QUANTITY AND COMPOSITION OF BLOOD 



965 



ingenious method adopted by Haldane for this determination in 

 the living animal may be here described. The method depends on the 

 fact that carbon monoxide gas when inhaled combines with haemo- 

 globin, expelling the oxygen from the oxy haemoglobin. If therefore 

 we allow a man to breathe a certain volume of carbon monoxide 

 until it is entirely absorbed and then find that one-fifth of the haemo- 

 globin in his blood is saturated with carbon monoxide, we know that 

 the whole blood could take up five times the bulk of carbon monoxide 

 which the man has inspired. We therefore in this way determine 

 the total ' carbonic oxide capacity ' of the blood, and since CO- 

 haemoglobin contains the same volume of carbon monoxide as oxy- 



FIG. 365. Haldanc's CO method for determining total blood volume in man. 



haemoglobin does of oxygen, the same figure gives us the total ' oxygen 

 capacity.' The total oxygen capacity enables us to determine the 

 total amount of haemoglobin in the body, and if we know the per- 

 centage amount of haemoglobin in the blood it is easy to calculate 

 the total volume of circulating fluid. 



Before the carbonic oxide is administered the percentage oxygen capacity, i.e. 

 the volume of oxygen capable of being taken up by the haemoglobin of 100 c.c. 

 of the blood, is determined as follows : The oxygen capacity of a sample of 

 fresh ox blood is accurately determined by the ferricyanide method (v. p. 969). 

 The ox blood is then compared colorimetrically with blood obtained in the 

 ordinary way by means of a hsemoglobinometer needle from the finger of the 

 subject of the experiment, and the oxygen capacity of the latter blood calculated 

 from the result of the comparison. The subject is now made to breathe through 

 a mouthpiece A (Fig. 365) into a bladder B of about 2 litres capacity. The 

 carbon dioxide produced during the experiment is absorbed by the soda lime vessel 

 between the mouthpiece and the bladder. The oxygen as it is used up is replaced 



