8.2 Ventilation
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Ventilation is the process of moving air in and out of our lungs. Ventilation occurs because there is a difference in air pressure in the thorax and the atmosphere. The air pressure will want to be the same, both in the thorax and in the atmosphere. Air will move from a place where there is higher pressure to a place where there is lower pressure, thus equalising air pressure. If the air pressure in the thorax is lower than the air pressure outside of the body, air will enter the lungs (this is the process of inhalation). If the air pressure is greater in the thorax than in the atmosphere, air will leave the lungs (this process is known as exhalation). When we breath we alter the pressure of the air in the thorax, which determines if air enters or leaves the lungs.
Inhalation
There are two different movements that bring about ventilation. These are the movement of the ribs and the movement of the diaphragm. When we breathe in:
There are two different movements that bring about ventilation. These are the movement of the ribs and the movement of the diaphragm. When we breathe in:
- Our ribs move upwards and outwards. They move upwards and outwards because of the intercostal muscles. There are two sets of muscles in the intercostal muscles; the external and internal intercostal muscles. In order for the ribcage to move upwards and outwards, the external intercostal muscles contract and the internal intercostal muscles relax.
- The diaphragm moves down and becomes flat because the muscles in the diaphragm contract.
Exhalation
The opposite happens when we breathe out.
Both of these movements, result in the volume of the thorax decreasing, which causes the air pressure of the thorax to increase to a level that is above atmospheric air pressure. This results in air leaving the lungs because air moves from a place of high pressure to a place of low pressure. Furthermore, the lungs are elastic and this helps exhalation.
The opposite happens when we breathe out.
- Our ribcage moves inwards and downwards. In order for our chest to move in this direction, the two sets of intercostal muscles do the opposite of what they did during inhalation. For exhalation, the internal intercostal muscles contract and the external intercostal muscles relax.
- The diaphragm goes back to a dome shape because the muscles in the diaphragm relax.
Both of these movements, result in the volume of the thorax decreasing, which causes the air pressure of the thorax to increase to a level that is above atmospheric air pressure. This results in air leaving the lungs because air moves from a place of high pressure to a place of low pressure. Furthermore, the lungs are elastic and this helps exhalation.