders the nasopharynx and the oral cavity.


The tonsil located at the back of the throat; also known as the adenoid when swollen.


A tube formed by skeletal muscle and lined by mucous membrane that is continuous with that of the nasal cavities; also known as the throat.


An abnormal collection of fluid between the thin layers of tissue (pleura) lining the lung and the wall of the chest cavity.


A severe inflammation of the lungs in which the alveoli (tiny air sacs) are filled with fluid.


An abnormal collection of air in the space between the thin layer of tissue that covers the lungs and the chest cavity that can cause all or part of the lung to collapse.


Simultaneous and continuous monitoring of several parameters during sleep to study normal and abnormal sleep.


Describes the back or direction toward the back of the body.


A doctor who has special training in creating and interpreting pictures of areas inside the body.


A procedure that uses x-rays to take pictures of areas inside the body.


The use of radiation or other imaging technologies to diagnose or treat disease.


The respiratory zone includes structures that are directly involved in gas exchange.


A plastic surgical operation on the nose, either reconstructive, restorative, or cosmetic.


Excess nasal drainage; also called a “runny nose.”


Located at the posterior portion of the nasal cavity and consists of muscle tissue.


A computer picture of areas inside the body created by high-energy sound waves.


The measurement of volume of air inhaled or exhaled by the lung.


Mucus and other matter brought up from the lungs by coughing.


An instrument is used to hear sounds produced by the heart, lungs, or other parts of the body.


The division of the nervous system involved in our fight-or-flight responses. It continuously monitors body temperature and initiates appropriate motor responses.


Removal of fluid from the pleural cavity through a needle inserted between the ribs.


A thin tube-like instrument used to examine the inside of the chest.


Infection of the nasal cavity, pharynx and larynx cause by a virus.


A small bulbous, teardrop-shaped structure located at the apex of the soft palate.


Bureau of Labor Statistics. (2021). Respiratory therapists. In Occupational outlook handbook. U.S. Department of Labor. https://www.bls.gov/ooh/healthcare/respiratory-therapists.htm


CrashCourse. (2015, August 24). Respiratory system, part 1: crash course A&P #31 [Video]. YouTube. https://youtu.be/bHZsvBdUC2I


CrashCourse. (2015, August 31). Respiratory system, part 2: crash course A&P #32 [Video]. YouTube. https://youtu.be/Cqt4LjHnMEA


National Cancer Institute. (n.d.). Thoracic surgeon definition. National Institutes of Health, U.S. Department of Health. https://www.cancer.gov/publications/dictionaries/cancer-terms/def/thoracic-surgeon


National Cancer Institute. (2021a). Non-small cell lung cancer treatment (PDQ®)-Patient version. National Institutes of Health, U.S. Department of Health. https://www.cancer.gov/types/lung/patient/non-small-cell-lung-treatment-pdq


National Cancer Institute. (2021b). Small cell cancer treatment (PDQ®)-Patient version. National Institutes of Health, U.S. Department of Health. https://www.cancer.gov/types/lung/patient/small-cell-lung-treatment-pdq


National Heart, Lung, and Blood Institute. (n.d.). COPD. National Institutes of Health, U.S. Department of Health. https://www.nhlbi.nih.gov/health-topics/copd


Oxford Medical Education. (2012, May 13). Peak flow and spirometry – Lung function tests [Video]. YouTube. https://www.youtube.com/watch?v=M4C8EInOMOI


The lymphatic system is a series of vessels, ducts, and trunks that remove interstitial fluid from the tissues and return it to the blood. The lymphatic vessels are also used to transport dietary lipids and cells of the immune system. Cells of the immune system, lymphocytes, all come from the hematopoietic system of the bone marrow. Primary lymphoid organs, the bone marrow and thymus gland, are the locations where lymphocytes proliferate and mature. Secondary lymphoid organs are the site in which mature lymphocytes congregate to mount immune responses. Many immune system cells use the lymphatic and circulatory systems for transport throughout the body to search for and then protect against pathogens.


This chapter begins by describing the anatomy and physiology of the lymphatic system, whose immune functions lead us into a discussion of the body’s multifaceted defenses, which together make up the immune system. Since the lymphatic system shares organs with a number of other body systems, the pathology discussed near the end of this chapter mainly focuses on disorders of the immune system.


immune functions lead us into a discussion of the body’s multifaceted defenses, which together make up the immune system. Since the lymphatic system shares organs with a number of other body systems, the pathology discussed near the end of this chapter mainly focuses on disorders of the immune system.


Media 11.1 Lymphatic System: Crash Course A&P #44 [Online video]. Copyright 2015 by CrashCourse.