Читать книгу Possible Worlds and Other Essays - J. B. S. Haldane - Страница 12

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THE source of almost all the energy developed in the human body is the combination of food with oxygen. We can replace one kind of food by another, but oxygen cannot be replaced. The combination occurs in all the tissues, and both food and oxygen must be supplied to them by the blood. All organs are sooner or later damaged by want of oxygen, but the brain is by far the most sensitive. The first symptoms of oxygen lack are always mental, and five minutes of complete deprivation will kill the brain, whereas the heart will survive for as many hours. You, reader, will die of oxygen want. Your lungs, your heart, or that part of your brain which controls your respiratory muscles will cease to play its part in oxygen supply, and the energy transformations which make up your conduct will cease.

Oxygen may be cut off suddenly from the tissues by such means as drowning, strangling, or beheading, but the physiologist, psychologist, and doctor will find more to interest them in the effects of partial but prolonged shortage. This generally arises in one of three ways: shortage of oxygen in the air, interference with its passage through the lung membrane into the blood, or failure of the blood to carry it to the tissues. The air breathed may contain too little oxygen if it is diluted with some other gas, or if the oxygen is partially removed from it. On ships, to take a single example, scores of men die every year by entering compartments from which the oxygen has been removed by paint, coal, or grain. On the other hand, the composition of the air may be unaltered, but its pressure reduced; so that a given volume of air contains a less weight of oxygen than at sea-level. In this case the blood can hold less oxygen, just as soda water can hold less carbon dioxide when we lower the pressure on it. The oxygen in blood is mostly combined with haemoglobin, which gives it its red colour, and the compound formed does not break up appreciably till the pressure of oxygen in the air has been considerably lowered; so that a small drop in oxygen pressure causes no noticeable effect. The effects of a larger drop have been studied not only in balloons and aeroplanes and on mountains, but by the artificial production of low pressure.

In the factory of an enterprising firm of diving-dress makers in South London is a steel cylinder about seven feet high and five in diameter. It communicates with the outside by a manhole, a small window of very thick glass, and two pipes. With a companion I crawl in through the manhole, which is closed behind us by a formidable series of screws. An engine begins to suck the air out through one of the pipes. The air becomes cold and fills with mist. In five minutes we have reached a pressure of 350 millimetres of mercury, or less than half an atmosphere, corresponding to a height of 22,000 feet above sea-level. I look at the barometer, and open the inlet valve so as to keep the pressure steady. And now I have time to observe my own symptoms. I am breathing rapidly and deeply, and my pulse is at 110; but the breathing soon calms down, and I feel much better, though perhaps my writing is a shade wobbly. But why cannot my companion behave himself? He is making silly jokes and trying to sing. His lips are rather purple, the colour of haemoglobin when uncombined with oxygen. I feel quite unaffected; in fact, I have just thought of a very funny story. It is true I can’t stand without some support. My companion suggests some oxygen from the cylinder which we have with us. To humour him I take a few breaths. The result is startling. The electric light becomes so much brighter that I fear the fuse may melt. The noise of the pumping engine increases fourfold. My notebook, which should have contained records of my pulse-rate, turns out to be filled with the often repeated but seldom legible statement that I am feeling much better, and remarks about my colleague, of which the least libellous is that he is drunk. I put down the oxygen tube and relapse into a not unpleasant state of mental confusion. An hour later, in spite of our indignant protests, the engine is stopped, and we return to normal pressure, no worse off except for a slight and transitory headache.

