Air is moved into and out of the lungs, as it is carried to and from the exchange surfaces of the alveoli.
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.
Which are the main muscles involved in normal breathing?
The diaphragm and (external) intercostal muscles
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.
Exchange of gases in fish is very efficient because of:
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.