VARIATIONS IN THE COMPOSITION OF HEALTHY BLOOD 127 



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 purple variety (deoxidized, or purple 

 hemoglobin). 



Arterial blood coagulates somewhat more quickly. 



Arterial blood contains more oxygen than venous, and less carbonic 

 acid. 



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 

 be seldom exactly the same. Speaking generally, and without considering 

 the sugar and other soluble matters which may have been absorbed from 

 the alimentary canal, this blood appears to be deficient in solid matters, 

 especially in colored corpuscles, owing to dilution by the quantity of water 

 absorbed, to contain an excess of proteid matter, and to yield a less tenacious 

 kind of fibrin than that of blood generally. 



The blood of the portal vein, combining the peculiarities of its two factors, 

 the splenic and mesenteric venous blood, is usually of lower specific gravity 

 than blood generally, is more watery, contains fewer colored corpuscles, 

 more proteids, and yields a less firm clot than that yielded by other blood, 

 owing to the deficient tenacity of its fibrin. 



Guarding (by ligature of the portal vein) against the possibility of an 

 error in the analysis from regurgitation of hepatic blood into the portal vein, 

 recent observers have determined that hepatic venous blood contains less 

 water, proteids, and salts than the blood of the portal veins ; but that it yields 

 a much larger amount of extractive matter, in which is one constant element, 

 namely, grape-sugar, which is found, whether saccharine or farinaceous 

 matter has been present in the food or not. 



