400 OF KESPIRATION. 



and skin in Man, varied from ? 1 -th (in an adult) to r ^ g d (in a child) of the 

 oxygen consumed, the former proportion agreeing very well with that deduced 

 by MM. Regnault and Reiset from their experiments on animals. 



[The alterations effected in the Blood bv Respiration have already been 

 fully considered. See 193-194.1 



31S. Exhalation and Absorption through the Lungs. The Air expired from 

 the lungs differs from that which was introduced into them, not merely in 

 the altered proportions of its Oxygen, Nitrogen, and Carbonic acid, but also 

 in having received (under ordinary circumstances at least) a large addition 

 to its watery vapor. This it doubtless acquires in accordance with physical 

 laws, through its exposure to the warm blood which is spread out over a 

 very extensive surface, the intermediate membrane being extremely per- 

 meable ; and the variations in its amount will depend upon the physical 

 conditions under which that exposure takes place. The air expired in ordi- 

 nary i-espiratiou is charged with as much watery vapor as saturates it at the 

 temperature of the body; 1 and consequently the amount of watery vapor 

 thus exhaled will vary (for equal volumes of air at any given temperature) 

 in the inverse proportion to that which the air previously contained. But 

 when the air is very cold and very dry, and the respiration is unusually 

 rapid, it may not remain sufficiently long in the air-cells to be raised to the 

 temperature of the body, or to be fully saturated with moisture. The 

 amount of watery vapor exhaled, moreover, will of course depend in part 

 upon the quantity of air which passes through the lungs. And from these 

 causes of difference, it happens that the amount of watery vapor exhaled in 

 twenty-four hours may vary from about 6 oz. to 27 oz. ; its usual range, how- 

 ever, being between 7 and 11 oz. Weyriclr estimates the amount of insen- 

 sible perspiration by the skin and lungs for a man weighing 125 Ibs. at 

 14,500 grains or 2 Ibs. avoirdupois per diem, and the proportion of water 

 discharged by the skin to that by the lungs as 2:3. Dr. Ed. Smith found 

 that during a long fast the quantity of vapor exhaled by the lungs was 2.02 

 grs. per minute, or .548 gr. in every 100 cub. in. of expired air. With food 

 and at rest, the quantity varied from 3 grs. to 3.4 grs. per minute. The in- 

 halation of alcoholic vapors increased the quantity of vapor exhaled ; when 

 alcohol was drunk the quantity was also increased, but it was decreased 

 under the action of gin. Of the fluid ordinarily exhaled with the breath, a 

 part doubtless proceeds from the moist lining of the nostrils, fauces, etc. ; but 

 it is indisputable that the greater proportion of it comes from the lungs, 

 since, when the respiration is entirely performed through a canula intro- 

 duced into the trachea, the amount of watery vapor which the breath con- 

 tains is still very considerable. Of the proper pulmonary exhalation, there 

 can be no doubt that the greater part is the mere surplus water of the blood, 

 and especially of the crude fluid which has been newly introduced into the 

 circulating current by the process of nutritive absorption. But there is 

 strong evidence that Hydrogen as well as Carbon undergoes combustion in 

 the system, and that a portion of the exhaled aqueous vapor is the product 

 of that combustion. For of the hydrogen which the food contains, not 

 more than from ^th to j'oth passes off' by the other excretions, the remain- 

 ing -|ths or T 9 ths being exhaled in the condition of watery vapor from the 

 lungs. A portion of the oxygen which this vapor contains is supplied by 

 the food; but there is usually a considerable surplus of hydrogen, and this 



1 The tension of tho watery vapor of expired air amounts!, according to \Veyrieh, 

 when red need to C. and 700 mm. pressure to 4.86 mm. of mercury, which is 

 nearly that of air saturated with aqueous vapor (4.5 mm.). 



2 Observations, etc., Dorpat, 1805, 8vo. 



