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      Respired Air
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Physiological Importance of Oxygen in Air

It has already been mentioned about Physiological Importance of Oxygen in Air by chief physiological that function of oxygen is to aerate the system and thereby ensure the removal of waste material in the form of carbon dioxide, which escapes into the air through the lungs. The oxidation processes involved cause considerable heat evolution, and it is through this means that the body temperature is maintained. Berthelot concluded that six-sevenths of the heat developed by respiration is liberated in various parts of the body other than the lungs, one-seventh only being liberated in the lungs. This pulmonary heat was found to be almost completely compensated by the absorption of heat due to liberation of carbon dioxide and water. It would appear, therefore, that upon the temperature of the inspired air would depend whether or not the lung temperature rises. In any case the variation would be small.

Respired air is saturated with moisture, after removal of which it contains normally some 4 per cent, of carbon dioxide and 16 to 17 per cent, of oxygen. These amounts vary both with the individual and with circumstances. Thus Thomson found that the expired air of the average Manchester citizen contained 4 per cent, of carbon dioxide, whereas an average of 5 per cent, was observed in country districts, reaching to 5.4 per cent, on high ground near Buxton. Under normal conditions the rate of breathing is subconsciously regulated so that the proportion of carbon dioxide in the arterial blood leaving the lungs contains a definite equilibrium pressure of carbon dioxide. A very slight increase in the amount of carbon dioxide excites the nervous centre controlling the breathing and stimulates respiration. Hence, during physical exercise or in cold weather, when more carbon dioxide is being produced, the proportion of this gas in expired air remains substantially the same, but the volume of air passing through the lungs increases proportionately, the breathing being deeper and more rapid.

During sleep, when both mental and physical activity are at a minimum, the amount of carbon dioxide produced is less than normal, and the rate of breathing is proportionately reduced.

When at rest, the average man consumes some 18 litres of oxygen per hour, an amount which may increase to 60 litres when walking at about three miles per hour, whilst in cases of more violent exercise such as running or jumping even 100 litres may not be quite sufficient.

The Physiological Influence of Excess of Oxygen

This has been made the subject of a considerable number of researches, and the conclusions arrived at by different investigators are reasonably concordant. It would appear from experiments on the cat and on man that the inhalation of pure oxygen does not materially augment the quantity of that gas in the blood, nor affect its average carbon dioxide content. Again, in a series of experiments on men at rest, performed some twelve hours after the last meal, no noticeable difference could be detected, either in the gaseous metabolism or in the character, depth, or frequency of respiration when the men breathed air containing 40, 60, and 90 per cent, of oxygen. The only difference that could be detected lay in the pulse rate which fell as the percentage of oxygen rose. It is very important to remember, however, that these experiments were only conducted for relatively short periods of time, and it has yet to be discovered whether or not a permanent increase in the percentage of oxygen in inspired air would have an influence upon the system in the long run. Thus J. L. Smith, in 1899, drew attention to the fact that oxygen, at the tension of the normal atmosphere, stimulates the lung cells to active absorption; but his experiments on mice indicated that at higher tensions inflammation might be produced.

The Physiological Influence of Reduced Oxygen Tension

The effect upon respiration of a reduced oxygen tension is one of much greater importance from a practical point of view than the problem just considered, inasmuch as the main difficulty in practice lies not in reducing the amount of oxygen in buildings, but in raising it to the normal. It is thus of the greatest importance to determine whether or not 21 per cent, of oxygen is essential to vigorous human life, and if not, what is the minimum amount of oxygen that may be safely permitted. A moment's consideration will show that no perfectly exact answer, at any rate to the second of these problems, can be arrived at, for not only do the needs of different persons vary, but those of the same individual are likewise influenced by the state of health and extent of physical and mental activity at the time of experiment. Further, after prolonged exposure to certain abnormal conditions, unless these latter are too severe, the body adapts itself to meet the new requirements. Thus persons who habitually live in ill-ventilated buildings are much less affected on any particular occasion than those who enter such buildings after a life in the open. This adaptive tendency is extremely well illustrated by the researches of Douglas, Haldane, Henderson, and Schneider, who stayed at Pike's Peak, Colorado, for five weeks at an altitude of 14,000 feet, the barometer standing at 45.7 cm. A careful study of their persons showed that they gradually became accustomed to the altered conditions, except that hyperpnoea upon exertion lasted longer than usual.

