How do fish breathe underwater?

Fish are aquatic animals that breathe underwater through a process called respiration. Unlike humans and other mammals, fish cannot extract oxygen from the air and rely on dissolved oxygen in the water to survive.

Most fish breathe using gills, which are specialized organs that extract oxygen from water. Gills are located on either side of a fish's head, behind its eyes, and consist of filaments that are rich in blood vessels. As water passes over the gills, oxygen is extracted from the water and absorbed into the fish's bloodstream. At the same time, carbon dioxide, which is a waste product of respiration, is released from the fish's bloodstream and expelled into the water.

The process of breathing through gills is made possible by a countercurrent exchange system. Water flows over the gills in one direction, while blood flows in the opposite direction through the filaments. This allows for efficient exchange of gases, with oxygen diffusing from the water into the fish's bloodstream, and carbon dioxide diffusing from the fish's bloodstream into the water.

Some fish, such as lungfish and some catfish, are also capable of breathing air in addition to extracting oxygen from water. These fish have evolved to live in environments with low oxygen levels or stagnant water, where it may be difficult to extract enough oxygen from the water to survive. Lungfish have lungs that allow them to breathe air, while catfish are able to gulp air into a specialized organ called a labyrinth organ, which extracts oxygen from the air and distributes it to the bloodstream.