How do gills work?

Gills help aquatic animals breathe by allowing gases to move between water and blood. Their feathery, branched structure is filled with superficial blood vessels, which gives them a large surface area for this exchange. This design brings the circulatory system very close to the water, making it easier for oxygen to enter the blood and carbon dioxide to leave.

In fish, water enters through the mouth and flows over gill filaments supported by gill arches. Oxygen passes into the blood while carbon dioxide is released into the water. The blood in the gill capillaries flows in the opposite direction of the water, a process called countercurrent exchange. As the water and blood move in opposite directions, the blood always meets water that has a slightly higher oxygen content. Since the oxygen level in the water remains higher than in the blood, oxygen continues to diffuse into the bloodstream along the full length of the gill filament. This setup improves the fish’s ability to absorb oxygen efficiently.

Different aquatic animals have special adaptations to maximize the function of their gills. Cephalopod mollusks such as squids and octopuses have gills that extract a high percentage of oxygen from the water. Crustaceans such as crabs and shrimps have protective chambers around their gills and use special appendages to move water over them. These adaptations help animals survive in waters where oxygen levels may fluctuate.