What role do capillaries play in the alveoli?

Capillaries play a crucial role in the lungs, specifically in the alveoli, by participating directly in gas exchange. This process involves the transfer of oxygen from the alveoli into the capillary blood while simultaneously allowing carbon dioxide to move from the blood into the alveoli to be exhaled. The walls of the capillaries and the walls of the alveoli are both extremely thin, which facilitates this efficient exchange of gases.

Oxygen in the alveoli diffuses across the alveolar walls and enters the capillary blood, where it binds to hemoglobin in red blood cells for transport throughout the body. Conversely, carbon dioxide, which is a waste product of cellular metabolism, diffuses from the blood into the alveoli, allowing it to be expelled from the respiratory system. This fundamental role of capillaries is essential for maintaining the body's oxygen supply and removing carbon dioxide, thus supporting cellular respiration and overall metabolic function.