Page:EB1911 - Volume 23.djvu/208

Rh the interior against enormous pressure. Thus, in the case of a
 * fish caught at a depth of 4500 ft., the partial pressure of the

oxygen present in the swimming bladder at this depth was 127 atmospheres, whereas the partial pressure of oxygen in sea-water is only about O~2 atmosphere. Diffusion can therefore have nothing to do with the passage of gas inwards, which is known to be under the control of the nervous system. The cells lining the interior of the swimming bladder are developed from the same part of the alimentary tract as those lining the air-cells of the lungs, so that it seems not unlikely that the lungs should possess the power of actively secreting or excreting gases. The question whether such a power exists, and is normally exercised, has been investigated by more than one method; and although it is not possible to go into the details of the experiments, there can be no doubt that the balance of the evidence at present available is in favour of the View that diffusion alone is incapable of explaining either the absorption of oxygen or the excretion of carbon dioxide through the lining cells of the lungs. The partial pressure of oxygen appears to be always higher, and of carbon dioxide often lower, in the blood leaving the lungs than in the air of the air-cells; and this result is inconsistent with the diffusion theory. As to the causes of the passage of oxygen and carbonic acid through the walls of the capillaries of the general circulation, we are at present in the dark. Possibly diffusion may explain this process.

II. Although we cannot trace the exact changes which occur when oxygen passes into living cells, yet it is possible to obtain a clear general view of the origin and destiny of the material concerned in the process, and of the physiological conditions which determine it.

The oxidizable material within the body consists, practically speaking, of proteids (albumen-like substances, with which the collagen of connective tissue may be included), fats and carbohydrates (sugars and glycogen). All of these substances contain carbon, hydrogen and oxygen in known, though different, proportions, and the former also contains a known amount of nitrogen and a little sulphur. Nitrogen is constantly leaving the body as urea and other substances in the urine and faeces; and a small but easily measurable proportion of carbon passes off in the same manner. The rest of the carbon passes out as carbon dioxide in respiration. Now carbohydrates and fats are oxidized completely in the body to carbon dioxide and water. This follows from the fact that, practically speaking, no other products into which they might have been converted leave the body except carbon dioxide and water. Moreover, a given weight of carbohydrate requires for its oxidation a definite weight of oxygen, and produces a definite weight of carbon dioxide. There is thus a definite relation between the weight of oxygen used up and the weight of carbon dioxide formed in this oxidation. The same is true for the oxidation of fat and of proteid, allowing in the latter case for the fact that the nitrogen, together with part of the carbon and hydrogen, passes out as urea, &c., in an incompletely oxidized form. From all this it follows that if we measure over a given period (1) the discharge of nitrogen from the body, (2) the intake of oxygen and (3) the output of carbonic acid, We can easily calculate exactly what the ultimate destiny of the oxygen has been, and at the ultimate expense of what material the carbonic acid has been formed. What the intermediate stages may have been we cannot say, but this in no way affects the validity of the calculation. If, during the period of measurement, food is taken, the basis of the calculation is still substantially the same, as the oxidizable material in food consists of practically nothing else except proteids, carbohydrates and fats.

Liberation of Energy.-From experiments made outside the body, we know that in the oxida-tion of a given weight of proteid, carbohydrate or fat, a definite amount of energy is liberated. In the article on Dmrnrrcs it is shown that precisely the same liberation of energy occurs in the living body, due allowance being made for the fact that the oxidation of proteid is not quite complete. The following table shows the respiratory quotients (the respiratory quotient being the ratio between the volume of carbon dioxide formed and that of oxygen used up) and energy expressed in units of heat (calories) liberated per gramme of carbon dioxide produced and oxygen consumed in the living body during the oxidation of proteid, fat and a typical carbohydrate:-

7 Calories per Calories per

Substance oxidized. Rgiggziffy §§ '§ ;';fof'f gr;'X“;, ';§ n°f duced. consumed.

1;';“*“' .; li ;:;§ I 3:23

Cane-sugar. I ~oo 2- 59 3~ 56

In the oxidation of non-living substances the rate varies, within wide limits, according to that at which oxygen is supplied. Thus a fire burns the faster the more air is supplied, and the higher the percentage of oxygen in the air. It was for long believed that in the living body also the rate of oxidation must vary according to the oxygen supply. It has been found, however, that this is not the case. Provided that a certain minimum of oxygen is present in the air breathed, or in the blood supplied to the tissues, it is, practically speaking, indifferent whether the oxygen supply be increased or diminished: only a certain amount is consumed. It might be supposed that the reason for this is that the available oxidizable material in the body is limited, and that if the food supply were increased there would be a corresponding increase in the rate of oxidation. This hypothesis is apparently supported by the fact that, when an increased supply of proteid is given as food, the amount of nitrogen discharged in the urine is almost exactly correspondingly increased, so that evidently the oxidation of proteid increases correspondingly with the supply. Similarly, when carbohydrate food is given, the alteration in the respiratory quotient shows that more carbohydrate than before is being oxidized. Closer investigation in recent times has, however, brought out the very striking fact that, if oxidation be measured in terms of energy liberated by it in the body, it makes but little difference, other things being equal, whether the animal is fasting or not. If more proteid or carbohydrate is oxidized at one time, correspondingly less fat is oxidized, but the total energy liberated as heat, &c., in the body is about the same, unless the diet is very excessive, when there is a slight increase of oxidation. Even after many days of st.arvation, the rate of oxidation per unit of body weight has been found to remain sensibly the same in man. When more food is taken than is required, the excess is stored upfchiefiy in the form of fat, into which carbohydrate and possibly also proteid are readily converted in the body. When less food is taken than is needed, the stock of fat is drawn upon, and supplies by far the greater proportion of the energy requirements of the body.

During the performance of muscular work oxidation is greatly increased, and may amount to ten times the normal or more. Even the slight exertion of easy walking increases oxidation to three times. When the energy represented by the external work done in muscular exertion is compared with the extra energy liberated by oxidation in the body, it is found, as would be expected, that the latter value largely exceeds the former. In other words, much of the energy liberated is wasted as heat. Nevertheless the muscles are capable of working with less waste than any steam or gas engine. In the work of climbing, for instance, it has been found in the case of man that 3 5 % of the energy liberated is represented in the work done in raising the body. Muscular work, if at all excessive, leads to fatigue, and consequent rest. On the other hand, unnatural abstinence from muscular activity leads to restlessness and consequent muscular work. Hence on an average of the twenty-four hours the expenditure of energy by different individuals, with different modes of life, does not as a rule differ greatly. The rate of oxidation per unit of body weight varies considerably according to size and age. If we compare different warmblooded animals, we find that the rate of oxidation is relatively