What is the primary mechanism by which gas exchange occurs in the lungs?

• A. Ventilation
• B. Diffusion
• C. Absorption
• D. Perfusion

Answer: (B)

The primary mechanism for gas exchange in the lungs is diffusion. This process occurs when oxygen and carbon dioxide move across the alveolar membrane, which separates the air in the alveoli from the blood in the capillaries.

Diffusion relies on the principles of concentration gradients; oxygen moves from an area of higher concentration in the alveoli to a lower concentration in the blood, while carbon dioxide moves in the opposite direction—from the blood, where it is in higher concentration, to the alveoli, where it is at a lower concentration. This bidirectional exchange is essential for maintaining proper oxygen levels in the blood and facilitating the removal of carbon dioxide, a metabolic waste product.

Ventilation refers to the mechanical movement of air in and out of the lungs, which is crucial for maintaining the concentration gradients necessary for diffusion, but it is not the direct mechanism of gas exchange itself. Absorption is a term more applicable to the uptake of substances in the digestive system rather than respiration, and perfusion relates to the flow of blood through the pulmonary capillaries, providing the necessary environment for gas exchange but not describing the actual movement of gases.