Which component is essential for the gas exchange between alveoli and blood capillaries?

The essential component for gas exchange between alveoli and blood capillaries is oxygen. During the process of respiration, oxygen from the air in the alveoli diffuses across the alveolar-capillary membrane into the blood in the capillaries. This diffusion occurs due to the concentration gradient, where oxygen concentration is higher in the alveoli compared to the blood. Once in the bloodstream, oxygen binds to hemoglobin in red blood cells and is transported to tissues throughout the body, where it is utilized for metabolic processes.

The other options do play roles in the respiratory system but are not directly responsible for the gas exchange process: water vapor helps humidify the air we breathe, carbon monoxide is a toxic gas that binds with hemoglobin more effectively than oxygen, thereby inhibiting oxygen transport, and surfactant reduces surface tension in the alveoli to help maintain their structure and prevent collapse, but it is not a gas exchanged in the blood. Each of these has a specific importance in the respiratory system, but oxygen is the fundamental element necessary for the vital process of gas exchange.