present article attention will be specially confined to the case of
the higher vertebrates, and in particular to man.
Air is brought into the lungs by the movements of breathing (see above, Movements of Respiration). Oxygen from this air passes through the delicate lining membrane of the air-cells of the lungs into the blood, where it enters into loose chemical combination with the haemoglobin of the red corpuscles (see Blood). In this form it is conveyed onwards to the heart, and thence through the arteries to the capillaries, where it again parts from the haemoglobin, and passes through the capillary walls to the tissues, where it is consumed. Carbon dioxide passes out from the tissues into the blood in a corresponding manner, , enters into loose combination as bicarbonate, and possibly in other ways, in the blood, and is conveyed by the veins to the lungs, whence it passes out in the expired air. Pure atmospheric air contains 20·93% of oxygen, ·03% of carbon dioxide and 79·04% of nitrogen (with which is mixed about 0·9% of argon). The dried expired air in man contains about 3·5% of carbon dioxide and 17% of oxygen, so that roughly speaking the carbon dioxide is increased by about 3·5% and the oxygen diminished by 4%. Expired air as it leaves the body contains about 6%, of moisture, compared with usually about 1% in the inspired air. The added moisture and higher temperature of expired air make it decidedly lighter than pure air.
Owing to the unpleasant effects often produced in badly ventilated rooms it was for long supposed that some poisonous volatile “organic matter” is also given off in the breath. Careful investigation has shown that this is not the case. The unpleasant effects are partly due to heat and moisture, and partly to odours which are usually not of respiratory origin. The carbon dioxide present in the air of even very badly ventilated rooms is present in far too small proportions to have any sensible effect.
The average volume of air inspired per minute by healthy adult men during rest is about 7 litres or ·25 cub. ft. In different individuals the frequency of breathing varies considerably—from about 7 to 25 per minute, the depth of each breath varying about inversely as the frequency. During muscular work the volume of air breathed may be six or eight times as much as during rest. The volume of carbon dioxide given off varies from about half a cubic foot per hour during complete rest to 5 cub. ft. during severe exertion, but averages about 0·9 cub. ft. per hour, and will reach or exceed 1 cub. ft. per hour during even very light exertion. The volume of oxygen consumed is about a seventh greater than that of the carbon dioxide given off.
The breathing is regulated from a nervous centre situated in the medulla oblongata, which is the lowest part of the brain. If this centre is destroyed or injured the breathing stops and death rapidly results. From the respiratory centre rhythmic efferent impulses proceed down the motor nerves supplying the diaphragm, intercostals and other respiratory muscles. Afferent impulses through various nerves may temporarily affect the rhythm of the respiratory centre. Of these afferent impulses by far the most important are those which proceed up the vagus nerve from the lungs themselves. On distention of the lungs with air the inspiratory impulses from the respiratory centre are suddenly arrested or “inhibited”; on the other hand, collapse of the lung strongly excites to inspiratory effort. On section of the vagus nerve these effects disappear, and the breathing becomes less frequent and much more laboured. The vagus nerve is thus the carrier of both inhibitory and exciting stimuli.
As the physiological function of breathing is to bring oxygen to and remove carbon dioxide from the blood, it would naturally be expected that breathing would be regulated in accordance with the amount of oxygen required and of carbon dioxide formed; but until quite recently the actual mode of regulation was by no means clear. It was commonly supposed that afferent nervous impulses in some way regulated the otherwise automatic action of the centre, want of oxygen or excess of CO2 in the blood being only an occasional and relatively unimportant factor in the regulations. The phenomenon of “apnoea” or complete cessation of natural breathing which occurs after forced breathing, was attributed mainly to the already mentioned distension effect through the vagus nerves. To go further back still, it was even supposed that the rate and depth of breathing, and the percentage of oxygen in the inspired air, determine the consumption of oxygen and formation of carbon dioxide in the body, just as the air-supply to a fire determines the rate of its combustion. This old belief is still often met with-for instance, in the reasons given for recommending “breathing exercises” as a part of physical training.
It is evident that if the breathing did not increase correspondingly with the greatly increased consumption of oxygen and formation of CO2 which occurs, for instance during muscular work, the percentage of oxygen in the air contained in the lung cells or alveoli (alveolar air) would rapidly fall, and the percentage of carbon dioxide increase. The inevitable result would be a very imperfect aeration of the blood. Investigation of the alveolar air has furnished the key to the actual regulation of breathing. Samples of this air can be obtained by making a sudden and deep expiration through a piece of long tube, and at once collecting some of the air contained in the part of this tube nearest the mouth. By this means it has been found that during normal breathing at ordinary atmospheric pressure the percentage of carbon) dioxide (about 5·6% on an average for men) is constant for each individual, though different persons vary slightly as regards their normal percentage. The breathing is thus so regulated as to keep the percentage of carbon dioxide constant; and under normal conditions this regulation is surprisingly exact. The ordinary expired air is a mixture of alveolar air and air from the “dead space” in the air passages. The deeper the breathing happens to be, the more alveolar air there will be in the expired air, and the higher, therefore, the percentage of carbon dioxide in it, so that the expired air is not constant in composition, though the alveolar air is. If air containing 2 or 3% of carbon dioxide is breathed, the breathing at once becomes deeper, in such a way as to prevent anything but a very slight rise in the alveolar carbon dioxide percentage. The difference is scarcely appreciable subjectively, except during muscular exertion. The effect of 1% of carbon dioxide in the inspired air is so slight as to be negligible, and there is no foundation for the popular belief that even very small percentages of carbon dioxide are injurious. With 4 or 5% or more of carbon dioxide, however, much panting is produced, and the alveolar carbon dioxide percentage begins to rise appreciably, since compensation is no longer possible. As a consequence, headache and other symptoms are produced. If, on the other hand, the percentage of carbon dioxide in the alveolar air is abnormally reduced by forced breathing, the condition of apnoea is produced and lasts until the percentage again rises to normal, but no longer. Forced breathing with air containing more than about 4% of carbon dioxide causes no apnoea, as the alveolar carbon dioxide does not fall.
If oxygen is breathed instead of air there is no appreciable change in the percentage of carbon dioxide in the alveolar air, and no tendency towards apnoea. Want of oxygen is thus not a factor in the regulation of normal breathing. During muscular work the depth and .frequency of breathing increase in such a way as to prevent the alveolar carbon dioxide from rising more than very slightly. It is still the carbon dioxide stimulus that regulates the breathing, although with excessive muscular work other accessory factors may come in to some extent.
Under increased barometric pressure the percentage of carbon dioxide in the alveolar air no longer remains constant; it diminishes in proportion to the increase of pressure. For instance, at a pressure of 2 atmospheres it is reduced to half, and at 6 atmospheres to a sixth; while at less than normal atmospheric pressure it rises correspondingly unless symptoms of want of oxygen begin to interfere with this rise. These results show that it is not the mere percentage, but the pressure (or “partial pressure”) of carbon dioxide in the
