How Sharks Breathe

Sharks don’t have lungs. They pull oxygen straight out of the sea.

Sharks have gills, and gills are a brilliant trick. On each side of a shark’s throat are several gill slits — usually five, sometimes six or seven — and behind each slit sits a stack of thin, feathery gill filaments packed with blood vessels.

Water comes in through the mouth, washes over those filaments, and flows out through the slits. As it passes, oxygen from the water slips into the shark’s blood, and carbon dioxide slips back out.

The counter-current trick

Here’s what makes gills so good at their job. Inside the gills, blood flows in the opposite direction to the water. So blood that has already picked up some oxygen meets water that still has plenty, and it keeps grabbing more. If blood and water flowed the same way, a shark would get far less oxygen out of every mouthful of water. Two flows, one going each way, is the whole secret.

Sharks aren’t the only sea animals built this way. Their flat cousins, the rays and skates, breathe the same way — except their gill slits are on the underside of their bodies, since they spend so much time on the sea floor.

Some sharks get an extra tool: a spiracle, a small opening just behind the eye. It lets them draw in clean water even with their mouth buried in the sand while they hunt. Think of it as a snorkel you’re born with.

One more thing worth knowing: breathing water is expensive. Water holds far less oxygen than air does, so a shark has to push a lot of it across its gills to get what it needs. That’s part of why so many sharks never really stop working.

Did you know? Some sharks’ gill slits are so fine and thin they look almost like a fringe — which is exactly how the frilled shark got its name.