For longer experiments a mountain is desirable; and to avoid the disturbing influences of fatigue on the one hand, and athletic training on the other, it should be ascended by rail. The only railways ascending over 14,000 feet are in the Rockies and the Andes, and it is here that the most complete investigations of prolonged oxygen want have been made. After a few hours nine people out of ten who have ascended rapidly from near sea-level suffer from sickness and headache and may faint. These symptoms are at once cured by a few minutes of oxygen inhalation, and have nothing to do with the low pressure as such. Later a quarrelsome stage generally supervenes. One of the dozen or so permanent residents on the top of Pike’s Peak is a sheriff, who is needed to deal with visitors. Later the body begins to adapt itself, and the symptoms pass off more or less completely. The bone marrow manufactures new red blood corpuscles until the blood can hold 20 or 30 per cent. more oxygen than normal when saturated, and slightly more even at a high altitude. The kidney holds back acid which it would normally excrete, and thus goads the respiratory centres in the brain to increased activity. And something seems to happen in the lungs which also occurs in athletic training. The attempts to decide the nature of this change, if any, constitute the most interesting of our inter-’varsity sports. Oxford has pinned its faith to the view that the lung learns to force oxygen into the blood as the gut forces food. Cambridge holds that the gas soaks in as it would through a dead membrane. The contest has now been raging for more than sixteen years, but in spite of American and Danish participation, is still undecided.

As compared with many other mammals, man is very efficient at adaptation. Cats generally die at 14,000 feet, while cows die at 15,000, and give no milk above 13,000, even in the tropics. There is, however, a limit to human adaptability, and it is an open question whether the summit of Mount Everest lies above or below this limit.

The mountaineer has generally time to adapt himself during his approach to the final stages. The airman only spends a few hours a week at most above 10,000 feet, and therefore cannot adapt himself. Moreover, the mountaineer will be warned of his danger by shortness of breath, but the airman will be lured higher and higher by an increasing and unreasonable conviction that he is all right, until he suddenly loses consciousness. When Sully-Prudhomme sang the courage of Sivel and Croce-Spinelli, who died on a balloon ascent in 1875, he was celebrating the psychological effects of oxygen want. Even at 10,000 feet the airman’s judgment would probably be improved by oxygen. Above 16,000 feet it is an absolute necessity, and the chief participants in the war supplied the crews of their high-flying aeroplanes and airships with compressed or liquefied oxygen and more or less efficient breathing apparatus.

In various diseases the oxygen cannot pass quickly enough into the blood. Thus, in croup and bronchitis the air passages to the lungs are narrowed, and in lobar pneumonia the membrane through which oxygen passes into the blood is thickened by inflammation. In either case the blood leaves the lungs without its full complement of oxygen. Oxygen has a great future in medicine, and could probably halve the death-rate in pneumonia. But as generally administered it has little more therapeutic value than extreme unction, and is much more expensive. If it is merely blown in the direction of the patient’s mouth, he or she does not get enough to soak through the thickened membrane. If his head is enclosed in a box into which oxygen is blown, he rebreathes the carbon dioxide of his expired air and suffers severely. It must be given continuously, sometimes for three days and nights on end. To give it intermittently is like dragging a drowning man to the surface once a minute. It should not be breathed pure, as it is poison, though a rather slow one. Further, the treatment should be started before, and not after, the patient shows signs of approaching death. These conditions are best fulfilled if the gas is administered through a suitable mask, such as that designed for the treatment of war-gas pneumonia. When it is properly administered, the patient’s mental state, colour, and other symptoms improve within five minutes.

Finally, the blood may be unable to carry enough oxygen. In heart disease the tissues generally get enough as long as their demand is restricted by keeping the patient at rest. In anaemia and carbon monoxide poisoning a given volume of blood can hold less than its normal amount of oxygen. In the former case the heart increases its output when at rest, but has no reserve of power to fall back on during exercise. In the latter the poison not only displaces some of the oxygen from the haemoglobin, with which it itself forms a compound, but makes the removal of what little is carried unusually difficult, and the brain especially feels the shortage. Carbon monoxide is the poisonous constituent of coal-gas, the gas from charcoal and coke fires, and that produced by explosions in collieries. The abolition of the legal restrictions on the proportion of it in lighting gas is now making the symptoms of poisoning by it familiar.

It might be thought that oxygen would have a good effect on normal people. As a matter of fact, it is poisonous if breathed either at high pressure or for days at a time, while the only effects of non-poisonous doses are to slow down the heart’s rate, and, if given for long enough, to diminish the amount of haemoglobin in the blood. The result of these changes is that the tissues get as much oxygen as normally, and no more. Their supply is accurately regulated, and though the physician can sometimes help nature to re-establish the normal state of affairs, he can never improve upon it.