When you inhale
- the intercostal muscles contract, expanding the ribcage
- the diaphragm contracts, pulling downwards to increase volume of the chest
- pressure inside the chest is lowered and air is sucked into the lungs
When you exhale
- the intercostal muscles relax, ribcage drops inwards and outwards
- the diaphragm relaxes, moving back upwards to decrease volume of the chest
- pressure inside the chest increases and air is forced out
Lots of things happen when you breathe, both inside and outside the body. Internally, you need oxygen to oxidize food to provide energy to run the body, and you need to get rid of excess carbon dioxide which would otherwise build up in the blood in the form of carbonic acid. The animal kingdom, of which human beings are a part, has a kind of symbiosis with the plant kingdom; plants perform photosynthesis, which produces oxygen and consumes carbon dioxide, while our metabolism produces carbon dioxide and uses oxygen.
