Page images
PDF
EPUB

one, and that of expiration passive. In inspiration, the glottis is opened by contraction of the posterior crico-arytenoid muscles. (Fig. 75.) These muscles originate from the posterior surface of the cricoid cartilage, near the median line; and their fibres, running upward and outward, are inserted into the external angle of the arytenoid cartilages. By the contraction of these muscles, during the movement of inspiration, the arytenoid cartilages are rotated upon their articulations with the cricoid, so that their anterior extremities are carried outward, and the vocal chords stretched and separated from each other. (Fig. 74.) In this way, the size of the glottis may be increased from 0.15 to 0.27 square inch.

In expiration, the posterior crico-arytenoid muscles are relaxed, and the elasticity of the vocal chords brings them back to their former position.

The motions of respiration consist, therefore, of two sets of movements: viz., those of the chest, and those of the glottis. These movements, in the natural condition, correspond with each other both in time and intensity. It is at the same time and by the same nervous influence, that the chest expands to inhale the air, while the glottis opens to admit it; and in expiration, the muscles of both chest and glottis are relaxed, while the elasticity of the tissues, by a kind of passive contraction, restores the parts to their original condition.

CHANGES IN THE AIR DURING RESPIRATION.

The atmospheric air, as it is drawn into the cavity of the lungs, is a mixture of oxygen and nitrogen, in the proportion of about 21 per cent., by volume, of oxygen, to 79 per cent. of nitrogen. It also contains about one-twentieth per cent. of carbonic acid, a vary ing quantity of watery vapor, and some traces of ammonia. If collected and examined, after passing through the lungs, it is found to have become altered in the following essential particulars, viz:1st. It has lost oxygen.

2d. It has gained carbonic acid. And

3d. It has absorbed the vapor of water.

Beside the two latter substances, there are also exhaled with the expired air a very small quantity of nitrogen, over and above what was taken in with inspiration, and a little animal matter in a gaseous form, which communicates a slight but peculiar odor to the breath. The air is also somewhat elevated in temperature, by

contact with the pulmonary mucous membrane. By far the most important part, however, of the above changes suffered by the air, consists in its loss of oxygen, and its absorption of carbonic acid.

The oxygen which disappears from the inspired air is not entirely replaced in the carbonic acid exhaled; that is, there is less oxygen in the carbonic acid which is returned to the air by expiration than has been lost during inspiration.

There is even more oxygen absorbed than is given off again in both the carbonic acid and aqueous vapor together, which are exhaled from the lungs. There is, then, a constant disappearance of oxygen from the air used in respiration, and a constant accumulation of carbonic acid.

The proportion of oxygen which disappears in the interior of the body, over and above that which is returned in the breath under the form of carbonic acid, varies in different kinds of animals. In the herbivora, it is about 10 per cent. of the whole amount of oxygen inspired; in the carnivora, 20 or 25 per cent., and even more. It is a very remarkable fact, also, and an important one, as regards the theory of respiration, that, in the same animal, the proportion of oxygen absorbed, to that of carbonic acid exhaled, varies according to the quality of the food. In dogs, for instance, while fed on animal food, according to the experiments of Regnault and Reiset, 25 per cent. of the inspired oxygen disappeared in the body of the animal; but when fed on starchy substances, all but 8 per cent. reappeared in the expired carbonic acid. It is already evident, therefore, from these facts, that the oxygen of the inspired air is not altogether employed in the formation of carbonic acid.

CHANGES IN THE BLOOD DURING RESPIRATION.

If we pass from the consideration of the changes produced in the air by respiration to those which take place in the blood during the same process, we find, as might have been expected, that the latter correspond inversely with the former. The blood, in passing

through the lungs, suffers the following alterations:

1st. Its color is changed from venous to arterial. 2d. It absorbs oxygen. And

3d. It exhales carbonic acid and the vapor of water.

'Lehmann's Physiological Chemistry, Philada. ed., vol. ii. p. 432.

The interchange of gases, which takes place during respiration between the air and the blood, is a simple phenomenon of absorp tion and exhalation. The inspired oxygen does not, as Lavoisier once supposed, immediately combine with carbon in the lungs, and return to the atmosphere under the form of carbonic acid. On the contrary, almost the first fact of importance which has been established by the examination of the blood in this respect is the fol lowing, viz: that carbonic acid exists ready formed in the venous blood before its entrance into the lungs; and, on the other hand, that the oxygen which is absorbed during respiration passes off in a free state with the arterial blood. The real process, as it takes place in the lung, is, therefore, for the most part, as follows: The blood comes to the lungs already charged with carbonic acid. In passing through the pulmonary capillaries, it is exposed to the influence of the air in the cavity of the pulmonary cells, and a transudation of gases takes place through the moist animal membranes of the lung. Since the blood in the capillaries contains a larger proportion of carbonic acid than the air in the air-vesicles, a portion of this gas leaves the blood and passes out through the pulmonary membrane; while the oxygen, being more abundant in the air of the vesicles than in the circulating fluid, passes inward at the same time, and is condensed by the blood.

