Question

During external respiration, what happens to the PCO₂ in alveolar capillaries?

1) It increases from 45 mm Hg to 40 mm Hg
2) It decreases from 45 mm Hg to 40 mm Hg
3) It remains constant at 45 mm Hg
4) It cannot be determined

Answer 1

The PCO₂ in alveolar capillaries decreases from 45 mm Hg to 40 mm Hg during external respiration due to the pressure gradient that drives the diffusion of carbon dioxide from the blood into the alveoli.

Step-by-step explanation:

During external respiration, partial pressure of carbon dioxide (PCO₂) in the blood of the pulmonary capillaries decreases. This happens because carbon dioxide diffuses from the blood where its partial pressure is higher (about 45 mm Hg) into the alveoli where its partial pressure is lower (about 40 mm Hg). This diffusion is driven by the pressure gradient between the capillaries and alveoli. Therefore, option 2) It decreases from 45 mm Hg to 40 mm Hg is correct. Carbon dioxide's solubility is also much greater than that of oxygen, by a factor of 20, aiding in its efficient transfer across the respiratory membrane.