In external respiration, where does oxygen diffuse?

In external respiration, oxygen diffusion primarily occurs from the alveoli into the capillaries. This process takes place in the lungs, where oxygen is inhaled and enters the alveoli, the tiny air sacs that facilitate gas exchange. The alveoli have a high concentration of oxygen due to fresh air being inhaled, while the blood in the capillaries has a lower concentration of oxygen. This difference in concentration creates a gradient that drives the diffusion of oxygen across the thin membrane of the alveoli into the capillary blood.

Once in the capillaries, oxygen binds to hemoglobin in red blood cells, which then transports it throughout the body. This mechanism of gas exchange is critical for ensuring that oxygen is delivered to tissues and organs that require it for cellular respiration and energy production.

The other options do not accurately describe the process of external respiration. For instance, diffusion from capillaries into alveoli would imply that oxygen is moving against its concentration gradient, which is incorrect. Similarly, the movement of blood from vein to artery pertains more to the circulatory process rather than the specific mechanism of gas exchange in the lungs. Lastly, the diffusion of oxygen from the blood into tissues occurs during internal respiration, a separate process from external respiration occurring in the