Question

Gas exchange in the gills of a teleost fish is enhanced by having the blood flow in direction opposite to the direction of water flow, a process known as

• A. Counter current exchange
• B. Ventilation
• C. Facilitated diffusion
• D. Active respiration

Answer 1

Answer: (A)

Counter current exchange

• Fish use gills for gas exchange. Gills have numerous folds that give them a very large surface area. The arrangement of water flowing past the gills in the opposite direction to the blood (called countercurrent flow) means that they can extract oxygen at three times the rate a human can.
• Countercurrent flow - As the blood flows in the opposite direction to the water, it always flows next to the water that has given up less of its oxygen. This way, the blood is absorbing more and more oxygen as it moves along. Even as the blood reaches the end of the lamella and is 80% or so saturated with oxygen, it is flowing past water which is at the beginning of the lamella and is 90% or 100% saturated.
• Therefore, even when the blood is highly saturated, having flowed past most of the length of the lamellae, there is still a concentration gradient and it can continue to absorb oxygen from the water.

Thus, option A is correct.