The reduced tension of the oxygen was counteracted -
  1. By increased lung ventilation.
  2. A considerable increase in the red corpuscles and haemoglobin of the blood, the extent of which, however, varied with the individual. The volume of the blood likewise increased slightly, except during the first week.
  3. Finally, an increased secretory activity of the pulmonary epithelium was observed.
Inasmuch as all these adaptations take considerable time to develop, they would not occur in rapid balloon or aeroplane ascents. On coming down from Pike's Peak, the normal state of the body began to assert itself, and in the course of four weeks all traces of the change had disappeared.

There is abundant evidence to show that the percentage of oxygen in the air may be reduced very considerably without producing any unpleasant symptoms. Dr. Whalley, in his report on the ventilation of Scottish coal mines, alludes to one in which considerable quantities of black-damp were evolved. "A light," he writes, "would not burn 1½ feet from the floor . . . but the men had no fault to find with the atmosphere, and the foreman told me it was better than usual." Upon analysis, the air on the pavement was found to contain only 13.13 per cent, of oxygen, and that at the coal face 18.97 per cent. Valenzuela caused his consumptive patients to breathe an artificial atmosphere, containing only 17 per cent, of oxygen, and noted that this exerted a marked stimulating action upon respiration, increasing the chest expansion, and liberation of carbon dioxide, whilst the nutrition was not adversely affected. This apparently indicates that in a normal atmosphere we consume more oxygen than we need, just as we ordinarily partake of more food than is really necessary. In the case of a person at rest, the percentage of oxygen may be reduced to 11 without anything very unusual being experienced, and the respiratory exchange remains the same. Below 10.5 per cent, the body loses its compensatory power, and the amount of carbon dioxide increases. The breathing now becomes deeper and slightly laboured. By reducing the supply to 7 per cent., the face becomes leaden in hue, and the senses deadened, and a further slight reduction results in sudden loss of consciousness.

Closely connected but not absolutely identical with this problem of the minimum partial pressure of oxygen in the atmosphere is that of the effect of reducing the total pressure of the air. This, for example, is experienced in balloon ascents and in mountaineering. The average individual does not feel himself inconvenienced at an altitude of 9000 feet, in which circumstances the barometer stands at approximately 50 cm., and the pressure of oxygen is correspondingly reduced to about 14 per cent, of an ordinary atmosphere. Above this altitude the average European begins to observe something peculiar during periods of physical exertion, and at 14,000 feet the effect is very marked, the amount of oxygen being equivalent to that of about 12 per cent, in an ordinary atmosphere at sea-level.

In view of the foregoing, it would appear that 11 per cent. of oxygen is the lowest limit to which it is safe to go. Below this the air is dangerous, and at 7 per cent, may prove fatal.

When taking physical exercise, however, these limits are probably too low for the average person, and 14 per cent, of oxygen may then be taken as the lowest that can be breathed with safety.

The greatest height ever reached by an investigator in a balloon is probably that attained by Berson and Suring in July 1901, namely, 35,400 feet (10,789 metres), although in 1862 Glaisher and Coxwell ascended over Wolverhampton to about 29,000 feet, when they became unconscious and are believed to have risen to nearly 36,000 feet for a short time. The two greatest heights recorded for aeroplanes are those of Rohlfs, who, in September 1919, ascended to about 32,418 feet (9880.5 metres), and of Schroeder, in February 1920, who reached approximately 31,184 feet (9505 metres).
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