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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, so as to dissolve out 

 the haemoglobin still adherent to the tissues and especially contained in 

 the red marrow. These washings are filtered and 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. According 

 to Haldane this estimate is 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 

 ch as anaemia and heart disease, the ingenious method adopted by Haldane 

 or this determination in the living animal may be here described. The 

 ethod depends on the fact that carbon monoxide gas when inhaled 



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