How do fish breathe?

Fish breathe through their gills, which are located on the sides of their head. Gills are made up of thin, feathery structures called filaments, which contain a network of tiny blood vessels called capillaries. As water flows over the gills, oxygen from the water diffuses into the capillaries and is carried to the fish's bloodstream. At the same time, carbon dioxide from the fish's bloodstream diffuses into the water and is carried away.

Fish use a process called countercurrent exchange to efficiently extract oxygen from the water they breathe. Water flows over the gills in one direction, while blood flows through the capillaries in the opposite direction. This allows oxygen to diffuse from the water into the blood as efficiently as possible.

It's important for fish to maintain a constant flow of water over their gills in order to keep oxygen levels high. Many fish have a specialized organ called the operculum, which helps to pump water over their gills even when they are not actively swimming.