Air is moved into and out of the lungs, as it is carried to and from the exchange surfaces of the alveoli.

The diaphragm and rib cage move to create a lower air pressure in the lungs than that of the air outside the body. Air then rushes into the lungs.

The most important muscle when we inhale normally is the diaphragm. The external intercostal muscles are the second most important muscles. Breathing is a passive process resulting from pressure changes in the lungs.

This diagram shows the structure of the respiratory system

Breathing in

When we inhale and exhale more forcibly, other muscles are also used



Which are the main muscles involved in normal breathing?

The diaphragm and (external) intercostal muscles

Gills in fish

Water is capable of holding only low concentrations of oxygen, so fish need a different type of exchange system.

The exchange surfaces in fish are gills.

An image showing the gills from a fish and how they operate

Fish gills

Water is taken in through the fish's mouth, passes over the gills, and then out under the operculum

Exchange of gases in fish is very efficient because of:

  • the large surface area of the gills
  • the large surface area of the blood capillaries in each gill filament
  • the short distance required for diffusion – the outer layer of the gill filaments and the capillary walls are just one cell thick
  • the efficient ventilation of the gills with water - there is a counter current flow of water and blood

The moving blood and ventilated gill surfaces mean that gases exchanged are continually removed – oxygen enters the blood, and carbon dioxide removed to the water. High concentration gradients can be maintained.

Gas exchange is more difficult for fish than animals that live on land. The concentration of oxygen in water is dependent on the temperature of the water. The concentration of oxygen in water is only around 1% at 15°C. This compares with 21% in the air at any temperature. The efficiency of gills has to be - and is - very high - 70 to 80% of the available oxygen is extracted from the water.

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