QUANTITY AND COMPOSITION OF BLOOD 



855 



monoxide gas when inhaled combines with haemoglobin, expelling the 

 oxygen from the oxyhsemoglobin. 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 haemoglobin 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- 

 hsemoglobin does of oxygen, the same figure gives us the total * oxygen 

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



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FIG. 369. Haldane's CO method for determining total blood volume in man. 



amount of haemoglobin in the body, and if we know the percentage 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. 859). The ox blood is then compared 

 colorimetrically with blood obtained in the ordinary way by means of a hsemoglobin- 

 ometer 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. 369) 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 from an oxygen cylinder through the tube c. D is a graduated vessel contain- 

 ing pure carbonic oxide gas. While the subject is breathing in and out of the bag a 

 given volume of carbon monoxide is admitted into the bag, being driven out from the 

 tube D by allowing water to flow through the tap E. The requhed volume of carbon 

 monoxide is gradually driven in from the measuring cylinder at the rate of about 30 c.c. 

 every two minutes. When the required quantity has been driven in and pushed forward 

 by the oxygen an interval of two or three minutes is allowed to elapse. After this a 



