Gas exchange between lungs and tissues

Gas composition and exchange between the air and alveoli
Barometric pressure at sea level is 760 mm Hg. Different gases contribute different proportions to it. However, the composition of gas is different between the atmosphere, alveoli, and tissue cells. Gases diffuse from regions where their partial pressure is higher to those where it is lower. CO2 diffuses out of the lungs into the air, and O2 to diffuses from the air into the lungs.

Gas exchange between alveoli and capillaries
O2 also diffuses across the alveolar-capillary membrane due to a difference in partial pressures, though this difference is rather small. It also depends on the lung diffusing capacity DL. Fick’s Law can be used to find the volumetric flow rate:

V̇x = DL*ΔP

Where V̇x is the volumetric flow rate, ΔP is the pressure difference, and DL is the diffusing capacity.

DL describes the permeability of the barrier between alveoli and capillaries. It is directly proportional to the diffusion coefficient of the gas and surface area, and inversely proportional to thickness.

Gas exchange between capillaries and tissue
Gas exchange between capillaries and tissues depends on


 * Partial pressure gradients
 * Surface area
 * Blood perfusion of the tissue