Where does the transfer of oxygen and nitrogen into the bloodstream occur?

The transfer of oxygen and nitrogen into the bloodstream occurs in the alveoli, which are tiny air sacs located in the lungs. The primary function of the alveoli is to facilitate the exchange of gases during respiration. When you inhale, oxygen enters the alveoli and diffuses across the thin walls of these air sacs into the surrounding capillaries, which are a network of small blood vessels. Meanwhile, carbon dioxide, which is a waste product of metabolism, diffuses from the blood in the capillaries into the alveoli to be exhaled.

While capillaries play a crucial role in transporting blood and facilitating gas exchange, the actual transfer of oxygen into the bloodstream happens at the alveolar level. Veins and arteries are primarily responsible for transporting blood to and from the heart; they do not participate directly in the gas exchange process. Therefore, the correct answer focuses on the alveoli as the specific site where this vital exchange occurs.