Countercurrent exchange in the gills of a fish
a. speeds up the flow of water through the gills
b. maintains a gradient that enhances diffusion
c. enables the fish to obtain oxygen while swimming backward
d. means that blood and water flow in the same direction
e. interferes with the efficient absorption of oxygen
The correct answer and explanation is:
The correct answer is b. maintains a gradient that enhances diffusion.
Explanation:
Countercurrent exchange is a highly efficient mechanism in fish gills that optimizes the absorption of oxygen from water. The gills of fish are specialized for gas exchange, where oxygen from the water diffuses into the blood, and carbon dioxide from the blood diffuses into the water. This exchange happens across thin gill membranes.
In countercurrent exchange, the flow of water over the gills and the flow of blood through the gills occur in opposite directions. This setup maintains a constant concentration gradient between the water and the blood, which is crucial for efficient gas exchange. As oxygen diffuses from the water into the blood, the difference in oxygen concentration between the water and blood remains high across the entire length of the gill filament. This maximizes the amount of oxygen that can be transferred into the blood.
By contrast, if the blood and water flowed in the same direction (as in choice d), the concentration gradient would diminish along the length of the gill, reducing the efficiency of oxygen uptake. The key to countercurrent exchange is that it ensures that the oxygen concentration in the water is always higher than that in the blood, which allows for a continuous flow of oxygen into the bloodstream.
This mechanism is not directly related to the speed of water flow (choice a) or any specific behavior of the fish, like swimming backward (choice c). It is also not a hindrance to oxygen absorption (choice e), but rather the opposite, promoting a high-efficiency gas exchange process.