150 CHANGES OF AIR AND BLOOD IN RESPIRATION. 



substances, are capable of relieving the system of some of these by exhalation 

 when they find their way into the veins. 



Exhalation of Nitrogen. The most accurate direct experiments, particu- 

 larly those of Regnault and Reiset, show that the exhalation of a small quan- 

 tity of nitrogen is a nearly constant respiratory phenomenon. As the result 

 of a large number of experiments, these observers came to the conclusion that 

 when animals are subjected to their habitual regimen, they exhale a quantity 

 of nitrogen equal in weight to y^ or -fa of the weight of oxygen consumed. 

 In birds, during inanition, they sometimes observed an absorption of nitrogen, 

 but this was rarely seen in mammals. Boussingault, estimating the nitrogen 

 taken into the body and comparing it with the entire quantity discharged, 

 arrived at the same results in experiments upon a cow. Barral, by the same 

 method, confirmed these observations by experiments on the human subject. 

 Notwithstanding the conflicting testimony of physiologists, there can be little 

 doubt that under ordinary physiological conditions, there is an exhalation of 

 a small quantity of nitrogen by the lungs. 



CHANGES OF THE BLOOD IN RESPIRATION (H^MATOSIS). 



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 disassimilation, or 

 wear of the tissues, connected with the lymphatic system, and exposed to the 

 action of the air in the lungs, should present important differences in compo- 

 sition 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 arteries and in the veins ; the change 

 from venous to arterial blood being effected almost instantaneously in its pas- 

 sage through the lungs. The blood which goes to the lungs is collected from 

 all parts of the body and presents great differences in its composition in dif- 

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

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

 constituents and the corpuscles are diminished ; in the portal vein, during di- 

 gestion, it contains matters absorbed from the alimentary canal ; and finally, 

 there is every reason to suppose that parts which require different substances 

 for their nutrition and produce different excrementitious matters exert differ- 

 ent influences on the constitution of the blood which passes through them. 

 After this mixture of different kinds of blood has been collected in the right 

 side of the heart and passed through the lungs, it is returned to the left side 

 and sent to the system, thoroughly changed and renovated, and as arterial 

 blood, it has a nearly uniform composition. The change, therefore, which 

 the blood undergoes in its passage through the lungs, is the transformation 

 of the mixture of venous blood from all parts of the organism into a fluid of 

 uniform character which is capable of nourishing every tissue and organ of 

 the body. 



The capital phenomena of respiration, as regards the air in the lungs, are 

 loss of oxygen and gain of carbon dioxide, the other phenomena being com- 

 paratively unimportant. As the blood is capable of absorbing gases, the 



