What occurs during external respiration?

External respiration refers specifically to the process of gas exchange that occurs in the alveoli of the lungs. During this process, oxygen from inhaled air diffuses across the thin walls of the alveoli into the bloodstream, while carbon dioxide from the blood diffuses into the alveoli to be exhaled. This vital exchange is essential for replenishing oxygen in the blood for delivery to tissues throughout the body and for removing carbon dioxide, which is a waste product of cellular metabolism.

The other choices describe related but distinct processes. For instance, the exchange of gases between cells and blood pertains to internal respiration, which occurs at the cellular level. Blood circulation through the heart is crucial for transporting oxygenated blood from the lungs to the rest of the body but does not involve the actual gas exchange process. Lastly, the intake of oxygen through the mouth is part of the broader respiratory process but does not encapsulate the actual exchange of gases that defines external respiration.