380 RESPIRATION [CH. XXIV. 



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 ; 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, 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 oxyhsemoglobin 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 combina- 

 tion 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. The instrument used in such deter- 

 minations is called an aerotonometer (see p. 381). 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 

 to form disodium phosphate (Na 2 HP0 4 ). 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. 



