CH. XXVI.] GASES OP ARTERIAL AND VENOUS BLOOD 371 



considerable degree of reduction when it is in contact with fluid 

 containing oxygen at a pressure of 20 to 30 mm. of mercury. 



Carbonic Acid in Blood. If blood is divided into plasma and 

 corpuscles, it will be found that both yield carbonic acid, but the 

 yield from the plasma is the greater. If we place blood in a vacuum 

 it bubbles, and gives out all its gases; addition of a weak acid 

 causes no further liberation of carbonic acid. When plasma or 

 serum is similarly treated the gas also comes off, but about 5 per 

 cent, of the carbonic acid is fixed that is, it requires the addition of 

 some stronger acid, such as phosphoric or tartaric acid, to displace it. 



One hundred volumes of venous blood contain about forty-six 

 volumes of carbonic acid. Whether this is in solution or in chemical 

 combination is determined by ascertaining the tension of the gas in the 

 blood. One hundred volumes of blood plasma would dissolve about 

 an equal volume of the gas from an atmosphere of carbonic acid, if its 

 solubility in plasma were equal to that in water. If, then, the carbonic 

 acid were in a state of solution, its tension would be very high, but, as 

 we have seen, it proves to be only equal to about 5 per cent, of an 

 atmosphere (that is approximately equal to 40 mm. of mercury). 

 This means that when venous blood is brought into an atmosphere 

 containing 5 per cent, of carbonic acid, the blood neither gives off 

 any carbonic acid nor takes up any from that atmosphere. Hence 

 the remainder of the gas, 41 per cent., is in a condition of chemical 

 combination. 



Of the carbonic acid present in chemical composition, the greater 

 part is present as sodium bicarbonate (NaHC0 3 ). About a third is 

 in loose combination with the proteins of the blood, and a small 

 quantity exists as normal sodium carbonate (Na 2 C0 3 ). 



Differences bet-ween Arterial and Venous Blood. The aver- 

 age quantity of gas that can be extracted from arterial and venous 

 blood respectively is : 



Arterial blood. Venous blood. 



Oxygen 20 8 to 12 



Nitrogen .... 1 to 2 1 to 2 



Carbonic acid ... 40 46 to 50 



It will be noticed that the amount of nitrogen which is simply 

 dissolved in the blood from the air is small in amount. It has no 

 physiological significance, and is the same in both varieties of blood. 

 The important distinction between arterial and venous blood is in 

 the other two gases, and as the table shows, on the average every 

 100 c.c. of blood which pass through the lungs gain 8 c.c. of oxygen 

 and lose 6 c.c. of carbonic acid. We will now study the mechanism 

 by which this gaseous interchange is effected. 



