The countercurrent exchange system found in fish gills maximizes gas exchange efficiency by:

A) producing rapid water flows across the gill surface.
B) allowing water and blood to come in direct contact to maximize oxygen exchange.
C) having a high concentration gradient of oxygen at the start of the capillaries.
D) maintaining a high concentration gradient of oxygen along the entire length of capillaries.
E) decreasing the oxygen content of the blood leaving the gill filaments.

Countercurrent Exchange

A mechanism by which fluids flowing in opposite directions exchange properties, optimizing heat or mass transfer.

Gas Exchange Efficiency

The effectiveness with which an organism exchanges oxygen and carbon dioxide with its environment, crucial for respiration.

Capillaries

The smallest blood vessels in the body's circulatory system, responsible for the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste substances between blood and surrounding tissues.