Fig. 8.

A scheme of the simplified model of the lung. The model implies numerous simplifying assumptions: steady state, diffusion occurring across a flat homogeneous alveolar–capillary barrier, and along a direction D that is perpendicular to the barrier; linear blood dissociation curve (constant gas transport coefficient for blood, β); homogeneous membrane and blood flow ($\dot{Q}$); absence of shunt. The graph describes the exponential increase of gas partial pressure inside the capillary, as blood proceeds along it. The flow across the barrier follows the pressure gradient. $d\dot{M}$ infinitesimal molar flow of the gas, P_A alveolar gas pressure, $P\overline{v}$ mixed venous gas pressure, P_a arterial gas pressure, P_c capillary gas pressure, x distance along the capillary, 0 beginning of pulmonary capillary, X₀ end of capillary, where $P_{c^{\prime }}{O}_2$ is attained

[From Piiper and Scheid (1981)]