The respiratory process
Cells get the energy they need to maintain metabolism through cellular respiration.
In this process, organic food molecules react with oxygen gas (O2), producing molecules and water and carbon dioxide (CO2), Besides energy.
With the exception of some oxygen-independent intestinal worms to survive, most animals need to get oxygen gas from their environment and drive it to cells for use in aerobic metabolism.
Oxygen uptake and carbon dioxide removal, that is, gas exchange by animals characterize what is known as respiration.
The place of the animal's body where the gas exchange with the environment occurs is called the respiratory surface. Oxygen gas from the medium diffuses through the membranes of the cells lining the respiratory surface and is then distributed to every cell in the body; Carbon dioxide goes the other way.
In small organisms and / or with lower metabolic activity living in the aquatic environment, gas exchange is not a problem. They simply occur on the surface of the body, for simple diffusion. This is what happens with the only cell of protozoa and with invertebrates as sponges, cnidaria, flatworms and nematelmints.
In animals of more complex organization, often larger in size and more active, the distance between the innermost cells and the environment increases, which is a limiting factor for the diffusion of gases throughout the body. In this case several adaptations, represented by the respiratory organs, such as skin, trachea, gills and lungs, facilitate the occurrence of gas exchange. In them a basic characteristic is maintained: gas exchange continues by simple diffusion through thin, moist and permeable surfaces. Gases must be in solution in water to get in or out of cells, so the gas exchange surface must always be moistened.
If respiratory gas exchange occurs throughout the body surface, skin respiration is referred to. Animals that have this type of breathing are usually small and have a cylindrical or flat body. Your skin is richly vascularized and contains numerous blood capillaries scattered throughout it, which considerably expands the capacity for gas exchange.
Skin breathing may be present in both aquatic animals (porifers, coelenterates and aquatic flatworms) as in land animals (terrestrial worms, earthworms and amphibians). The moist environment is critical for skin respiration to occur as the body surface must be moistened to allow gas diffusion. In amphibians skin respiration complements pulmonary respiration.