alveolar air that regulates breathing. The pressure exercised
by the carbon dioxide in the alveolar air is of course proportional
to its percentage, multiplied by the total atmospheric
pressure. It follows from this law that at a pressure of
6 atmospheres 1% of carbon dioxide in the inspired air would
have the same violent effect as 6% at the normal pressure of
I atmosphere. To take a concrete practical application, if a diver
whose head was just below water were supplied with sufficient
air to keep the carbon dioxide percentage in the air of his helmet
down to 3% at most, he would be quite comfortable. But if,
with the same air supply as measured at surface, he went down
to a depth of 170 ft., where the pressure is 6 atmospheres, he
would at once experience great distress culminating in loss of
consciousness, owing, not to the pressure of the water, which
has trifling effects, but to the pressure of carbon dioxide in the
air he Was breathing. The air supply must be increased in
proportion to the increase of pressure if these effects are to be
avoided, and ignorance of this has led to the common failure
of diving work at considerable depths.
The foregoing, facts enable us to understand the regulation of breathing under normal conditions. The pressure of carbon dioxide in the alveolar air evidently determines that of the carbon dioxide in the arterial blood, and the latter in its turn determines the carbon dioxide pressure in the respiratory centre, which is very richly supplied with blood. The centre itself is extremely sensitive to the slightest increase or diminution in carbon dioxide pressure; and thus it is that the alveolar carbon dioxide pressure is so important. That the stimulus of carbon dioxide is from the blood and not through nerves is proved by many experiments. The function of the vagus nerves in regulating the breathing is apparently to, as it were, guide the centre in the expenditure of each separate inspiratory or expiratory effort; for as soon as inspiration or expiration is completed the inspiratory or expiratory effort is cut short by impulse proceeding up the vagus nerve, and much waste of muscular work and risk of injury to the lungs is thereby prevented. Under ordinary conditions the regulation of carbon dioxide pressure in the alveolar air ensures at the same time a normal pressure of oxygen, since absorption of oxygen and giving off of carbon dioxide normally run parallel to one another. If, however, air containing abnormally little oxygen is breathed, the normal relation between oxygen and carbon dioxide in the alveolar air is disturbed. A similar state of affairs is brought about by any considerable diminution of atmospheric pressure. Not only does the partial pressure of oxygen in the inspired air fall, but this fall is proportionally much greater in the alveolar air; and the effects of want of oxygen depend on its partial pressure in the alveolar air. It has been known for long that any great deficiency in the proportion of oxygen in the air breathed increases the depth and frequency of the breathing; but this effect is not apparent until the percentage of oxygen or the barometric pressure is reduced by more than a third, which corresponds to a reduction of more than half in the alveolar oxygen pressure. In contrast with this an increase of a fiftieth in the alveolar carbon dioxide pressure has a marked effect on the breathing. Along with the increased breathing caused by deficiency of oxygen there is more or less blueness of the skin and abnormal effects of various kinds, such as partial loss of sensibility, memory and power of thinking. Long exposure often causes headache, nausea, sleeplessness, &c.—a train of symptoms known to mountaineers as “mountain sickness.” That the primary cause of “mountain sickness” is lack of oxygen owing to the low atmospheric pressure there is not the slightest doubt. Lack of oxygen is thus not only an important, but also an abnormal form of stimulus to the respiratory centre, since it is accompanied by quite abnormal symptoms. A further analysis of the special effect of lack of oxygen on the respiratory centre has shown that this effect still depends on the partial pressure of carbon dioxide in the alveolar air. The lack of oxygen appears, in fact, to have simply increased the sensitiveness of the centre to carbon dioxide, so that a lower partial pressure of carbon dioxide excites the centre, and the breathing is correspondingly increased. By prolonged forced breathing so much carbon dioxide is washed out of the body that the subsequent apnoea lasts until the oxygen in the alveolar air is nearly exhausted. The subject of the experiment becomes very blue in the face and is partially stupefied by want of oxygen before he has any desire to breathe. The probable explanation of these facts is that want of oxygen does not itself excite the centre, but that some substance very probably lactic acid, which is known to be formed abundantly —is produced abnormally in the body during exposure to want of oxygen and aids the carbon dioxide in exciting the centre. Itis known that the blood becomes less alkaline at high altitudes, and that acids in general excite the centre. A person on a high mountain thus gets out of breath much more easily than at sea-level. The extra stimulus to the centre during work still comes from the extra carbon dioxide formed, but has a greater effect than usual on the breathing. If the extra stimulus came directly from Want of oxygen the person on the mountain would probably turn blue and lose consciousness on the slightest exertion. By analysing the alveolar air it can be shown that after a time even a height of 5000 to 6000 ft., or a diminution of only a sixth in the barometric pressure, distinctly increases the sensitiveness of the respiratory centre to carbon dioxide, so that there seems to be a. slow accumulation of acid in the blood. The effect also passes off very slowly on returning to normal pressure, although the lack of oxygen is at once removed. The blueness of the skin (“ cyanosis ”) produced by lack of oxygen is due to the fact that the haemoglobin of the red corpuscles is imperfectly saturated with oxygen. Haemoglobin which is fully saturated with oxygen has a bright red colour, contrasting with the blue colour which it assumes when deprived of oxygen. According to the existing evidence the saturation of the haemoglobin is practically complete under normal conditions in the lungs, or when thoroughly shaken at the body temperature and normal atmospheric pressure with air of the same composition as normal alveolar air. As the partial pressure of the oxygen in this air falls, however, the saturation of the haemoglobin becomes less and less complete, and the arterial blood assumes amore and more blue tinge, which imparts a blue or leaden colour to the skin, accompanied by the symptoms, already referred to, of lack of oxygen. Normal arterial blood in man yields about 19 volumes of physiologically available oxygen for each 100 volumes of blood. Of these 19 volumes about 18% are loosely combined with the haemoglobin of the red corpuscles, the small remainder being in simple solution in the blood. Venous blood, on the other hand, yields only about 12 volumes. The combination of haemoglobin with oxygen is only stable in the presence of free oxygen at a pressure of about that in normal alveolar air. As this pressure falls the compound is progressively dissociated. From this it can be readily understood why the blood loses its oxygen in passing through the tissues, which are constantly absorbing free oxygen, and regains it in the lungs. The marked effects produced by abnormal deficiency in the pressure of oxygen in the alveolar air are also readily intelligible; for even although the arterial blood still contains sufficient oxygen to cover the normal difference between the oxygen content of arterial and that of venous blood, yet this oxygen is given off to the tissues less readily—i.e. at a lower pressure, and thus fails to supply their demands completely. It is evident also that in pure air at normal pressure increased ventilation of the lungs does not appreciably increase the supply of oxygen to the blood, whereas in air largely deprived of its oxygen, or at low pressure, the increased alveolar oxygen pressure produced by deep breathing helps greatly in saturating the blood with oxygen, and may thus relieve the symptoms of Want of oxygen. Hence it is that the increased sensitiveness of the respiratory centre to carbon dioxide, and consequent increased depth of breathing, at high altitudes compensates to a large extent for deficiency in the oxygen pressure. Addition of carbon dioxide to the inspired air produces exactly the same result. Indeed
