Questions on breathing

Which two sets of muscles bring about the increase in volume of the thorax at the start of inhalation, and how do they do so?

• The intercostal muscles and the diaphragm increase the volume of the thorax.
• The intercostal muscles pull the ribcage up and outwards.
• The diaphragm flattens out.

Which set of muscles is most important in your own breathing at rest, and what happens to the relative importance after exercise?

• The diaphragm muscles are used mainly when you at rest. After exercise the intercostal muscles are used equally.

It is essential that the thorax is airtight. Explain why this is so and why the lungs are in two, separate, airtight compartments.

• The thorax should be airtight so that the change in pressure forces us to breathe air in and out.

What actually causes air to flow into the lungs during inhalation?

• Higher pressure outside the body than in the thorax.

What is the principal driving force for exhalation at rest?

• The diaphragm relaxing and bulging up.

Exhalation can be an active or forced process (although it is not normally so). Try blowing out as much air as possible from your own lungs to find out which sets of muscles are involved, and then explain what happens.

• In exhalation, the intercostal muscles relax and move downwards and move in and the diaphragm relaxes and bulges up.

We have conscious control over the muscles in our arms and in our legs but the muscles of the intestine are controlled unconsciously. What, therefore, is rather unusual about the muscles associated with breathing?

• They can be controlled consciously and unconsciously.
• For the efficient exchange of gases (uptake of oxygen and release of carbon dioxide), the air around the actual exchange surface must not be allowed to become stale.
• I.e. the air must be ventilated. This process involves drawing in fresh air, inhalation, and removing some of the used air, exhalation.
• Air will be pushed into the lungs, which are in an airtight chamber called the thorax, if the pressure inside falls below the pressure outside.
• This is achieved by increasing the volume of the chest by pulling down the normally domed diaphragm and raising and moving out the ribcage.
• Both of these are achieved by the contraction of muscles.
• The air entering the lungs stretches them, and when the intercostal and diaphragm muscles relax the elastic tissues in the lungs recoil, forcing the air out.