Breathing under water

The idea

Because dissolved oxygen in water is fairly low compared with oxygen in air, aquatic organisms breathe much faster than terrestrial ones; fishes force water from the mouth past the gills, where blood takes up the dissolved oxygen.

Air and water

Air has plenty of oxygen. Some oxygen is also dissolved in water.

Which do you think holds more oxygen, air or water?

• Air
• Water
• They hold exactly the same

Air, water and a fish

Press Next step.

Why fishes breathe fast

Water holds fairly little dissolved oxygen compared with air, so an animal living in water must take in water at a faster rate to get enough oxygen. A fish takes water in through its mouth and forces it past its gills. There the blood takes up the dissolved oxygen.

Because dissolved oxygen in water is fairly low compared with oxygen in air, aquatic organisms breathe much faster than terrestrial ones; fishes force water from the mouth past the gills, where blood takes up the dissolved oxygen.

Notes

Dissolved oxygen in water is fairly low compared with air, so aquatic organisms breathe much faster; fishes force water past the gills, where blood takes up the dissolved oxygen.

Check yourself

Why do aquatic organisms breathe much faster than terrestrial ones?

What do fishes do with the water they take in through their mouths?

Compared with terrestrial organisms, the rate of breathing in aquatic organisms is ...

Where is the dissolved oxygen taken up by the blood in a fish?

• Water holds more oxygen than air. The amount of dissolved oxygen in water is fairly low compared with the amount in air.
• The oxygen dissolved in water is fairly low compared with oxygen in air — correct. Yes!
• Aquatic organisms do not need oxygen. Aquatic animals need the oxygen dissolved in water.
• Store it in the stomach to digest it. The water is forced past the gills, where the blood takes up the dissolved oxygen.
• Push it out of the mouth again at once without using it. The oxygen dissolved in the water is taken up by the blood at the gills.
• Force it past the gills, where blood takes up the dissolved oxygen — correct. Yes!
• much faster — correct. Yes!
• much slower. Dissolved oxygen is low in water, so the rate of breathing in aquatic organisms is much faster.
• exactly the same. Dissolved oxygen is low in water, so the rate of breathing in aquatic organisms is much faster.
• In the stomach. The stomach is where gastric glands release acid, pepsin and mucus.
• At the gills — correct. Yes!
• In the large intestine. The large intestine absorbs water from unabsorbed food.