CH. xxiv.] GASES OF THE BLOOD. 379 



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 they undergo dissociation, carbonic acid passing into 

 the alveolar air, where the tension of the gas is comparatively 

 low, though it is greater here than in 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 ; addi- 

 tion 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, like phosphoric acid, 

 to displace it. Fresh red corpuscles will, however, take the place 

 of the phosphoric acid, and thus it has been surmised that 

 oxy haemoglobin 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 

 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 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 manes. 

 If carbonic acid is in excess, we get sodium carbonate (NaaCOs), 

 and mono-sodium phosphate (NaHjPO^ ; but if the carbonic acid 

 is diminished, the phosphoric acid obtains the greater share of 

 sodium to form disodium phosphate (NagHPO^. In this way, as 

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



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

 than in pure water. 



