However, the skin’s share in man’s breathing as a whole is negligible compared with that of the lungs. This is understandable if we take into account the fact that the total surface of man’s skin is scarcely two square metres, while the surface of the lungs with all their seven hundred million alveoli spread flat is at least 90 to 100 square metres, that is 45 to 50 times as much. (Alveoli are minute thin-walled sacs through whose surfaces the respiratory exchange between the environment and the blood occurs.)
Breathing by means of the skin can only provide very small animals with an adequate amount of oxygen. Therefore, right from the very beginning Nature employed a method of trial and error to find an adequate means for this purpose. The organs of digestion were the first to be selected for the test.
The Coelenterata consist only of two layers of cells. The external layer takes oxygen from the environment, while the internal layer draws it from the water which freely enters the intestinal cavity. Even flat worms, which have developed more complex digestive organs, could not employ them for respiration. They had to remain flat as in large volume diffusion is unable to supply the deep-lying tissues with adequate amounts of oxygen.
The many species of Annelida which emerged on the Earth following the flat worms also manage to breathe through the skin, but this only proved possible as a result of the circulatory organs which they had evolved to distribute oxygen throughout the body. Incidentally, some species of Annelida provided themselves with the gills, the first special organ for taking in oxygen from the atmosphere.
In all the subsequent animals similar organs mainly followed two patterns. If oxygen was obtained from water, special outgrowths or protrusions, which were directly in contact with the water, were developed, while depressions or cavities — from a simple sac, such as the respiratory organ of the edible snail “or the lungs of the newt and salamander, to exceedingly complex blocks of minute vesicles resembling clusters of grapes, like the lungs of the mammals — have been evolved to obtain oxygen from the ambient atmosphere.
The conditions for respiration in water and on land differ greatly. One litre of water, even under the most favourable conditions, contains as little as ten cubic centimetres of oxygen, while one litre of the atmospheric air contains 210 cubic centimetres, i. e. twenty times as much. It might, therefore, seem strange that the respiratory organs of aquatic animals cannot obtain an adequate amount of oxygen from such an oxygen-rich environment as the atmospheric air. The structure of the gills would allow them to cope successfully with their task in the air, too, but the fine plates (laminae) of the gills stick to one another and soon dry up without the support and protection provided by water. The blood ceases to circulate and the breathing function is thus arrested.