SECTION V 



THE QUANTITY AND COMPOSITION OF THE 

 BLOOD IN MAN 



A. THE TOTAL QUANTITY OF BLOOD IN THE BODY 



THE amount of blood contained in the body can be estimated by Welcker's 

 method. It is not sufficient simply to open one of the blood-vessels and 

 allow the animal to bleed to death, because it is not possible in this way 

 to obtain the whole of the blood present in the body, and the blood which 

 is obtained gradually becomes more dilute in consequence of absorption 

 from the tissue spaces as bleeding continues. A small sample of blood is 

 therefore taken from a blood-vessel and diluted 100 times with distilled 

 water to serve as a standard of comparison. The animal is then bled from 

 a cannula placed in a large artery, while at the same time normal salt 

 solution is led into a vein so as to maintain the vascular system as full as 

 possible and allow of its being washed out by the action of the heart. When 

 the heart ceases to beat, the blood-vessels are thoroughly washed out by 

 a stream of normal salt solution from the aorta. The animal is then minced 

 up thoroughly and extracted with distilled water, and filtered so as to 

 dissolve out the hsemoglobin still adherent to the tissues and especially 

 contained in the red marrow. These washings are mixed with the whole 

 diluted blood and the strength of the mixture in hemoglobin is compared 

 with that of the standard solution. In this way it is possible to estimate 

 the total hsemoglobin present in the body in terms of the sample and so 

 find the total amount of circulating fluid. It has been found that the dog 

 contains about 7-7 per cent, of his body weight as circulating blood, and 

 although smaller figures were obtained on other animals, such as the rabbit, 

 the number of one-thirteenth has been taken as applicable to man on the 

 basis of two observations made long ago on executed criminals. Haldane 

 has shown recently that this estimate is much too high, the average amount 

 of blood in man being only about 4-9 per cent, of the body weight, i.e. 

 about one-twentieth ; in some cases, as in fat individuals, it may be as 

 little as one-thirtieth. Since the determination of the total volume of the 

 circulating blood plays an important part in the consideration of the 

 pathology of certain diseases such as anaBmia and heart disease, the ingenious 

 method adopted by Haldane for this determination in the living animal 

 may be here described. The method depends on the fact that carbon 



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