In small aquatic animals such as the slipper or hydra, gas exchange is a very simple process: dissolved oxygen diffuses from the surrounding water into the cells, and carbon dioxide diffuses out, and there is no need for any special respiratory system. This type of breathing is called direct breathing, since the cells of the body directly exchange oxygen and carbon dioxide with the environment.
With the advent of more highly organized, complex forms, direct gas exchange between each animal cell and the environment became impossible.
Some type of indirect breathing was required, involving body structures specialized for this function. The specialized organ must have a thin wall (the membrane of this wall must be semi-permeable) so as not to impede diffusion; it must always have a wet surface so that oxygen and carbon dioxide can be dissolved in the water; finally, it must be well supplied with blood. Fish, crabs, crayfish and many other animals have developed gills for indirect respiration; higher vertebrates - reptiles, birds and mammals - developed lungs; The earthworm uses its moist skin to breathe; insects have tracheae - tubes that penetrate the entire body and communicate with the external environment through pores.
During indirect respiration, gas exchange between the cells of the body and the environment includes two phases - external and internal. External respiration consists of the exchange of gases by diffusion between the external environment and the blood using a specialized respiratory organ, such as the lung in mammals. Internal respiration involves the exchange of gases between the blood and body cells.
The transfer of gases between these two phases of respiration is carried out by the circulatory system.