Pulmonary alveolus

An alveolus (plural:alveoli), is an anatomical structure that has the form of a hollow cavity. In the lung, the alveoli are spherical outcroppings of the respiratory bronchioles and are the primary sites of gas exchange with the blood.

Exchange between blood and gas

The blood that enters the pulmonary capillaries is the systemic venous blood which enter the lungs via the pulmonary arteries.

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Due to differences in partial pressures across the alveolar-capillary membrane, O2 diffuses into the blood and CO2 diffuses out. Thus, the blood that returns to the heart has nearly the same PO2 and PCO2 as the alveolar air. The more pulmonary capillaries participating in this process, the more total O2 and CO2 that can be exchanged. The magnitude of the difference between the alveolar PO2 and arterial PO2 can be used to detect the presence of some lung diseases.

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