RESPIRATION. 277 



influence of the pressure of muscles: the force by which it is accom- 

 plished,, and the course of the blood are alike simple. 



Changes in the Blood. The most obvious change which the blood of 

 the pulmonary artery undergoes in its passage through the lungs is 1st, 

 that of color, the dark crimson of venous blood being exchanged for the 

 bright scarlet of arterial blood. The cause of this has been already 

 shown to be that the arterial blood contains a greater quantity of scarlet 

 or oxyhaemoglobin; 2d, and in connection with the preceding change it 

 gains oxygen ; 3d, it loses carbon dioxide. It was incidentally mentioned 

 in the Chapter on the Blood that the carbon dioxide which is carried 

 by the blood to be eliminated by the lungs is not simply dissolved in 

 the plasma. It is combined with some substance in the blood, and 

 when it is carried to the lungs this substance must undergo decomposi- 

 tion. What is the nature of the compound it forms is not known, but 

 it appears most likely that the gas is combined in the plasma with the 

 sodium carbonate which it contains. It has also been suggested that as 

 the carbon dioxide of the entire blood is more easily given up to the 

 vacuum of a mercurial air-pump than is the gas of the serum correspond- 

 ing to the blood taken, that the corpuscles of the blood exercise some 

 power in promoting the decomposition of the substance with which the 

 gas is combined in the plasma. The plasma or serum will not give up 

 the whole of its carbon dioxide until the addition of an acid, when the 

 last portion, 2 to 5 per cent, comes off, the entire blood gives up the 

 whole of its carbon dioxide to the action of the mercurial pump, and 

 does not require the action of an acid. It may be mentioned that, ac- 

 cording to some, the carbon dioxide is combined with proteid, either in 

 the plasma -or in the red blood-corpuscles; th, it becomes slightly 

 cooler; 5th, it coagulates sooner and more firmly, apparently containing 

 more fibrin. The oxygen absorbed into the blood from the atmospheric 

 air in the lungs is combined chemically with the haemoglobin of the 

 red blood -corpuscles. In this condition it is carried in the arterial blood 

 to the various parts of the body, and brought into near relation or con- 

 tact with the tissues. In these tissues, a certain portion of the oxygen,, 

 which the arterial blood contains, disappears, and a proportionate quan- 

 tity of carbon dioxide and water is formed. The venous blood, contain- 

 ing the new-formed carbon dioxide, returns to the lungs, where a portion 

 of the carbon dioxide is exhaled, and a fresh supply of oxygen is taken in. 



In what way these changes are brought about will be next discussed. 



Respiratory Changes in the Tissues. 



The changes which occur in the composition of the blood during its 

 circulation are believed to take place in the tissues, and particularly in 

 the muscles. The changes are, as we have just mentioned, chiefly the 



