362 



RESPIRATION. 



On the manner in which the air in the upper 

 and lower parts of the respiratory apparatus 

 becomes intermixed. The respiratory qualities 

 of the other parts of the inner surface of the 

 air-passages must be very feeble when com- 

 pared with the membrane of the air-cells of 

 the lungs ; and there can be no doubt that 

 almost all the carbonic acid present in the 

 expired air is derived from the blood circula- 

 ting in the capillary blood-vessels of the air- 

 cells ; and that this evolution of carbonic acid 

 gas is continuous, going on during expiration 

 as well as during inspiration. As a portion 

 only of the atmospheric air, probably not 

 much more than a fourth or a fifth part, is re- 

 newed at each ordinary respiratory movement 

 when the body is in a state of rest, the air 

 expelled during expiration will chiefly consist 

 of that occupying the larynx, trachea, and the 

 larger bronchial tubes ; so in the same man- 

 ner, the air drawn in by inspiration will 

 chiefly occupy the same parts of the respira- 

 tory apparatus. It is well known that the 

 air expelled in the first part of an expiration 

 contains less carbonic acid than that expelled 

 towards its close ; thus the air in the deeper 

 parts of the respiratory apparatus must be 

 richer in carbonic acid and poorer in oxygen 

 than that in the upper parts. The amount of 

 intermixture of the gases in the different parts 

 of the respiratory apparatus effected by the 

 muscular movements of the chest would, in 

 all probability, be too imperfect for the proper 

 arterialisation of the blood, were this not 

 aided by the well-known tendency of gases to 

 diffuse themselves through each other. As 

 the air in the air-cells differs from that in the 

 higher parts of the respiratory apparatus in 

 containing more carbonic acid and less oxy- 

 gen, the nitrogen being nearly the same in 

 both, this diffusion of gases is probably chiefly 

 confined to the two former. From the oxy- 

 gen being of lighter specific gravity than the 

 carbonic acid gas, the descending current of 

 oxygen gas will exceed the ascending current 

 of carbonic acid, and 81 parts of carbonic 

 acid will be replaced by 95 of oxygen, for 

 according to the law regulating the diffusion- 

 volumes of gases under such circumstances, 

 established by Graham, in the case of each 

 gas this is inversely proportional to the square 

 root of its density.* 



On the nature of the actions between the 

 blood find the atmospheric air in the lungs, by 

 which a quantity of oxygen is removed from the 

 inspired air, and a quantity of carbonic acid gas 

 added to the expired air. Four views have 

 been maintained on this point. 1. That of 

 Lavoisier, La Place, and others ; that the 

 oxygen which disappears from the inspired 

 air unites directly in the lungs with hydro- 

 carbon furnished by the venous blood, and 

 forms the carbonic acid gas and watery vapour 

 that escape along with the expired air.-j- 



* Edinburgh Transactions of Eoyal Society, vol. 

 xii. p. 573. 1834. 



t Seguin and Lavoisier " Sur la Transpiration 

 des Animaux," in Memoires de 1'Academie des 



2. That of La Grange and Hassenfratz ; 

 that free carbonic acid gas is present in a state 

 of solution in the venous blood before it 

 arrives at the lungs, where this gas is ex- 

 haled ; that nearly the whole of the oxygen gas 

 abstracted from the inspired air is absorbed 

 at the lungs, and held in solution by the 

 arterial blood ; and that the combination of 

 the oxygen with the carbon and formation of 

 carbonic acid chiefly take place when the 

 blood is passing through the capillaries of the 

 systemic circulation.* 



3. That the oxygen that disappears from 

 the inspired air enters into chemical combina- 

 tion with one or more of the constituent 

 parts of the blood in its course through the 

 lungs, that in the passage of the blood through 

 the capillaries of the systemic circulation this 

 oxygen leaves the substance or substances to 

 which it had united itself, and combines with 

 carbon to form carbonic acid, or with carbon 

 and hydrogen to form carbonic acid and 

 water, and that the carbonic acid thus formed 

 does not combine chemically with any of the 

 constituent parts of the venous blood, but is 

 held in solution by it, and is evolved while 

 passing through the capillaries of the lungs. 



4. That not only the oxygen that disap- 

 pears from the inspired air is united chemi- 

 cally in the arterial blood, but also the 

 carbonic acid formed during its circulation 

 through the systemic capillaries enters into 

 chemical combination with some one of the 

 constituent parts of the venous blood ; that 

 the combination thus formed is decomposed 

 in the pulmonic capillaries by the agency of 

 the absorbed oxygen, and the carbonic acid 

 thus set free is evolved and escapes in the 

 expired air. 



The first view, viz. that the carbonic acid 

 that appears in the expired air is formed in the 

 lungs by the combination of part of the oxy- 

 gen of the inspired air with the carbon of the 

 venous blood, must now be regarded as unte- 

 nable. The existence of free gases in the 

 blood, the evolution of carbonic acid from the 

 blood at the lungs in animals made to breathe 

 gases devoid of oxygen, the small increase of 



Sciences for 1790, p. 601. It is still maintained by 

 some chemists and physiologists, who appear to re- 

 gard the function of respiration simply as a process 

 of combustion, but who do not uphold the opinion 

 that this combustion takes place in the lungs and 

 that the watery vapour in the expired air is imme- 

 diately derived from this source, that a part of the 

 oxygen that disappears from the inspired air unites 

 with hydrogen to form water. No satisfactory evi- 

 dence is offered in support of this opinion, and in 

 the present state of our knowledge it must be re- 

 garded as a mere conjecture. 



* This doctrine, as propounded by Hassenfratz 

 (Annales de Chiniie, torn. ix. p. 261. 1791), which 

 has received various modifications since his time, 

 was based on the view that the purple colour of the 

 venous blood is the result of the combination of 

 oxygen with the carbon and hydrogen of the blood, 

 while the scarlet colour of arterial blood is caused 

 by the solution of oxygen gas in it, and consequently 

 there can be little combination of the carbon and 

 hydrogen of the blood with the atmospheric air in 

 the lungs. 



