10 68 PHYSIOLOGY 



reaction of blood as normally taken depends as a matter of fact on the 

 indicator which is used. Thus blood-plasma is acid to phenolphthalein, 

 but alkaline to litmus. On the other hand, the carbon dioxide and proteins 

 in combination with the sodium may be easily replaced by stronger acid 

 radicals, and if the carbon dioxide be allowed to escape, very little change 

 in the reaction, i.e. in the concentration of H or OH ions respectively, 

 will be produced. We may thus speak of blood-plasma' as actually neutral, 

 though potentially alkaline in that it can neutralise a considerable amount 

 of acid. This potential alkalinity is more important than the actual 

 alkalinity, since on it depends the power possessed by the blood of carrying 

 carbon dioxide from the tissues to the lungs. By adding a fixed acid to the 

 blood we may use up this potential alkalinity and finally arrive at a point at 

 which each addition of acid makes a proportionate increase in the actual 

 acidity, i.e. in the concentration of H ions. From this time forward, 

 however, the plasma has lost its power of binding carbon dioxide and can 

 carry this gas only in simple solution. On exposure of such plasma to carbon 

 dioxide the tension of the gas rapidly rises in the fluid, which becomes 

 saturated when only a relatively small amount of gas has entered into the 

 fluid. Any diminution of the so-called alkalinity of the blood or blood- 

 plasma will seriously impair the function of the blood in respiration. An 

 example of such a condition is the acidosis which occurs in diabetes, or in 

 any other form of carbohydrate starvation. 



EXCHANGE OF GASES IN THE LUNGS 



A fluid gives of? gas to or takes up gas from any other medium with which 

 it is in contact, according to the relative pressures of the gas. The question 

 arises whether the physical conditions in the lungs are such as to account for 

 the absorption of oxygen and the evolution of carbon dioxide by the blood in 

 its passage through these organs. In order to answer this question we must 

 know the partial pressures or tensions of oxygen and of carbon dioxide in the 

 alveolar air, in the venous blood coming to the lungs, and in the arterial 

 blood leaving the lungs. In the alveoli the pressures are given by the analysis 

 of alveolar air. The determination of the gaseous tensions in the blood 

 presents, however, considerable difficulty. It is necessary to bring the 

 blood in contact with gaseous mixtures containing various proportions 

 of the gas whose tension in the blood it is desired to measure. By making 

 various experiments a gaseous mixture will be found with which the 

 blood is in equilibrium. If we know beforehand the amount of gas in 

 this mixture, we know its tension and therefore the tension of the gas in 

 the liquid. 



Pfliiger's aerotonometer (Fig. 501) consists of two glass tubes, R and R, contained in a 

 vessel filled with water at the temperature of the body. The upper ends of the tubes 

 are connected by the tube a with the artery or vein in which it is desired to estimate 

 the tension of the blood-gases. If, for instance, we wish to determine the tension of 

 C0 2 in venous blood, where we expect the tension to amount to about 4 per cent, of an 

 atmosphere, one tube R is filled with a gaseous mixture containing 3 per cent. CO 2 , and 



