CHANGES IN THE BLOOD IN RESPIRATION. 155 



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. Notwith- 

 standing the conflicting testimony of the older physiologists, there can now be no doubt 

 that, under ordinary physiological conditions, there is an exhalation by the lungs of a 

 small quantity of nitrogen. 



Changes of the Blood in Respiration (Hcematosis). 



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 decay of the tissues, con- 

 nected with the lymphatic system, and exposed to the action of the air in the lungs, 

 should present important differences 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 arteries 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 nitrogenized constituents and cor- 

 puscles are diminished ; in the portal vein, during digestion, it contains materials absorbed 

 from the alimentary canal ; and, finally, there is every reason to suppose that parts which 

 require different materials for their nutrition and produce different excrementitious prin- 

 ciples exert different 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, as far as can be ascertained, in all parts of the system. The change, there- 

 fore, 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 and sustaining the function of 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 carbonic acid, the other phenomena being accessory and comparatively un- 

 important. As the blood is capable of holding gases in solution, in studying the essential 

 changes which this fluid undergoes in respiration, we look for them in connection with 

 the proportions of oxygen and carbonic acid before and after it has passed through the 

 lungs. In respiration, the most marked effect on the venous blood is change in color. 



Difference in Color between Arterial and Venous Blood. "We have already considered 

 this in treating of the properties of the blood, and shall take up in this connection only 

 the cause of the remarkable change in the color of the blood in the lungs. This change 

 is instantaneous, and, long before the discovery of oxygen by Priestley, was recognized 

 by Lower, Goodwyn, and others, as due to the action of the air. 



The influence of air in changing the color of venous blood may be noted in blood 

 which has been drawn from the body, as is exemplified by the red color of that portion 

 of a clot, or the surface of defibrinated venous blood, which is exposed to the air. If we 

 cut into a clot of venous blood, the interior is almost black, but it becomes red on ex- 

 posure to the air for a very few seconds. 



We have been in the habit of illustrating the physiological influence of the air on 

 venous blood by the following simple experiment : Removing the lungs of an animal (a 

 dog) just killed, the nozzle of a syringe is secured in the pulmonary artery by a ligature, 

 and a canula, connected with a rubber tube which empties into a glass vessel, is secured 



