How Do Fish Breathe Underwater?

Step 1 of 5

Water has oxygen dissolved in it.

Not the oxygen bound up in H₂O, but separate oxygen gas mixed in from the air and from plants. That is what fish are after.

Step 2 of 5

There is very little of it.

Water holds around thirty times less oxygen than air, and it is heavy to move. Fish are extracting a scarce resource from a difficult medium.

Step 3 of 5

Gills are enormously folded.

Each gill has stacks of filaments covered in microscopic folds, giving a huge surface area in a small space — the same trick as lungs.

Step 4 of 5

Blood flows the opposite way to the water.

Blood runs counter to the water flow, so it always meets water slightly richer in oxygen than itself. This extracts far more than same-direction flow.

Step 5 of 5

Out of water, gills collapse.

Air cannot hold the delicate filaments apart. They stick together, the surface area vanishes, and a fish suffocates surrounded by oxygen.

Counterflow lets fish take up to 80 percent of the oxygen from passing water. Our lungs manage about 25.