78 A TEXTBOOK OF PHYSIOLOGY 



The Amount of Blood. This varies according to the method by 

 which it is determined and for different animals. The following table 

 gives some of the results which have been obtained : 



Dog . . . . 

 Guinea-pig . . 

 Rabbit .. 

 Cat . . . , 

 Horse . . 



*\ of body -weight Pig . . . . -aViV of body-weight 



r,' a Bird . . iV 



^ Frog .. TT 



New-born child ." . j 1 ,, 



It is stated in man to vary from 3* to 4 litres. 



The old method (that of Welcker) of obtaining the amount of 

 blood of an animal was (1) to bleed the animal into a Aveighed flask, 

 (2) remove all traces of blood by perfusing the vessels thoroughly 

 with water, (3) chopping up the body with the exception of the in- 

 testines and washing the choppings in water. The washings are 

 measured and the amount of blood in them gauged by determining 

 the amount of haemoglobin by finding how much the blood must 

 be diluted to correspond in depth of tint to that of the washings. 

 This method is not very exact, since during the death from haemorrhage 

 the tissue fluids pass into the blood, giving too high a result. 



In man the amount has been ascertained by the carbon monoxide 

 method. In this method the subject breathes, through a tin of soda 

 lime to absorb exhaled C0 2 , in and out of a bag containing a known 

 volume of carbon monoxide mixed with a sufficiency of oxygen. 

 Carbon monoxide has a strong affinity for haemoglobin, and 

 combines with this, displacing oxygen. After sufficient time for 

 the whole of the CO to be absorbed a sample of the subject's 

 blood is taken and the percentage saturation of the blood with carbon 

 monoxide determined. The carbon monoxide gives the blood, suit- 

 ably diluted, a pink colour, and the determination is effected by 

 comparing the tint of (1) the sample, with (2) a sample of the subject's 

 normal blood, (3) a sample of the subject's blood saturated with CO, 

 all three samples being diluted 1 in 200. A standard carmine solution 

 is added to (1) and (2) till the tint of each equals that of (3). Suppose 

 twice as much carmine has to be added to (2) as to (1), then (1) is 

 hah* saturated with CO. The amount of O 3 or CO which can 

 combine with 100 c.c. of the subject's blood is found by the use of the 

 Haldane-Gowers hsemoglobinometer. The amount of CO absorbed 

 is known, and thus, if it be found that blood is 25 per cent, saturated 

 and the person has absorbed 150 c.c. of CO, it is obvious that all the 

 blood will require 600 c.c. of CO to saturate it. If it is found that 

 100 c.c. of the blood are saturated by 20 c.c. of carbon monoxide or 



oxygen, the total volume of the blood is - =3,000 c.c., etc. 



a\) 



By this method the amount of blood in man is reckoned to be 

 about -.Jjj of the body weight (^ for fat men). Generally speaking, 

 this method gives a lower value than the other, and there has been a 

 considerable amount of discussion recently as to the accuracy of the 

 CO method. CO is taken up by the haemoglobin in the muscles; the 



