Question

What is the mechanism of gas exchange between alveoli and pulmonary capillaries? Compare the gas pressures in the pulmonary capillaries to the gas pressures in the alveoli, in a normal person at rest.

Answer 1

• Gas exchange is the delivery of oxygen from the lungs to the bloodstream i.e. inhaled oxygen moves from the alveoli of lungs to the blood in the capillaries, and the elimination of carbon dioxide from the bloodstream to the lungs i.e. carbon dioxide moves from the blood in the capillaries to the air in the alveoli. Gas exchange takes place in the millions of alveoli in lungs and a network of tiny blood vessels called capillaries. The walls of the alveoli actually share a membrane with the capillaries in which oxygen and carbon dioxide move freely between the respiratory system and the bloodstream.

• A gas will move from an area where its partial pressure is higher to an area where its partial pressure is lower. In addition, the greater the partial pressure difference between the two areas, the more rapid is the movement of gases. In a normal person at rest, carbon dioxide diffuses in the opposite direction since the partial pressure is greater in the blood entering the lungs than it is in the alveolar air. Again, the gases moves from a high to low partial pressure. The partial pressure of oxygen is lower in the blood than in alveoli, so it diffuses into the blood. This actual exchange of gases occurs due to simple diffusion and these gases follow pressure gradients that allow them to diffuse.