RESPIRATION 269 



The following was estimated to be the distribution of C0 2 in 

 100 c.c. of arterial blood, the pressure of C0 2 being equal to 

 30 mm. Hg: Physically absorbed, 1-9 c.c. (0-7 c.c. in corpuscles); 

 as bicarbonate, 12 c.c.; as organic compounds in plasma, 11-8 c.c.; 

 in haemoglobin, about 7-5 c.c.; as bicarbonate in corpuscles, about 

 6-8 c.c. total, 40 c.c. 



The Pressure of the Gases in the Alveolar Air, Blood, and Tissues. 

 In order that we may discuss the processes by which the interchange 

 of gases takes place between the alveolar air and the blood in the 

 lungs, and between the blood in the capillaries and the tissue fluids, 

 it is necessary that we know the pressure of the gases in these various 

 parts. It is possible that such an interchange is a physical process 

 following well-known physical laws; on the other hand, it is possible 

 that, owing to the incessant demand of the tissues for oxygen, and the 

 need of freeing them from excess of C0 2 , that secretory processes, 

 unexplainable by known physical laws, may play a part in this gaseous 

 interchange. Such a process would be the passage of a gas from a 

 region of lower concentration, through a layer of. cells wet with 

 tissue lymph, to a region of higher concentration. If this took 

 place, it Avould be fair to assume that the intervening cells and the 

 blood were in some way aiding the process by virtue of their own 

 special vital metabolism. It might be said that the cells were actually 

 secreting the gas into the region of higher concentration, just as the 

 kidney secretes urea from the blood, where it may be 1 per 1,000, 

 into the urine, where it may be 20 per 1,000. 



Both views have been advocated, and are still advocated, in regard 

 to the processes of respiration. One school of thought believes that 

 all the processes may be explained by known physical laws; another 

 school holds that, under some circumstances at any rate, the pro- 

 cesses of gaseous interchange are aided by active intervention on the 

 part of the body cells, particularly those of the alveoli. To ascertain 

 the merits of these conflicting views, it is necessary to know the partial 

 pressure of the gases concerned in this interchange in the various parts 

 of the body, and also to study the accuracy of the methods by which 

 these pressures are calculated. 



Partial Pressure of Gases in the Blood. Many experiments have 

 been made by various researchers to determine exactly the partial 

 pressure of the gases in both arterial and venous blood. In general, 

 the method employed has been to bring the circulating blood into 

 contact with a gas mixture of known composition, and by analysis at 

 the end of the experiment to ascertain the composition of this mixture. 

 The blood, thoroughly shaken with the mixture, gets into equilibrium 

 with it, and the blood gases then have the same partial pressure as 

 those found in the mixture. The instruments employed for this 

 purpose are known as " aerotonometers. " The microtonometer is 

 the apparatus now most generally employed. In this apparatus 

 (Fig. 141), a bubble of air (2) of 2 millimetres diameter is brought into 

 contact with the blood coming from an artery or vein. The blood 

 enters by a fine point (1), and keeps the bubble in constant movement, 



