All animals require oxygen, but they don’t all get it the same way.
Animals get oxygen in a number of ways. Aquatic life, like fish, typically breathe through gills. Gills are flaps located on both sides of the fish or in its mouth. As water flows into its gills, a fish absorbs oxygen through its red blood cells.
Another breathing mechanism some animals rely on is diffusion. Sea butterflies, a part of the snail family, are thin and transparent creatures that live in oceans. Because their bodies are so thin, they can take up – or diffuse – oxygen from water. Other small water-dwelling animals also depend on diffusion to survive.
Lastly, mammals breathe using lungs. Lungs are a pair of spongy, hollow organs on either side of the heart. The lungs have thousands of capillaries, which are a type of blood vessel that help the body absorb oxygen. The muscle of the lung expands to let air in and relaxes to let out air.
It’s amazing how many ways animals can breathe and how these mechanisms are so different depending on the size, shape, and habitat of an animal. How wild it is that something seemingly so simple is actually so complex.
[Source: How Animals Work]