RESPIRATION. 



363 



temperature the blood acquires in its change 

 from the venous to the arterial condition *, 

 and the result of observations made upon the 

 blood out of the body, when subjected to 

 alternate applications of oxygen and carbonic 

 acid gas, are all opposed to the supposition 

 that the formation of carbonic acid gas takes 

 place to any great extent in the lungs. The 

 existence of a quantity of free carbonic acid 

 in the venous blood, more than sufficient to 

 furnish the whole of this gas thrown off at the 

 lungs, and the avowedly conjectural explana- 

 tion of the manner in which the carbonic acid 

 is combined and the agency by which its com- 

 binations are decomposed in the lungs, given 

 by those who advocate this view, justify the 

 adoption of the opinion that the carbonic 

 acid gas evolved at the lungs exists in a free 

 state in the venous blood before it reaches 

 the lungs. 



An interchange, therefore, takes place be- 

 tween the air in the cells of the lungs and 

 the blood in the pulmonic capillaries, the 

 latter receiving oxygen and giving up part of 

 the free carbonic acid held by it in solution. 

 These gases, from their solubility, readily per- 

 meate the thin moist membranes interposed 

 between the blood and the atmospheric air 

 contained in the cells of the lungs. We have 

 already mentioned that Valentin and Brunner 

 have concluded from their experiments that 

 this interchange of oxygen and carbonic acid 

 gas is regulated by the law of the diffusion of 

 gases established by Graham ; but besides 

 the objections that may be urged against this 

 view, drawn from the considerable diversity in 

 the relative proportions of these gases inter- 

 changed during respiration as ascertained by 

 different experimenters, the conditions under 

 which the two gases are placed in respiration 

 are very different from those in the experi- 

 ments instituted by Graham.f In respiration 

 the gases are separated by moist animal mem- 

 branes, and one of these, viz. the carbonic 



* Dr. J. Davy ascertained (Lond. Philos. Trans, 

 for 1838, p. 298) that oxygen gas shaken with ve- 

 nous blood out of the body raised the temperature 

 of the latter from 1 to 2 Fahr. Marchand (Journal 

 fur praktische Chemie, Band xxxv. S. 400) adduces 

 reasons for believing that this increase in temper- 

 ature arose from the mere absorption of the gas, and 

 not from any chemical action between it and the 

 blood. 



f Graham's first experiments, from which he de- 

 duced his law that " the diffusive velocities of dif- 

 ferent gases are inversely as the square root of their 

 densities," were made by interposing a porous sep- 

 tum of stucco between the gases experimented upon 

 and the external air. The equivalent diffusion-vo- 

 lumes of oxygen and carbonic acid calculated ac- 

 cording to this theory, with which the experimental 

 resuks closely agree, are air being equal to 1, 

 oxygen 0-9487, and carbonic acid 0-8091. (Trans- 

 actions of Royal Society of Edinburgh, vol. xii. 

 p. 222. 1834.) In some later experiments Mr. Gra- 

 ham ascertained that this law also held when gases 

 pass through minute apertures in a thin plate into a 

 vacuum, while, on the other hand, the discharge of 

 the same gases through tubes into a vacuum has 

 no uniform relation to the density of the gases. 

 (Philosophical Transactions of London for 1846, 

 p. 373.) 



acid, is held in solution in a fluid subjected to 

 an increased pressure caused by the action of 

 the heart.* 



We are not, in the present state of our 

 knowledge, in a condition to form any thing 

 like an accurate estimate of the various cir- 

 cumstances which regulate this interchange 

 between the oxygen of the air and the car- 

 bonic acid gas of the blood, but it is obvious 

 that it will be affected in a most important 

 manner by the relative proportion of these 

 gases in the air contained in the air-cells of 

 the lungs and in the blood, and by the quan- 

 tities of atmospheric air and blood trans- 

 mitted through the respiratory apparatus. 



We have seen, from the experiments of 

 Vierordt, that when the air is rapidly renewed 

 in the lungs, though the percentage of car- 

 bonic acid in the expired air is diminished, 

 yet the total amount of this gas thrown off 

 from the lungs within a given time is pro- 

 portionally increased ; while, on the other 

 hand, when the respirations are diminished 

 below the natural standard, though the per- 

 centage of carbonic acid in the expired air is 

 increased, yet the total quantity thrown off 

 from the lungs in a given time is propor- 

 tionally diminished. When the atmospheric 

 air in the lungs is rapidly renewed by an in- 

 creased frequency of the respiratory move- 

 ment, the diffusion of the oxygen in the 

 higher, and of the carbonic acid in the deeper, 

 parts of the air tubes will proceed more 

 rapidly, and the air in the deeper parts or in 

 the air-cells will contain a less percentage of 

 carbonic acid, and a greater percentage of 

 oxygen, than when the respirations are carried 

 on with the usual frequency and force. This 

 diminution of the usual quantity of carbonic 

 acid gas and increase of oxygen in the deeper 

 parts of the lungs will accelerate the inter- 

 change between the oxygen of the air and the 

 carbonic acid of the blood, provided the blood 

 holds its normal amount of free gases in solu- 

 tion, and a larger quantity than usual of car- 

 bonic acid will be separated from the blood at 



* The passage of gases through moist membranes 

 is not simple diffusion, as it is influenced by the 

 solubility of these gases in the fluids of "the mem- 

 branes. In the case of respiration it will also pro- 

 bably be affected by the attractive force of the 

 constituents of the blood for the gases. The relative 

 rapidity of the passage of different gases through 

 membranous septa, as observed in the experiments 

 of Dr. Faust and of Mr. Mitchell (American Journal 

 of the Medical Sciences, Nov. 1830), and by other 

 experimenters, is not in accordance with the law of 

 the diffusion of gases, as determined from experi- 

 ments upon their diffusive velocities through porous 

 septa into the atmospheric air, and through minute 

 apertures in a thin plate into a vacuum. When a 

 bladder filled with oxygen gas is introduced into a 

 vessel full of carbonic acid gas, the latter passes so 

 much more rapidly through the coats of the bladder 

 than the former, that the bladder becomes gradually 

 distended, and at last may burst. In these last expe- 

 riments, equally as in those of Graham, the condi- 

 tions under which the diffusion of the gases occurs, 

 are not the same as those in respiration ; and we 

 find the carbonic acid gas passing in greater quantity 

 through the organic membranes than the oxygen, 

 the reverse of what takes place in respiration. 



