ELI5 - How do Fish Breathe?

Fish breathe under water using something called gills. Gills are special body parts that let fish take oxygen from water. Humans use lungs and air. Fish use gills and water. Water has oxygen in it. You cannot see it, but it is there. Fish need this oxygen to live, just like you do.

A fish breathes by opening its mouth. Water flows into the mouth. Then the fish closes its mouth and pushes the water out through its gills. The gills are on the sides of the fish’s head. They look like thin, feathery lines inside. Inside the gills are tiny parts called filaments. These filaments are full of very small blood vessels. When water flows over them, oxygen moves from the water into the fish’s blood. At the same time, waste gas called carbon dioxide moves out of the blood and into the water.

This works because oxygen likes to move from places where there is a lot of it to places where there is less. The water has oxygen. The fish’s blood needs oxygen. So the oxygen moves across the thin gill walls into the blood. Fish are very good at this. Their gills are designed to grab as much oxygen as possible. Even when water has much less oxygen than air, fish can still get what they need. This is also why fish cannot breathe on land. In air, their gills dry out and stick together. When that happens, oxygen cannot move into the blood anymore. That is why fish need to stay in water to survive.

Some fish breathe a little differently. For example, sharks must keep swimming so water keeps flowing over their gills. Other fish can sit still and pump water over their gills by moving their mouths.

There are also a few special fish that can breathe air too. They still have gills, but they can gulp air at the surface if the water does not have much oxygen. This helps them survive in muddy or warm water. So fish do not drown in water. Water is their air. Their gills are like underwater lungs. They quietly pull oxygen from water all day, every day, without you ever noticing.