SEC. 4. THE QUANTITY OF BLOOD, AND ITS 

 DISTRIBUTION IN THE BODY. 



38. The quantity of blood contained in the whole vascular 

 system is a balance struck between the tissues which give to, and 

 those which take away from the blood. Thus the tissues of the 

 alimentary canal largely add to the blood water and the material 

 derived from food, while the excretory organs largely take away 

 water and the other substances constituting the excretions. Other 

 tissues both give and take ; and the considerable drain from the 

 blood to the lymph spaces which takes place in the capillaries is 

 met by the flow of lymph into the great veins. 



From the result of a few observations on executed criminals it 

 has been concluded that the total quantity of blood in the human 

 body is about -^th ^ ^ ne body weight. But in various animals, 

 the proportion of the weight of the blood to that of the body has 

 been found to vary very considerably in different individuals ; and 

 probably this holds good for man also, at all events within certain 

 limits. 



In the same individual the quantity probably does not vary 

 largely. A sudden drain upon the water of the^ blood by great 

 activity of the excretory organs, as by profuse sweating, or a sudden 

 addition to the water of the blood, as by drinking large quantities 

 of water or by injecting fluid into the blood vessels, is rapidly 

 compensated by the passage of water from the tissues to the blood 

 or from the blood to the tissues. As we have already said the 

 tissues are continually striving to keep up an average composition 

 of the blood, and in so doing keep up an average quantity. In 

 starvation the quantity (and quality) of the blood is maintained 

 for a long time at the expense of the tissues, so that after some 

 days deprivation of food and drink, while the fat, the muscles, and 

 other tissues have been largely diminished, the quantity of blood 

 remains nearly the same. 



