CONDITION OF CO2 IN THE BLOOD 155 



sented in the CO2 exhaled from the lungs and skin, it is 

 certain that when various of the food stuffs are broken down 

 at least a part of it is appropriated by hydrogen to form 

 water. 



Source of Exhaled Carbon Dioxide. The increase of CO2 

 in expired air over the small amount contained in inspired 

 air is derived from the venous blood circulating through the 

 lungs. It exists in that blood under a constant tension, as is 

 demonstrated by its escape when the blood is placed in a 

 vacuum. The total amount escapes when the blood intact is 

 placed in vacua : when the corpuscles alone are so treated 

 they yield up all their OCte, though it is small in amount; 

 but the plasma alone in vacuo yields a less amount than 

 when it contains corpuscles. If now corpuscles be added to 

 the plasma the total amount of OO2 is forthcoming. The 

 corpuscles must, therefore, act as an acid causing the liber- 

 ation of this gas from the plasma. It is probably the hemo- 

 globin, or oxyhemoglobin, which has this effect, though in 

 the laboratory the phosphates and certain proteids of the 

 corpuscles produce a like reaction when brought in contact 

 with the carbonates and bicarbonates of soda. 



Condition of CCte in the Blood. About 5 per cent, of the 

 total amount of OCte in venous blood is in simple solution 

 in the plasma; about 75-85 per cent, is in loose chemical 

 combination in both corpuscles and plasma; the remaining 

 10-20 per cent, is in comparatively stable combination in the 

 plasma. Of the 75-85 per cent., by far the largest part is in 

 the plasma, probably in a condition of loose association with 

 sodium to form carbonates and bicarbonates ; the small part 

 in the corpuscles may exist in a similar state, but it is now 

 thought to exist in combination with the proteid portion of 

 hemoglobin. The total 75-85 per cent, in corpuscles and 

 plasma is so loosely combined that the mere diminution in 

 pressure in the lungs is probably sufficient to liberate it. 

 The 10-20 per cent, in firm chemical combination is that part 

 which cannot be extracted from plasma alone in vacuo, but 



