Which organ exchanges oxygen and carbon dioxide with water?

Which organ exchanges oxygen and carbon dioxide with water?

• A. Lungs
• B. Nostrils
• C. Gills
• D. Skin

Answer: (C)

Gas exchange with water requires a specialized organ that has a large, thin surface area in direct contact with the surrounding water and a good blood supply to pick up oxygen and release carbon dioxide. Gills fit this role perfectly in many aquatic animals. Water flows over the gills, oxygen dissolved in the water diffuses across the thin membranes into the blood, and carbon dioxide diffuses out into the water. In many species, a countercurrent exchange mechanism—where water and blood flow in opposite directions—keeps the diffusion gradient favorable along the entire gill length, maximizing oxygen uptake. Lungs, by contrast, are designed for gas exchange with air rather than water. Nostrils are just openings that bring air in; they don’t exchange gases themselves. Skin can support some diffusion in very moist, small organisms, but it isn’t the primary organ for exchanging gases with water, especially in animals with true aquatic respiration.

Gas exchange with water requires a specialized organ that has a large, thin surface area in direct contact with the surrounding water and a good blood supply to pick up oxygen and release carbon dioxide. Gills fit this role perfectly in many aquatic animals. Water flows over the gills, oxygen dissolved in the water diffuses across the thin membranes into the blood, and carbon dioxide diffuses out into the water. In many species, a countercurrent exchange mechanism—where water and blood flow in opposite directions—keeps the diffusion gradient favorable along the entire gill length, maximizing oxygen uptake.

Lungs, by contrast, are designed for gas exchange with air rather than water. Nostrils are just openings that bring air in; they don’t exchange gases themselves. Skin can support some diffusion in very moist, small organisms, but it isn’t the primary organ for exchanging gases with water, especially in animals with true aquatic respiration.