Breathing in animals

The exchange of gases

Aerobic respiration needs a steady supply of oxygen. It also makes carbon dioxide, which must be removed. Taking in oxygen and giving out carbon dioxide is called the exchange of gases.

Gases pass in and out by diffusion. A gas moves from where there is more of it to where there is less. A good surface for the exchange of gases has

• a large area, so that much gas can pass at once;
• a thin wall, so that the gas has only a short way to go;
• a moist lining, because the gases dissolve before they pass through;
• in most animals, a rich blood supply, which carries the gases to and from the cells.

Very small animals, such as Amoeba, exchange gases through their whole body surface. The earthworm uses its moist skin. Larger animals need special organs.

Breathing in water

Fish use the oxygen that is dissolved in water. Water holds far less oxygen than air. One litre of air holds about $210$ mL of oxygen. One litre of pond water holds only about $5$ to $10$ mL.

So a fish must move a lot of water over its gills. This is why animals that live in water breathe much faster than animals on land.

• Water enters through the mouth.
• It flows over the gills, which have many thin filaments full of blood vessels.
• Oxygen passes into the blood, and carbon dioxide passes out into the water.
• The water leaves from under the gill cover.

Breathing in air

• Insects, such as the grasshopper, have small holes called spiracles along the body. They open into air tubes called tracheae, which carry air right up to the cells.
• Frogs breathe with their lungs, and also through their moist skin.
• Reptiles, birds and mammals breathe with lungs. A system of tubes carries the air to them.

Air holds plenty of oxygen, so land animals can breathe much more slowly than fish.

Plants exchange gases too

Plants respire all the time, so they also need oxygen and must give out carbon dioxide. They have no special breathing organs.

• In leaves, gases pass through tiny pores called stomata.
• In woody stems and roots, gases pass through lenticels, small openings in the bark.
• The gases then diffuse from cell to cell. Every part of a plant is close to the air, so diffusion is enough.

By day, photosynthesis uses carbon dioxide faster than respiration makes it. So a leaf in light takes in carbon dioxide and gives out oxygen. At night there is no photosynthesis. Then the leaf gives out carbon dioxide and takes in oxygen. Respiration goes on all the time; only the net exchange changes.

You should find that the leaf takes in carbon dioxide only when the light is bright enough. In dim light the two processes balance. In the dark, the leaf gives out carbon dioxide.

Try it in Python

The program compares a fish with a land animal. Each uses $24$ mL of oxygen in an hour.

You should see $5.0$ L of water against about $0.57$ L of air. The fish moves about $9$ times as much. With less oxygen in the water, the fish has to move even more.

Practise with new numbers

Common mistakes

• Thinking fish use the oxygen inside water molecules. They use the oxygen gas dissolved in the water, not the O in $\text{H}_2\text{O}$.
• Thinking plants only give out oxygen. At night they give out carbon dioxide, and they respire by day too.
• Thinking stomata are the only openings. Woody stems and roots use lenticels.
• Mixing up breathing and the exchange of gases. Breathing moves air or water over the surface. The exchange is the diffusion across it.

Remember these

• A surface for the exchange of gases is large, thin and moist, and usually has a good blood supply.
• Water holds far less oxygen than air, so fish breathe faster than land animals.
• Fish use gills, insects use tracheae, and land vertebrates use lungs.
• Plants exchange gases through stomata and lenticels, by diffusion.
• By day a leaf takes in carbon dioxide; at night it gives it out. Respiration never stops.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Give three features of a good surface for the exchange of gases.

2. Why does a fish open and close its mouth so often?

3. Through which openings does a grasshopper take in air?

4. Why do plants give out carbon dioxide at night?

5. Why do plants not need lungs?