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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.
It was quiet in the operating-room. A young anesthetist was bending over a girl patient. Everything was ready for the operation.
The operation to be performed was quite a simple one. Nevertheless, it is still frightening to be on the operating table, so it is not surprising that the patient became particularly frightened when the first drop of ether reached her lunge and she attempted to take off the mask. The nurse had to hold the mask on by force and the young anaesthetist involuntarily gave the anaesthetic at a greater rate, which soon brought about the desired effect.
In a minute or two the patient’s muscles relaxed and she became quiet. But why was she so unnaturally rigid? The patient was not breathing. The next moment the anaesthetist hastily removed the mask and began to give artificial respiration. He asked the nurse in a tremulous voice for lobeline.
In the past, arrest of respiration, a hazardous complication, often occurred at the beginning of anaesthesia. It may develop if the amount of anaesthetic being administered is rapidly increased. Nowadays the technique used in anaesthesia almost completely precludes complications of this type and provides surgeons with reliable methods of combatting its consequences. Nevertheless, it is extremely unpleasant for a student anaesthetist just beginning his career to encounter such a complication, and especially if it is the result of his own carelessness.
This is why the anaesthetist was very energetic in administering artificial respiration. Two or three agonizing minutes had elapsed before the patient made her first inspiration, then the second, the third….
Now the surgeon stepped in and reassured his colleague, telling him that he had merely given the patient too much air. The long wait began once more. Finally the patient took another breath followed by another and yet another. Gradually her breathing became more frequent and regular.
‘Now go on with the anaesthetic before the patient wakes up completely, but do not rush,’ said the surgeon. Before long the people in the operating theatre resumed working at their usual pace. In another half hour the patient was back in the ward.
Why did the patient stop breathing twice during the operation? The reason for it the first time is clear: the excessive dose of a narcotic substance acted as a depressant on the respiratory centre of the medulla oblongata, and breathing ceased. The reason why breathing stopped the second time was more complicated. To comprehend this, we must first see how breathing is regulated. Three different receptor apparatus participate in controlling respiration. The first are the lung receptors which inform the respiratory centre in the brain of the extent to which they expand or contract. They send the signals to the brain informing it when to stop inhalation or expiration and vice versa.