Which of the following best characterizes alveolar ventilation and blood flow at the base, compared with the apex, of the lungs of a healthy standing person?
**Core Concept**
Alveolar ventilation and blood flow are distributed unevenly throughout the lungs, leading to differences between the base and apex. This phenomenon is known as the _ventilation-perfusion (V/Q) mismatch_.
**Why the Correct Answer is Right**
In a healthy standing person, gravity causes blood to pool in the lower regions of the lungs (base), increasing local blood flow. Conversely, the apex receives less blood flow. As a result, the base of the lungs receives more blood flow than the apex, while the apex receives more alveolar ventilation due to the effects of gravity on the diaphragm. This leads to a higher V/Q ratio at the apex compared to the base.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it implies a uniform distribution of blood flow and alveolar ventilation throughout the lungs, which contradicts the effects of gravity on pulmonary circulation and ventilation.
**Option B:** This option is incorrect because it suggests a higher V/Q ratio at the base, which is opposite to what occurs due to the effects of gravity.
**Option C:** This option is incorrect because it implies that the apex receives more blood flow than the base, which is not the case.
**Option D:** This option is incorrect because it suggests that the base of the lungs receives more alveolar ventilation than the apex, which is the opposite of what occurs due to the effects of gravity.
**Clinical Pearl / High-Yield Fact**
In a healthy standing person, the V/Q mismatch is a normal adaptation that helps to optimize oxygenation and carbon dioxide elimination. However, in patients with respiratory or cardiovascular disease, the V/Q mismatch can become abnormal and lead to hypoventilation or hyperventilation.
**Correct Answer:** C. The apex of the lungs receives more alveolar ventilation than blood flow, while the base receives more blood flow than alveolar ventilation.