26p ESSENTIALS OF PHYSIOLOGY. 



tension only about '2^ volumes of carbonic acid per 100 volumes of 

 blood; but since 100 volumes of blood, when exposed to a vacuum, 

 give off from 40 to 48 volumes of carbonic acid, the bulk of this must be 

 in a state of chemical combination. It is combined partly as sodium 

 bicarbonate, partly with the proteins of the blood plasma, and partly 

 with haemoglobin, the proportions in inorganic and organic combination 

 being approximately equal. The proteins in the plasma can act either 

 as weak acids or as weak alkalies, and as the blood passes through the 

 lungs it loses carbonic acid and the proteins combine with the sodium 

 thus set free ; when the blood takes up carbonic acid from the tissues, 

 the combination between sodium and protein is broken, and the carbonic 

 acid unites partly with the sodium and partly with the protein. In 

 this way, the reaction of the blood remains practically constant, in 

 spite of the varying amount of carbonic acid which it contains. 



Owing to this action of protein, when blood is exposed to a vacuum the 

 whole of the carbonic acid which it contains is evolved, whereas when a 

 pure solution of sodium bicarbonate is exposed to a vacuum only half 

 the carbonic acid is evolved. In the former case the sodium enters 

 into combination with the protein ; in the latter it forms sodium 

 carbonate, which is stable in vacuo. 



The dissociation curve of carbonic acid in blood, when exposed to 

 varying partial pressures of carbonic acid in the surrounding air, has 

 the same general form as that of oxyhsemoglobin, the dissociation being 

 most rapid when the partial pressure of carbonic acid varies from 

 to 30 mm. Hg. Oxygenated blood gives off its carbonic acid more 

 readily than deoxygenated blood. 



The blood also contains about 2| volumes of carbonic acid per 

 100 volumes of blood in solution. 



THE EXCHANGE OF GASES IN THE LUNGS. 



The air in the lungs is separated from the blood by the walls of the 

 pulmonary capillaries and by the alveolar epithelium ; and each of 

 these membranes is extremely thin. Two theories have been held as 

 to the means by which oxygen passes from the alveoli into the blood 

 and carbonic acid from the blood into the alveoli. On the one hand, it 

 has been supposed that the tension of oxygen in arterial blood is higher 

 than that in the alveolar air, and that the pulmonary epithelium must 

 therefore secrete oxygen into the blood. On the other hand, it is 

 widely held that the tension of oxygen in arterial blood is always lower 

 than that in the alveolar air, and that oxygen passes from the lungs 

 into the blood by the purely physical process of diffusion. These 

 divergent views arose from the fact that the observers employed 



