HI$ and $H_2$ discs, we calculated the midplane gas and star volume densities as a function of the radial distance $R$ for several spiral and (for comparison) $LSB$ galaxies. The former include: M33, M51, M81, M100, M101, M106 and our Galaxy. The central parts of galaxies where the bulge dominates and/or the observed rotation curve is uncertain were ignored. We assumed that the stellar and gaseous discs are axisymmetric being in hydrostatic equilibrium and that the pressure of gas is determined by its turbulent motion: $P_{gas}=\rho_{gas}\,C_{gas}^2$, where the velocity dispersion $C_{gas}$ was taken to be constant (although different for atomic and molecular gas). Both the self gravitation of gas and presence of dark halos which also influence the thickness of discs were taken into account. The equations were numerically solved using an iterative algorithm (see the details in Zasov & Abramova [@ZA_AZ]).

To estimate the stellar disc thickness, two models were employed: stellar velocity dispersion $C_z$ was assumed to be either a constant or proportional to the marginal radial velocity dispersion $C_r$, that provides stellar disc with gravitational stability (see the discussion in [@ZA_Bottema93; @ZA_ZasAll04]). Both models give rather similar results.

The obtained estimates of the midplane gas density $\rho_{gas}(R)$ for spiral and $LSB$ galaxies are illustrated in Fig. \[fig1\].

To compare the surface $(SFR_s)$ and volume $(SFR_v)$ star formation rates with the gas densities in spiral galaxies we used the estimates of $SFR_s$ from Boissier et al. [@ZA_main], based on the smoothed absorption-corrected UV profiles. The resulting diagrams are demonstrated in Figs. \[fig1\]a,b.

[**Main results**]{}.

***1***. Marginally stable stellar discs in all cases but M33 and our Galaxy increase their thickness significantly beyond $R\approx 2-3\,R_0$. ***2***. Gaseous discs of $LSB$ galaxies are thicker than the stellar ones, and $\rho_{gas}$ is about an order of magnitudes lower than in HSB spiral galaxies. ***3***. 