GENERAL CHARACTERISTICS OF THE BLOOD 161 



through the capillaries of this organ has enabled it to undergo a 

 chemical change, which, as will become evident later, consists in an 

 absorption of oxygen and a loss of carbon dioxid. As far as the color- 

 ing substance, hemoglobin, is concerned, its sojourn in the lungs ha^ 

 permitted it to acquire a certain amount of oxygen in place of that 

 which has previously been turned over to the tissues. For this reason, 

 the color of the blood may be employed as in index betraying the ex- 

 tent of the molecular union which has been effected between the hemo- 

 globin and the oxygen of the respired air. In other words, it betrays 

 the degree of aeration or oxygenation of the blood. 



This explanation also accounts for the differences in the color of 

 the blood in different parts of the vascular system. The most decided 

 contrast is noted centrally in the large arteries and veins, while periph- 

 erally all intermediary shades between crimson and purple may be 

 observed in accordance with the state of oxygenation of the blood in the 

 region examined. In this connection mention should also be made of 

 the fact that glandular and muscular activity is always associated 

 with a greater flow of blood through the active organ, in consequence 

 of which its venous discharge often assumes a color more like that of 

 its arterial supply. Thus, it may be noted at times that the blood 

 returned from the kidney, is much lighter than that of the inferior 

 vena cava with which it eventually intermingles. A more copious 

 supply of arterial blood is required by an organ or tissue when acti- 

 vated, because it needs more material for purposes of oxidation. 



The appearance of the different exposed regions of the body, such 

 as the lips, conjunctiva, nails, and mucous membrane of the mouth, 

 is frequently employed as an index of the degree of aeration of the 

 blood. Provided that the capacity of the blood-vessels of these parts 

 has not been materially altered, a pink color signifies an adequate 

 supply of oxygen, whereas a bluish hue suggests a poverty in oxygen 

 and a superfluity in carbon dioxid. A dark blue color may readily 

 be imparted to the circulating arterial blood by temporarily suspending 

 the respiratory movements, or by permitting the animal to breathe 

 air charged with carbon dioxid. Outside the body similar results may 

 be obtained by removing the oxygen from the arterial blood by means 

 of an air pump, or with the help of a reducing agent. During asphyxia 

 the blood assumes an almost chocolate-brown color. This change can 

 also be brought about locally by obstructing the venous return in a 

 mechanical way. As the oxygen is gradually abstracted from the 

 blood, the part experimented upon assumes a much darker appear- 

 ance. Marked alterations also follow the administration of poisonous 

 substances. Thus, carbon monoxid gives rise to a cherry-red color, 

 while phenylhydrazin produces a dark-brown appearance. Quite 

 similarly, venous blood may be made to assume a much lighter color 

 by instituting vigorous respirations, or by shaking the shed blood 

 in atmospheric air. Obviously, this change is brought about by an 

 absorption of oxygen. 

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