900 PHYSIOLOGY 



A simpler method of determining the total blood volume has been worked out by 

 Keith, Rowntree and Geraghty. The method consists in injecting a non-toxic, non- 

 diffusible dye substance into the blood stream and estimating its dilution. The dye 

 used is ' vital red,' a chemical compound belonging to the triphenyl-methane series. 

 In performing the test 6 to 8 c.c. of blood are removed from an elbow vein. From 

 10 to 18 c.c. of a 1-6 per cent, solution of the dye in distilled water is then slowly injected 

 by the same needle. Five minutes later a second specimen of blood is withdrawn into 

 a third syringe. The blood samples are prevented from clotting by the addition of 

 potassium oxalatc. A part of this is drawn into a hsematocrit tube and centrifuged 

 for twenty minutes at a high speed in order to determine the relative volume of cor- 

 puscles and plasma. The rest of both samples of blood are centrifuged in order to 

 obtain the plasma. Samples of plasma before and after are then compared in the 

 following mixture : 



1 1 part of the diluted dye solution. 

 Standard^ 1 part of the plasma before dye injection, 

 [2 parts 0-8 per cent. NaCl solution. 



T f 1 part of plasma after dye injection. 

 \3 parts 0-8 per cent. NaCl solution. 



The two solutions are compared in a colorimeter and the test solution read off as a 

 percentage of the standard. The following formula will give us the plasma volume : 



If R be the percentage reading of test solution. 



200 



- X c.c. dye injected X 100 = c.c. plasma. 

 R 



The blood volume is calculated from the hsematocrit reading. 



100 X c.c. plasma 

 Total blood volume = 



percentage plasma in blood. 



The total blood volume probably varies appreciably with alterations 

 in the condition of the animal, and may be found different on two suc- 

 ceeding days. It is ce"rtainly influenced by the height of the blood pressure 

 as well as by the oxygen tension in the air breafhed, and therefore alters 

 with the altitude. Some of these variations we shall have to consider more 

 fully in a later section. Any lowering of blood pressure causes an absorp- 

 tion of fluid from the tissues into the blood, so that the latter becomes more 

 dilute. The blood content during the last stages of bleeding may contain 

 little more than 50 or 60 per cent, of the haemoglobin which was present 

 in the first samples of blood, pointing to a corresponding dilution of the 

 blood during these few minutes by means of tissue lymph. By this means, 

 i. e. the absorption of fluid from tissues, the volume of circulating blood 

 after a limited haemorrhage is rapidly brought up to normal, so that there 

 is a circulation of a fluid impoverished in corpuscles. The latter are made 

 up in the course of a few weeks as a result of increased activity in the 

 bon?-marrow. 



Relative Amount of Plasma and Corpuscles. The relative amount of corpuscles 

 in a given sample of blood is most easily determined by Blix's method. The 

 blood is mixed with a definite amount of 2-5 per cent, potassium bichromate, and 

 thr mixture is put into small graduated capillary tubes, which are then placed in 

 a centrifuge revolving about 10,000 times per minute. The corpuscles rapidly 

 accumulate in an almost solid mans at the bottom of the tube, and their volume 



