Page 13 - 新思维科学学生用书8 样章

1 Respiration

Gas exchange in the air sacs

Inside the air sacs, oxygen from the air goes into the blood. Carbon dioxide from the blood goes into

the air. This is called gas exchange.

Think about the blood capillary on the left of the diagram. The blood inside this capillary comes
from the heart. Before reaching the heart, it came from the organs in the body. These organs contain
cells that respire, using up oxygen and making carbon dioxide. So the blood in this capillary contains
only a small amount of oxygen, and a lot of carbon dioxide.

Now think about the air inside the air sac. It came from outside the body, where the air contains a

lot of oxygen and only a small amount of carbon dioxide.
Inside the alveolus, this air is very close to the blood. There are only two very thin cells between the

air and the blood.

The oxygen particles in the air are a gas, so they are moving freely. They can easily move from the
air, through the thin-walled cells and into the blood. This is called diffusion. You can find out more
about diffusion in Topic 3.7. The oxygen particles move from where there are a lot of them (in the
air) to where there are fewer of them (in the blood).

When the oxygen gets into the blood, it dissolves. (You can find out about dissolving in Topic 2.1.)

The oxygen goes into the red blood cells where it combines with haemoglobin. You will find out
what happens to it after that in Topic 1.5.

Now think about the carbon dioxide. There is a lot of it in the blood in the capillary, and only a small
quantity in the air inside the air sac. So the carbon dioxide diffuses into the air in the air sac.

Activity 1.2.1

Gases in and out
Copy this diagram.
1 On your diagram, draw a green arrow to show how

oxygen diffuses from the air into the blood.
2 How many cells does the oxygen move through, as it
leaves the air and goes into the blood?
3 On your diagram, draw a blue arrow to show how carbon

dioxide diffuses from the blood into the air.

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