Which of the following best describes external respiration?

The correct answer accurately identifies external respiration as the process of gas exchange between the lungs and the blood. During this process, oxygen from the air in the alveoli (the small air sacs in the lungs) diffuses into the capillaries surrounding them. Simultaneously, carbon dioxide from the blood diffuses into the alveoli to be exhaled. This exchange is crucial for maintaining adequate oxygen levels in the blood and removing carbon dioxide, which is a waste product of cellular metabolism.

Other options describe different aspects of respiration or related processes. For instance, gas exchange within the cells pertains to internal respiration, where oxygen is utilized by cells to produce energy, and carbon dioxide is produced as a by-product. Oxygen uptake by the blood refers to the transfer of oxygen from the alveoli to the blood in capillaries, which is an element of external respiration, but it does not capture the entire process involved. Ventilation of the lungs specifically refers to the mechanical process of inhaling and exhaling air, which is distinct from the actual gas exchange process taking place in the lungs. Therefore, the most comprehensive description of external respiration is the exchange of gases between the lungs and the blood.