Breathing
In this topic you will:
- measure how much air you can push out of your lungs in one breath
- learn how the muscles between your ribs and your diaphragm move air into and out of the lungs
Key words
- breathing
- contract
- relax
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:
- 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:
- The intercostal muscles between the ribs relax (return to normal size). This allows 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.
Quick fact: pressure and volume
When the volume of the chest cavity increases, the pressure inside it decreases. This causes air to move into the lungs. When the volume decreases, the pressure increases, and air is pushed out of the lungs. This relationship between pressure and volume is a key principle in understanding how breathing works.
Common mistake
Some learners think that air is "sucked" into the lungs when we breathe in. In reality, air moves into the lungs because the pressure inside the chest cavity becomes lower than the air pressure outside the body. It’s a passive flow due to pressure difference — not an active suction.
Questions
Use these words: contract, relax
| Action | What do the diaphragm muscles do? | What do the intercostal muscles do? |
|---|---|---|
| Breathing in | ||
| Breathing out |
Show Answer
Breathing in: Diaphragm muscles contract; Intercostal muscles contract.
Breathing out: Diaphragm muscles relax; Intercostal muscles relax.
| Action | What do the diaphragm muscles do? | What do the intercostal muscles do? |
|---|---|---|
| Breathing in | contract | contract |
| Breathing out | relax | relax |
Use these words: decrease, increase, into, out of
Show Answer
When we breathe in, the muscles in the diaphragm and between the ribs increase the volume of the chest.
This makes air move into the lungs.
When we breathe out, the muscles in the diaphragm and between the ribs decrease the volume of the chest.
This makes air move out of the lungs.