How do fish breathe underwater?

Water contains dissolved oxygen, and fish have evolved gills to extract it. A fish takes water in through its mouth and pushes it out over the gills, which are made of thin, feathery filaments packed with tiny blood vessels. As water flows over these filaments, oxygen passes from the water into the blood, while carbon dioxide passes out. The arrangement is highly efficient, with blood flowing in the opposite direction to the water to capture as much oxygen as possible. Because water holds far less oxygen than air, gills must be very effective, and some fish must keep swimming to keep water moving over them.