188 THE BODY AT WORK 



the lungs and readily parting with it to the tissues is easily 

 and completely explained by the property which haemoglobin 

 possesses of forming an unstable compound with this gas. 



It is quite otherwise with regard to the liberation of carbonic 

 acid-. The problems presented by the solution of this gas in 

 blood and its elimination in the lungs are difficult to solve. 

 Less than one-tenth of the volume of carbonic acid which 

 can be extracted from blood by the air-pump is simply in solu- 

 tion. The remainder is in loose chemical combination, the 

 chief agents in holding it being the alkaline carbonates which 

 the plasma contains. With an excess of carbonic acid they 

 form acid carbonates, which give up carbonic acid and again 

 become normal carbonates in the lungs. About one-third of 

 the carbonic acid is, however, held by the blood-corpuscles 

 partly in virtue of their alkaline carbonates and phosphates, 

 partly in combination with their globulin. The affinity of these 

 several vehicles for carbonic acid is sufficient to enable them to 

 take it from the lymph, and to hold it while the blood is in the 

 veins. When they reach the capillaries of the lungs, they part 

 with their burden of carbonic acid to the air. It is in connec- 

 tion with this renunciation that certain difficulties remain to be 

 explained. The carbonic acid is given up with greater readi- 

 ness than our knowledge of the chemistry of the compounds 

 into which it enters in the blood would lead us to expect. 



Why does oxygen enter blood as it circulates through the 

 lungs, and carbonic acid leave it ? We have referred to the 

 immense surface which the lungs expose to air. If a soap- 

 bubble be filled with a mixture of oxygen, nitrogen, and 

 carbonic acid, and if the oxygen be in smaller proportion, and 

 the carbonic acid be in greater proportion, than in the air of the 

 room, oxygen will enter the bubble, and carbonic acid will leave 

 it, by diffusion. If, instead of filling a bubble with gas, we fill a 

 bladder with water charged with carbonic acid, but destitute 

 of dissolved oxygen, a similar exchange with the gases of the 

 air will take place. It is merely a question of " gaseous 

 tension." The tension of the gases in the lungs is measured by 

 passing a small tube down the trachea, and along one of the 

 two chief bronchi until it becomes blocked in a bronchus just 

 large enough to admit it. Respiration is carried on under 

 normal conditions in the remainder of the lung ; but in the lobe 



