Question

which process do organisms with gills use to extract oxygen from the water and eliminate carbon dioxide?
bulk flow
countercurrent exchange
active transport

Explanation:

• Bulk flow: Refers to the movement of fluids (like blood or water) due to pressure differences. While it's involved in transporting substances in organisms, it's not the specific process for gas exchange in gills.
• Countercurrent exchange: In gills, water flows over the gill filaments in one direction, and the blood within the gill filaments flows in the opposite direction. This creates a concentration gradient that maximizes the diffusion of oxygen from water to blood and carbon dioxide from blood to water.
• Active transport: Requires energy (usually ATP) to move substances against their concentration gradient. Gas exchange in gills (oxygen and carbon dioxide movement) occurs via diffusion (a passive process), not active transport.

Answer:

B. countercurrent exchange