Professor Angelo Mosso was led by observation of the beneficial
effects of carbon dioxide at low atmospheric pressure to attribute
mountain sickness to lack of carbon dioxide, a condition
which he designated by the word “ acapnia. ” When impure
air is vitiated, not only by deficiency of oxygen, but also by
carbon dioxide, the carbon dioxide causes panting, which not
only gives warning of any danger, but prevents the alveolar
oxygen percentage from falling in the way it would do if the
carbon dioxide were absent. In this Way the carbon dioxide
greatly lessens the danger. To give instances, air progressively
and very highly vitiated by respiration is much less likely to
cause danger if the carbon dioxide is not artificially absorbed,
and not nearly so dangerous as the great diminution of atmospheric
pressure (and consequently of oxygen pressure) which
occurs in a very high balloon ascent. Indeed the dangers of
a very high balloon ascent are notorious, and a number of
deaths or very narrow escapes are on record.
]ust as oxygen forms a dis sociable compound with the haemoglobin of the blood, so does carbon dioxide form dis sociable compounds. One of' these compounds appears to be with haemoglobin itself, and another is sodium bicarbonate, which is far more easily dissociated in the blood than in a simple watery solution, owing to the presence of proteid and possibly other substances which act as weak acids and thus help the dissociation process. The whole of the carbon dioxide can therefore be removed from the blood by a vacuum pump, just as the whole of the oxygen can. Venous blood contains roughly speaking about 40 volumes of carbon dioxide per 100 of blood, and arterial blood about 34 volumes. Of this carbon dioxide only about 3 volumes can be in free solution, the rest being loosely combined. The conveyance of carbon dioxide from the blood to the lungs is thus readily intelligible, as well as the fact that any increase or diminution of the pressure of carbon dioxide in the alveolar air will naturally lead to a damming back or increased liberation of carbon dioxide from the blood, and that by forced breathing carbon dioxide can be washed out of the blood to such an extent that a prolonged cessation of natural breathing (apnoea) follows, since even in the venous blood the partial pressure of carbon dioxide has become too low to excite the respiratory centre. It will be evident from the foregoing that in order to supply efficiently the respiratory requirements of the tissues not only must the breathing, but also the circulation, be suitably regulated. In hard muscular work the consumption of oxygen and output of carbon dioxide may be increased eight or ten times beyond those of rest. Unless, therefore, the blood supply to the active tissues were correspondingly increased, deficiency of oxygen would at once arise, since the amount of oxygen carried by a given volume of the arterial blood is very limited, as already explained. It is known that the supply of blood to each organ is always increased during its activity. This; increase can, for instance, readily be seen and measured in the case of contracting muscles or secreting glands; and the volume and frequency of the pulse are greatly increased during muscular work. But while it is evident enough that the flow of blood through the body is determined in accordance with the metabolic activities of each tissue, our knowledge is as yet very scanty as to the means by which this determination is brought about. Probably, however, carbon di.oxide may be nearly as important a factor in the regulation of the circulation as in that of breathing. Just as the rate of breathing was formerly supposed to determine, and not to be determined by, the fundamental metabolic processes of the body, so the circulation was supposed to be another independent determining factor; and under the influence of these mechanistic conceptions the direction of investigation into the phenomena of respiration and' circulation has been largely diverted to side issues.
Since the circulation, no less than the breathing, is concerned in the supply of oxygen to and removal of carbon dioxide from the tissues, it can readily be understood that defective circulation, such as occurs, for instance, in uncompensated valvular affections of the heart, may affect the breathing and hinder the normal respiratory exchange. Conversely, also, defects in the aeration or oxygen-carrying power of the blood may be compensated for by increase in the circulation. For instance, in the very common condition known as anaemia, where the percentage of haemoglobin, and consequently the oxygen-carrying power of the blood, is often reduced to a third or less, the respiratory disturbances may be so slight that the patient is going about his or her ordinary work. A miner suffering from the now well-known “ worm disease, ” or ankylostomiasis (g.v.), may be working underground, or a. housemaid suffering from chlorosis may be doing her work, with only a third of the normal oxygen-carrying power of the blood. There seems to be no doubt that in such cases an increased rate of blood circulation compensates for the diminished oxygen-carrying power of the blood. It is well known that at high altitudes a gradual process of adaptation to the low pressure occurs, and the shortness of breath and other symptoms experienced for the first few days gradually become less and less. This adaptation is .partly, at least, due to a. marked increase in the percentage of haemoglobin in the blood, though probably circulatory and perhaps other compensatory changes are also involved.
In connexion with respiration the action of certain poisons is of great interest. One of these, carbon monoxide, is of very common occurrence, and causes numerous cases of poisoning. Like oxygen, it has the property of combining with the haemoglobin of the blood, but its affinity for haemoglobin is far more strong than that of oxygen. In presence of air containing as little as -05% of carbon monoxide, the haemoglobin will become about equally shared between oxygen and carbon monoxide, so that, since air contains 20.9% of oxygen, the affinity of carbon monoxide for haemoglobin may be regarded as abou-t 400 times greater than that of oxygen. The blood of a person breathing even a small percentage of carbon monoxide may thus become gradually saturated to a dangerous extent, since the haemoglobin engaged by the carbon monoxide is for the time useless as an oxygen-carrier. Air containing more than about o.r % of carbon monoxide is thus more or less dangerous if breathed for long; but the blood completely recovers in the course of a few hours if pure air is again breathed. The poisonous action of carbon monoxide can be abolished by placing the animal exposed to it in oxygen at an excess pressure of about an atmosphere. The reason for this is that, in consequence of the increased partial pressure of the oxygen, the amount of this gas in free solution in the blood is greatly increased in accordance with Dalton's law, and becomes sufficient to supply the tissues with oxygen quite independently of the haemoglobin. Even at ordinary atmospheric' pressure the extra oxygen dissolved in the blood when pure oxygen is breathed is of considerable importance. Carbon-monoxide poisoning is the chief cause of death in Colliery explosions and fires, and the sole cause in poisoning by lighting gas and fuel gas of various kinds. Its presence in dangerous proportions may be readily detected with the help of a small bird, mouse or other small warm-blooded animal. In such animals the respiratory exchange is so rapid that symptoms of carbon-monoxide poisoning are shown far more quickly than in man. The small animal can thus be employed in mines, &c., to indicate danger from carbon monoxide. A lamp is useless for this purpose. There are various other poisons, such as nitrites, chlorate's, dinitrobenzol, &c., which act by disabling the haemoglobin, and so cutting 05 the oxygen supply to the tissues.
Between the air in the air-cells of the lungs and the blood of the lung capillaries there intervenes nothing but a layer of very thin, fattened cells, and until recently it was very generally believed that it was by diffusion alone that oxygen passes inwards and carbonic acid outwards through this layer. Similar simple physical explanations of processes of secretion and absorption through living cells have, however, turned out to be incorrect in the case of other organs. It is known, moreover, that in the case of the swimming-bladder of fishes oxygen is secreted into
