144 THE BLOOD 



for some weeks after birth. The quantity of solid matter then falls during 

 childhood below the average, rises during adult life, and in old age falls again. 



Diet. Such differences in the composition of the blood as are due to the 

 temporary presence of various matters absorbed with the food and drink, 

 as well as the more lasting changes which must result from generous or poor 

 diet, respectively, need be here only referred to. 



Effects of Bleeding. The result of bleeding is to diminish the specific 

 gravity of the blood, and so quickly that in a single venesection the portion 

 of blood last drawn has often a less specific gravity than that of the blood that 

 flowed first. This is, of course, due to absorption of fluid from the tissues 

 of the body. The physiological import of this fact, namely, the instant 

 absorption of liquid from the tissues, is the same as that of the intense thirst 

 which is so common after either loss of blood or the abstraction from it of 

 watery fluid, as in cholera, diabetes, and the like. 



For some little time after bleeding, the want of colored corpuscles is well 

 marked, but with this exception no considerable alteration seems to be 

 produced in the composition of the blood for more than a very short time; 

 the loss of the other constituents, including the colorless corpuscles, being 

 very quickly repaired. 



Variations in Different Parts of the Body. The composition of the blood, 

 as might be expected, is found to vary in different parts of the body. Thus 

 arterial blood differs from venous; and although its composition and general 

 characters are uniform throughout the whole course of the systemic arteries, 

 they are not so throughout the venous system, the blood contained in some 

 veins differing markedly from that in others. 



Differences between Arterial and Venous Blood. The differences between 

 arterial and venous blood are these: 



Arterial blood is bright red, from the fact that almost all its hemoglobin 

 is combined with oxygen, oxyhemoglobin, while the purple tint of venous 

 blood is due to the deoxidation of a certain quantity of its oxyhemoglobin, 

 and its consequent reduction to the hemoglobin. 



Arterial blood coagulates somewhat more quickly. 



Arterial blood contains more oxygen than venous and less carbon 

 dioxide gas. 



Some of the veins contain blood which differs from the ordinary standard 

 considerably. These are the Portal, the Hepatic, and the Splenic veins. 



Portal Blood. The blood which the portal vein conveys to the liver is 

 supplied from two chief sources; namely, from the gastric and mesenteric 

 veins, which contain the soluble elements of food absorbed from the stomach 

 and intestines during digestion, and from the splenic vein. It must, there- 

 fore, combine the qualities of the blood from each of these sources. 



The blood in the gastric and mesenteric veins will vary much according 

 to the stage of digestion and the nature of the food taken, and can therefore 



