The Living World 8th Edition by George Johnson

تمرين 5

In the gills of fish, countercurrent flow allows
A) the animal's blood to be continually exposed to water of lower oxygen concentration.
B) the animal's blood to be continually exposed to water of equal oxygen concentration.
C) the animal's blood to be continually exposed to water of higher oxygen concentration.
D) for a larger surface area for gas exchange to occur.