102 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



it has been surmised that oxyhaemoglobin has the propei'ties of an 

 acid. 



One hundred volumes of venous blood contain 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 

 more than an equal volume of the gas at atmospheric pressure, 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 it proves to be only equal to 5 per cent, of an atmosphere. 

 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, 95 per cent., is in a condition of chemical 

 combination. The chief compound appears to be sodium bicar- 

 bonate. 



The carbonic acid and phosphoric acid of the blood are in a state 

 of constant struggle for the possession of the sodium. The salts 

 formed by these two acids depend on their relative masses. If 

 carbonic acid is in excess, we get sodium carbonate (Na2C03), and 

 mono-sodium phosphate (NaH2P04) ; but if the carbonic acid is 

 diminished, the phosphoric acid obtains the greater share of sodium 

 to form disodium phosphate (Na2HP04). In this way, as soon as 

 the amount of free carbonic acid diminishes, as in the lungs, the 

 amount of carbonic acid in combination also decreases ; whereas in 

 the tissues, where the tension of the gas is highest, a large amount is 

 taken up into the blood, where it forms sodium bicarbonate. 



The tension of the carbonic acid in the tissues is high, but one 

 cannot give exact figures ; we can measure the tension of the gas in 

 certain secretions ; in the urine it is 9, in the bile 7 per cent. The 

 tension in the cells themselves must be higher still. 



The following figures (from Predericq) give the tension of carbonic 

 dioxide in percentages of an atmosphere : — 



Tissues . . . . . 5 to 9 ] 



Venous blood .... 3*8 to 5'4 [in dog. ¥ 



Alveolar air .... 2'8 i v 



External air . . . . 0'04 



The arrow indicates the direction in which the gas passes, namely, 

 in the direction of pressure from the tissues to the atmosphere. 



' To be exact, the solubility of carbon dioxide in plasma is a little less than 

 in pure water. 