In this double phenomenon of exhalation and absorption, which takes place in the lungs, both parts of the process are equally necessary to life. It is essential for the constant activity and nutrition of the tissues that they be steadily supplied with oxygen by the blood; and if this supply be cut off, their functional activity ceases. On the other hand, the carbonic acid which is produced in the body by the processes of nutrition becomes a poisonous substance, if it be allowed to collect in large quantity. Under ordinary circumstances, the carbonic acid is removed by exhalation through the lungs as fast as it is produced in the interior of the body; but if respiration be suspended, or seriously impeded, since the production of carbonic acid continues, while its elimination is prevented, it accumulates in the blood and in the tissues, and terminates life in a few moments, by a rapid deterioration of the circulating fluid, and more particularly by its poisonous effect on the nervous system.

The deleterious effects of breathing in a confined space will therefore very soon become apparent. As respiration goes on, the oxygen of the air constantly diminishes, and the carbonic acid, mingled with it by exhalation, increases in quantity. After a time

the air becomes accordingly so poor in oxygen that, by that fact alone, it is incapable of supporting life. At the same time, the carbonic acid becomes so abundant in the air vesicles that it prevents the escape of that which already exists in the blood; and the deleterious effect of its accumulation in the circulating fluid is added to that produced by a diminished supply of oxygen. An increased proportion of carbonic acid in the atmosphere is therefore injurious in a similar manner, although there may be no diminution of oxygen; since by its presence it impedes the elimination of the carbonic acid already formed in the blood, and induces the poisonous effects which result from its accumulation.

Examination of the blood shows furthermore that the interchange of gases in the lungs is not complete but only partial in its extent. It results from the experiments of Magendie, Magnus, and others, that both oxygen and carbonic acid are contained in both venous and arterial blood. Magnus' found that the proportion of oxygen to carbonic acid, by volume, in arterial blood was as 10 to 25; in venous blood as 10 to 40. The venous blood, then, as it arrives at the lungs, still retains a remnant of the oxygen which it had previously absorbed; and in passing through the pulmonary capillaries it gives off only a part of the carbonic acid with which it has become charged in the general circulation.

The oxygen and carbonic acid of the blood exist in a state of solution in the circulating fluid, and not in a state of intimate chemical combination. This is shown by the fact that both of these substances may be withdrawn from the blood by simple exhaustion with an air pump, or by a stream of any other indifferent gas, such as hydrogen, which possesses sufficient physical displacing power. Magnus found that freshly drawn arterial blood yielded by simple agitation with carbonic acid more than 10 per cent. of its volume of oxygen. The carbonic acid may also be expelled from venous blood by a current of pure oxygen, or of hydrogen, or, in great measure, by simple agitation with atmospheric air. There is some difficulty in determining, however, whether the carbonic acid of the blood be altogether in a free state, or whether it be partly in a state of loose chemical combination with a base, under the form of an alkaline bicarbonate. A solution of bicarbonate of soda itself will lose a portion of its carbonic acid, and become reduced to the

' In Lehmann, op. cit., vol. i. p. 570.

2 In Robin and Verdeil, op. cit., vol. ii. p. 34.

condition of a carbonate, by simple exhaustion under the air-pump. or by agitation with pure hydrogen at the temperature of the body. Lehmann has found' that after the expulsion of all the carbonie acid removable by the air-pump and a current of hydrogen, there still remains, in ox's blood, 0.1628 per cent. of carbonate of soda; and he estimates that this quantity is sufficient to have retained all the carbonic acid, previously given off, in the form of a bicarbonate. It makes little or no difference, however, so far ar regards the process of respiration, whether the carbonic acid of the blood exist in an entirely free state, or under the form of an alkaline bicarbonate: since it may be readily removed from this combination, at the tem perature of the body, by contact with an indifferent gas.

The oxygen and carbonic acid of the blood are in solution principally in the blood-globules, and not in the plasma. The researches of Magnus have shown that the blood holds in solution 2 times as much oxygen as pure water could dissolve at the same temperature; and that while the serum of the blood, separated from the globules, exerts no more solvent power on oxygen than pure water, defibrinated blood, that is, the serum and globules mixed, dissolves quite as much oxygen as the fresh blood itself. The same thing is true with regard to the carbonic acid. It is therefore the semifluid blood-globules which retain these two gases in solution; and since the color of the blood depends entirely upon that of the globules, it is easy to understand why the blood should alter its hue from purple to scarlet in passing through the lungs, where the globules give up carbonic acid, and absorb a fresh quantity of oxygen. The above change may readily be produced outside the body. If freshly drawn venous blood be shaken in a bottle with pure oxygen, its color changes at once from purple to red; and the same change will take place, though more slowly, if the blood be simply agitated with atmospheric air. It is for this reason that the surface of defibrinated venous blood, and the external parts of a dark-colored clot, exposed to the atmosphere, become rapidly reddened, while the internal portions retain their original color.

The process of respiration, so far as we have considered it, consists in an alternate interchange of carbonic acid and oxygen in the blood of the general and pulmonary circulations. In the pulmonary circulation, carbonic acid is given off and oxygen absorbed; while

1 Op. cit., vol. i. p. 393.

2 In Robin and Verdeil, op. cit., vol. ii. pp. 28-32.

« PreviousContinue »