CHAPTEE XIII. 



CHANGES OF THE BLOOD IN RESPIRATION. 



Difference in color between arterial and venous blood Comparison of the gases 

 in venous and arterial blood Observations of Magnus Analysis of the blood 

 for gases Relative quantities of oxygen and carbonic acid in venous and ar- 

 terial blood Nitrogen of the blood Condition of the gases in the blood 

 Mechanism of the interchange of gases between the blood and the air in the 

 lungs General differences in the composition of arterial and venous blood. 



IT is to be expected that the blood, receiving on the one 

 hand all the products of digestion, and on the other the 

 products of destructive assimilation or decay of the tissues, 

 connected with the lymphatic system, and exposed to the 

 action of the air in the lungs, should present important dif- 

 ferences in composition in different parts of the vascular 

 system. 



In the first place, there is a marked difference in color, 

 composition, and properties, between the blood in the arte- 

 ries and in the veins; the change from venous to arterial 

 blood being effected almost instantaneously in its passage 

 through the lungs. The blood which goes to the lungs is a 

 mixture of the fluid collected from all parts of the body ; 

 and we have seen that it presents great differences in its 

 composition in different parts of the venous system. In 

 some veins it is almost black, and in some nearly as red as in 

 the arteries. In the hepatic vein it contains sugar, and its 

 fibrin, albumen, and corpuscles are diminished ; in the portal 



