Exchange of gases

Alveoli are the primary sites of the exchange of gases. The exchange of ages also occurs between blood and tissues. It occurs b simple diffusion mainly based on pressure/concentration gradient.
Gases exchange between alveoli and blood: this exchange between lung alveoli and pulmonary capillaries is called external respiration. The alveolar wall is the thin, rich network of blood capillaries. It is called the respiratory membrane or diffusion membrane.

Figure: diffusion membrane

This membrane has a limit of gas exchange between alveoli and pulmonary blood, called diffusion blood called the diffusion capacity. CO2 can diffuse through the membrane per unit difference I partial pressure is much higher as compared to oxygen. pO2 in the alveoli is higher than that in the deoxygenated blood in the capillaries of pulmonary arteries.so the movement of O2 is from alveoli to the blood. Also, CO2 is higher in deoxygenated blood than in alveoli. Therefore CO2 passes from the blood to the alveoli.

Figure: exchange of gases