THE CHEMISTRY OF RESPIRATION 1191 



H or OH ions in blood than in distilled water. The 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 combi- 

 nation 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 propor- 

 tionate 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 off 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 pro- 

 portions 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. 



