Page 21 - 新思维科学学生用书8 样章

1 Respiration

Breathing

Look at the diagrams of the human respiratory system, in Topic 1.1. Find the ribs, and the intercostal
muscles between them. Find the diaphragm.

Remember that air is a gas. The pressure of a gas increases when the volume of its container is

decreased. You can find more about pressure in Topic 3.6.
When you breathe in, these things happen: air moves into lungs
• The intercostal muscles between the ribs
contract (get shorter). This pulls the ribs

upwards and outwards.
• The muscles in the diaphragm contract.
This pulls the diaphragm downwards.

• These two movements make more space inside
the chest cavity. They increase the volume
inside it.
• When the volume increases, the pressure
inside the chest cavity and lungs decreases.

• Air moves down through the trachea into the
lungs, to fill the extra space.

When you breathe out, these things happen: air moves out of lungs

• The intercostal muscles between the ribs relax
(return to normal size). This allow the ribs to
drop down into their natural position.
• The muscles in the diaphragm relax.

This allows the diaphragm to become
its normal, domed shape.
• These two movements make less space inside

the chest cavity. They decrease the volume
inside it.
• When the volume decreases, the pressure
increases. So air is squeezed out of the lungs.

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