Why animals need to breathe
Cells use oxygen to get energy from food. In this process they make carbon dioxide, a waste gas. So the animal must take in oxygen and send out carbon dioxide. This exchange of gases is called breathing or gas exchange.
For gases to pass in and out, the body surface must be thin and moist, and it needs a good supply of blood or air near it.
Breathing through the skin
Small, soft animals like earthworms breathe through the skin. The skin is thin and always wet with a little slime. Oxygen dissolves in the wet layer and moves into the blood just under the skin. Carbon dioxide goes out the same way. Frogs also use the skin, besides lungs.
This works only for small animals. A big animal has too little skin for its large body.
Gills in water animals
Water holds much less oxygen than air. Gills are thin, feather-like, blood-filled plates. A fish takes water in through its mouth, pushes it over the gills and lets it out through the gill slits. As the water passes, oxygen goes from the water into the blood and carbon dioxide goes into the water.
Many fishes, tadpoles, crabs and prawns breathe with gills.
Tracheae in insects
An insect, like a cockroach, ant or grasshopper, does not use blood to carry oxygen. It has tiny holes called spiracles along its sides. Each leads into a branching network of air tubes called tracheae. The tubes get thinner and end right at the cells. Air goes straight to where it is needed.
Lungs in land animals
Lungs are spongy bags inside the chest. They have a large wet surface where air and blood meet. When we breathe in, the chest muscles and the diaphragm move so the lungs expand and fill with air. Oxygen passes into the blood, and carbon dioxide passes out. When we breathe out, the lungs shrink and push the air out.
Reptiles, birds and mammals all have lungs. Whales and dolphins live in water but use lungs, so they rise to the surface to breathe.
Try it
In the 3D: go to the last step. Press each animal and match it with the organ. Watch the lungs grow and shrink.
At home: place your hands on your ribs and breathe in and out slowly. Do the ribs go out or in when you breathe in? Count your breaths for one minute, then run on the spot and count again. Why do you breathe faster?
Key formulas and definitions
- Breathing = taking in oxygen and giving out carbon dioxide
- Earthworm: moist skin. Fish: gills. Insect: spiracles + tracheae. Dog, human, bird: lungs
- A gas-exchange surface is thin, moist and close to blood or air
- Water has less oxygen than air, so fish gills must be large with much blood
Worked examples
1. Why does an earthworm come out of its burrow after heavy rain?
Rain water fills the burrow and the water has little oxygen. The worm breathes through its skin, so it needs air and comes up to the surface.
2. A cockroach has no lungs and no gills. How does it breathe?
It breathes through spiracles, little holes on its sides. They lead into tracheae, air tubes that carry air to every cell.
3. A fish taken out of water soon dies although there is plenty of air. Why?
The gill plates stick together without water. The surface for gas exchange shrinks and dries, so it cannot take in oxygen.
Common mistakes
- Thinking all animals have lungs. Many use skin, gills or tracheae.
- Saying that insects breathe through their mouth. They use spiracles on the sides of the body.
- Thinking that fish breathe water. They take oxygen out of the water; they do not breathe the water itself.
- Saying that a whale has gills. A whale has lungs and comes up for air.