524 OF RESPIRATION. 



561. The alterations in this air which are effected by Respiration, mainly 

 consist in the removal of a portion of its Oxygen, and the substitution of a 

 quantity of Carbonic acid, usually rather less in bulk than the oxygen which 

 has disappeared. The proportion of the air thus changed appears to vary 

 according to the frequency of the respirations. Thus Vierordt 1 found that, if he 

 only respired six times in a minute, the quantity of Carbonic acid was 5.5 per 

 cent, of the whole air exhaled; with twelve respirations, it was 4.2; with twenty- 

 four, it was 3.3 ; with forty-eight, it was 3.0; and with ninety-six, it was 2.6 

 per cent. In some of the experiments of Messrs. Allen and Pepys, it was as 

 much as 8 per cent. Probably about 4.35 per cent, may be taken as the average, 

 at the ordinary rate of respiration. It appears, however, from the researches 

 of the last-named experimenters, that, if the air be already charged in some 

 degree with Carbonic acid, the quantity exhaled is much less ; for, when 300 

 cubic inches of air were respired for three minutes, only 28 cubic inches (9$ 

 per cent.) of carbonic acid were found in it; although the previous rate of its 

 production, when fresh air was taken in at every respiration, was 32 cubic inches 

 in a minute. Knowing, then, the necessity of a free excretion of carbonic acid, 

 we are led by this fact to perceive the high importance of ventilation ; for it is 

 not sufficient for health, that a room should contain the quantity of air requisite 

 for the support of its inhabitants during a given time ; since after they have 

 remained in it but a part of that time, the quantity of carbonic acid which its 

 atmosphere will contain, will be large enough to interfere greatly with the due 

 aeration of their blood, and will thus cause oppression of the brain, and the 

 other morbid affections that result from the accumulation of carbonic acid in the 

 circulating fluid. It appears from the experiments of Dr. Snow, that the pre- 

 sence of Carbonic acid in the atmosphere acts more deleteriously upon the sys- 

 tem, in proportion as the normal quantity of Oxygen has been reduced. He 

 found that birds and mammalia, introduced into an atmosphere containing only 

 from 10J to 16 per cent, of oxygen, soon died, although means were taken to 

 remove the carbonic acid set free by their respiration, as fast as it was formed ; 

 whilst, on the other hand, an increase in the proportion of carbonic acid to 12 

 or even 20 per cent. the percentage of oxygen being kept to its regular standard 

 of 21 per cent. did not appear to enfeeble the vital actions more rapidly than 

 did the reduction of the oxygen in the experiments just referred to. Dr. Snow 

 concludes from his experiments on the lower animals, that 5 or 6 per cent, of 

 carbonic acid cannot exist in an atmosphere respired by Man, without danger to 

 life ; and that less than half this amount will soon be fatal, when it is formed 

 at the expense of the oxygen of the air. 2 



562. The reaction which thus takes place between the Air and the Blood, is 

 partly explicable upon physical principles. If the blood come to the lungs 

 charged with Carbonic acid, and be exposed in their cells to the influence of at- 

 mospheric air, which is a mixture of Oxygen and Nitrogen, an endosmose and 

 exosmose of gases will take place. 3 The carbonic acid of the blood will pass out, 

 to be replaced by oxygen and nitrogen ; and the quantity of the former which 

 enters will be much greater than that of the latter, on account of the superior 

 facility with which oxygen passes through porous membranes. If the venous 



1 "Physiologie des Athmens," pp. 102-149. 



2 "Edinb. Med. and Surg. Journal," 1846. 



3 See " Princ. of Phys., Gen. and Comp.," g 495, Am. Ed. It has been recently affirmed 

 by Dr. Bence Jones ("Medical Times," 1851, p. 169), that the law of the " diffusion of gases" 

 does not apply to the respiration of air-breathing animals : since a gas is on one side of the 

 septum and a liquid on the other. But it was long since shown by Dr. Mitchell of Phila- 

 delphia, that the tendency to mutual diffusion and replacement exists between atmospheric 

 nir and gases dissolved in water; and that this is not prevented by the interposition of a 

 permeable membrane. 



