External respiration best describes which of the following?

External respiration best describes which of the following?

• A. Oxygen diffuses into blood via capillaries
• B. Oxygen diffuses into tissues
• C. Carbon dioxide diffuses into alveolar air
• D. Nitrogen diffuses out of blood

Answer: (A)

External respiration is the exchange of gases between the air in the alveoli and the blood in the pulmonary capillaries. The key event is oxygen moving from the alveolar air into the blood, because the alveolar oxygen partial pressure is higher than that in the blood, allowing diffusion into the capillaries where it can bind to hemoglobin for transport to tissues. While carbon dioxide also moves from blood into alveolar air to be exhaled, the most direct and defining description of external respiration is the loading of oxygen into the blood. Nitrogen diffusion is not a major factor under normal conditions, since its diffusion across the respiratory membrane is minimal.

External respiration is the exchange of gases between the air in the alveoli and the blood in the pulmonary capillaries. The key event is oxygen moving from the alveolar air into the blood, because the alveolar oxygen partial pressure is higher than that in the blood, allowing diffusion into the capillaries where it can bind to hemoglobin for transport to tissues. While carbon dioxide also moves from blood into alveolar air to be exhaled, the most direct and defining description of external respiration is the loading of oxygen into the blood. Nitrogen diffusion is not a major factor under normal conditions, since its diffusion across the respiratory membrane is minimal.