RESPIKATION 129 



methylene blue ; on exposure to the air this is oxidised, and the 

 blue pigment is thus regenerated. 



Carbonic Acid in the Blood, What has been said for oxygen holds 

 good in the reverse direction for carbonic acid. Compounds are 

 formed in the tissues where the tension of the gas is high : these pass 

 into the lymph, then into the blood, and in the lungs the compounds 

 undergo dissociation, carbonic acid passing into the alveolar air where 

 the tension of the gas is comparatively low, though it is greater here 

 than iu the expired air. 



The relations of this gas and the compounds it forms are more 

 complex than in the case of oxygen. 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. If plasma or serum is 

 similarly treated the gas comes off, but from 10 to 18 per cent, of the 

 carbonic acid is fixed that is, the addition of some stronger acid, 

 such as phosphoric acid, is necessary to displace it. Fresh red cor- 

 puscles will, however, take the place of the phosphoric acid, and thus 

 it has been surmised that oxyhaemoglobin has the properties of an 

 acid. 



One hundred volumes of venous blood contain forty-six volumes 

 of carbonic acid. Whether this is in solution or in chemical com- 

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

 blood. One hundred volumes of blood plasma would 4 dissolve more 

 than an equal volume of the gas at atmospheric pressure[if its solu- 

 bility in plasma were equal to that in water. 1 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 about 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. Hence the remainder of the 

 gas, 95 per cent., is in a condition of chemical combination. The 

 chief compound appears to be sodium bicarbonate. 



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 (Na 2 C0 3 ), and 

 mono-sodium phosphate (NaH 2 P0 4 ) ; but if the carbonic acid is 

 diminished, the phosphoric acid obtains the greater share of sodium 



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

 in pure water. 



