EXCHANGE OF OXYGEN AND CARBONIC ACID. 583 



oxygen is exhausted ; thus showing that the respirability of air does not de- 

 pend so much upon the proportion of oxygen it contains, as upon its freedom 

 from contamination with carbonic acid or other poisonous gases. 



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

 is easily explained upon physical principles. If the Blood come to the Lungs 

 charged with Carbonic acid, and is exposed in their cells to the influence of 

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

 and exosmose of gases will take place, according to certain fixed laws.* The 

 Carbonic acid of the blood will pass out, to be replaced by Oxygen and Ni- 

 trogen ; 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 blood also contain Nitrogen as 

 well as carbonic acid, this also will pass out, to be replaced by the Oxygen of 

 the air. Thus, there will be a continual Exosmose of Carbonic acid and Ni- 

 trogen, and a continual Endosmose of Oxygen and Nitrogen. The exhalation 

 and absorption of Nitrogen appear usually to balance each other; so that the 

 amount of this gas in the respired air undergoes little or no change. But the 

 case is different in regard to the exchange of Carbonic acid and Oxygen. 

 According to the law of mutual diffusion of gases, the volume of Oxygen that 

 ffs taken in, should exceed that of the Carbonic acid which passes out, in the 

 proportion of 1174 to 1000. This calculation, deduced from the relative 

 densities of the two gases, corresponds so closely with the actual results of 

 experiments upon the respiration of Man, that the interchange may be regarded 

 as always taking place in accordance with the law of mutual diffusion ; so 

 that, from the amount of Carbonic acid exhaled, the quantity of Oxygen ab- 

 sorbed may be readily calculated.! Now as Carbonic acid contains its own 

 bulk of Oxygen, it follows that the amount of Oxygen absorbed exceeds that 

 which is given off, by 174 parts in every 1000 ; so that a quantity of oxygen, 

 equal to more than one-sixth of that which is converted into Carbonic acid, is 

 employed in the system for other purposes. It appears probable that a part 

 of this additional Oxygen is made to combine with Hydrogen furnished by the 

 food or by the disintegration of the tissues ; and that the water thus generated 

 forms part of that exhaled from the lungs; whilst another part will be applied 

 to the oxidation of the Sulphur and Phosphorus, which are taken in as such 

 in the food, and which, after forming part of the solid tissues, are excreted in 

 the condition of Sulphuric and Phosphoric acids, chiefly through the kidneys. 



767. The absolute quantity of Carbonic Acid exhaled from the Lungs is 

 liable to variation from so many sources, that no fixed standard can be 

 assigned for it. The mean of a great number of observations, however, 

 made in different modes, and under different circumstances, would give about 

 160 grains of Carbon per hour as the amount set free by a well-grown adult 

 man, under ordinary circumstances. Taking this as the average of the 

 twenty-four hours, the total quantity of Carbon thus daily expired from the 

 Lungs would be 3840 grains, or 8 oz. Troy. The chief causes of variation 

 are, the Temperature of the surrounding medium, Age, Sex, Development 

 of the body, state of .Health or Disease, Muscular Exertion or Repose, Sleep 

 or Watchfulness, Period of the Day, and state of the Digestive process. 

 These will now be considered in detail. 



a. Temperature of surrounding Medium. The amount of Carbonic Acid exhaled by 

 warm-blooded animals is greatly increased by external Cold, and diminished by Heat ; as is 

 shown by the following results of comparative experiments upon the quantity set free by 

 the same animals, at low, medium, and high temperatures, in periods of an hour (Letellier). 



See Principles of General and Comparative Physiology, 437 9. 

 t Valentin's Lehrbuch der Physiologic, vol. i., pp. 507-580. 



