[Campbell Biology P.990] According to Figure 42.22's diagram on countercurrent exchange, whi... | Practice Question
According to Figure 42.22's diagram on countercurrent exchange, which statement best explains why this mechanism is highly efficient for oxygen uptake in fish gills?
A: The partial pressure of O2 in the blood always remains significantly higher than that in the water throughout the capillary length, maximizing diffusion.
B: Oxygen diffuses from water to blood because a partial pressure gradient is maintained, with the water always having a higher PO2 than the blood it encounters, along the entire length of the capillary.
C: The PO2 in the water rapidly drops to match the PO2 in the blood early in the capillary, leading to equilibrium and preventing further diffusion.
D: Blood flow and water flow are in the same direction, which optimizes the initial steep O2 gradient for rapid diffusion.
Explanation
The diagram and text explain that in countercurrent exchange, the water always has a higher PO2 than the blood it encounters at any point, maintaining a continuous partial pressure gradient that drives diffusion of O2 from water to blood along the entire length of the capillary.