Breathing in humans
Breathing is a physical process driven by the diaphragm (a dome-shaped muscle below the lungs) and the intercostal muscles between the ribs. Changing the volume of the thoracic cavity changes the air pressure inside the lungs. Because air always moves from high to low pressure, this drives air in and out.
Breathing, also called ventilation, only moves air in and out of the lungs; it is not the same as gaseous exchange, which happens afterwards across the alveoli. Steady breathing keeps a supply of fresh, oxygen-rich air at the respiratory surface and removes stale air.
Inhalation (breathing in)
- The diaphragm contracts and flattens, moving downwards.
- The external intercostal muscles contract, pulling the rib cage upwards and outwards.
- The volume of the thoracic cavity increases.
- The air pressure in the lungs drops below atmospheric pressure, so air flows in.
Exhalation (breathing out)
- The diaphragm relaxes and returns to its dome shape, moving upwards.
- The external intercostal muscles relax, and the rib cage moves downwards and inwards.
- The volume of the thoracic cavity decreases.
- The air pressure in the lungs rises above atmospheric pressure, so air flows out.
Key idea
Volume up → pressure down → air rushes in. Volume down → pressure up → air pushed out. This is Boyle's law applied to the chest.
Breathing in fish and insects
A fish uses a pumping action: it opens its mouth to draw water into the buccal cavity, then closes the mouth and raises the floor so water is forced out over the gills. An insect such as a grasshopper contracts and relaxes its abdominal muscles to pump air in and out through the spiracles.
Example
Place your hands on your ribs and breathe in deeply: you feel the rib cage lift up and out as the thoracic volume grows.