Whales can’t breathe in water for the same reason we can’t. We have lungs and not gills. Lungs work fine on land because there is a lot of oxygen in air. About 20% of air is oxygen. Water has a lot less oxygen in it. The amount depends on things like water temperature, depth, and salinity (the amount of salt in the water).

Oxygen moves from the environment into the animal over surfaces on gills or in lungs. The lower the oxygen concentration, the more surface area the animal needs. Imagine that you needed to catch fish. If the fish were all close together, you could use a small net. If they were spread way out, you would need a bigger net. Animals that breathe water need a big “oxygen net.”

If you had a really big net and a small space to store it, what would you do? You would probably fold it up. Gills are big oxygen-catching membranes that are folded up, but the folds can’t be squashed onto each other. They have to be touching the water so that they can catch the oxygen out of the water. Fish gills have tiny folds, which are folded into small folds, which are folded into larger folds. You can see gill pictures at

Gills don’t work in air because all of these folds collapse into each other when they are not floating in water. Amphibians like frogs usually lay their eggs in water. Young frogs are called tadpoles or pollywogs. They live in water and have gills. As they grow up, they grow legs and lungs and they lose their gills. When other amphibians evolved into land animals hundreds of millions of years ago, they developed lungs. Lungs have little air pockets that don’t collapse in air. These pockets are called alveoli and you can see pictures here:

Whales cannot breathe underwater because they do not have the appropriate respiratory system to do so. To breathe underwater, they would require gills, which exchange the carbon dioxide and oxygen dissolved in water.

Great question! Whales can't breathe underwater for the same reason that you and I can't breathe underwater. Whales and people are mammals. The earliest mammals lived on land, and therefore used lungs for breathing (as do reptiles and amphibians). Over time, however, some mammals, including whales, transitioned to living in water. Other mammals that underwent similar shifts to living in water include seals and manatees. Since the ancestors of these aquatic mammals had lungs, that's the breathing system they're "stuck" with--that is, no animal that we know about has been able to "re-evolve" gills--which is how sharks, tunas, goldfish, etc. breathe. Animals with backbones lost their gills and developed lungs when they moved onto land. Those back-boned animals that returned, millions of years later, to living in water, maintain the breathing system (lungs) of their land-living cousins.

Whales cannot breathe underwater because they are mammals and not fish. This means that their breathing organs are lungs, rather than gills. So, like humans, whales need to come to the surface to take a deep breath of fresh air and when they are underwater they hold their breath.

A previous ScienceLine answer explained why whales are able to hold their breath for especially long times compared to humans:

Fish also need oxygen to breathe. Fortunately, there is always a small amount of oxygen mixed into water. Fish gills separate the oxygen from the water very efficiently and pass the oxygen into the blood of the fish. This is how a fish inhales.

That´s a terrific question! Whales are mammals just like you are (they have hair, feed their offspring milk, and maintain their body temperature). Whales cannot breathe in water because they need to get oxygen from the air, so they have to come up to the surface to breathe. You may know that fish also need oxygen, but they get their oxygen out of the water directly, in the form of dissolved oxygen. Why can´t whales also get their oxygen directly from the water? It turns out that whales are descended from other mammals that evolved on land, and these land mammal ancestors were breathing oxygen from the air using their lungs (like us). Whales and other marine mammals have kept this trait, even though they now live in the sea.

In case curious, fish breathe dissolved oxygen in this way: water passes over the fish gill system through extremely fine membranes. These gill membranes are made up of very small blood vessels that can take up the dissolved oxygen as the water moves past them. That´s why fish (unlike whales) cannot breathe outside of water - they need the water to move over their gills to absorb the oxygen!