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 con- 

 tinues. 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 haemoglobin 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 haemoglobin is 

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

 to estimate the total haemoglobin 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 circu- 

 lating blood plays an important part in the consideration of the 

 pathology of certain diseases such as anaemia and heart disease, the 



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