What is ventilation?

Ventilation, or breathing, consists of inspiration (breathing in) and expiration (breathing out).

It allows air to enter and leave the lungs, providing the body with oxygen and removing carbon dioxide.

Muscles involved in ventilation

The ribcage is made up of bones called ribs that enclose the thorax - the cavity where the lungs are located.

In mammals, ventilation is controlled by muscles that change the volume of the thorax. When the muscles attached to the ribcage contract and relax, they move the ribs to change the volume in the thoracic cavity. This affects the pressure in the lungs and controls ventilation.

There are two sets of muscles that act on the ribcage:

1. The diaphragm - a sheet of muscle separating the thorax from the abdomen that moves the ribcage up and out when it contracts.
2. The intercostal muscles - these are found between the ribs and pull the ribcage up and out when they contract.

Inspiration

Inspiration is an active process requiring energy for muscle contraction.

During inspiration:

1. The intercostal muscles contract moving the ribcage up and out.
2. The diaphragm contracts and flattens.
3. The volume of the thoracic cavity increases.
4. The lung pressure decreases below atmospheric pressure.
5. Air flows into the lungs down the pressure gradient.

Expiration

Normal expiration at rest is a passive process so it does not require energy.

During expiration:

1. The intercostal muscles relax allowing the ribcage to move down and in.
2. The diaphragm relaxes and rises.
3. The volume of the thoracic cavity decreases.
4. The lung pressure increases above atmospheric pressure.
5. Air is forced out of the lungs down the pressure gradient.