Blood Volume following Operations in Man. 



417 



them now in order that they may be compared witli these post-operative 

 changes. 



Since, then, there appears to be this interdependence of the changes in the 

 blood pressure and blood volume, it seemed necessary to explain it. An 

 artificial schema was used in which a raised reservoir of water supplied the 

 systolic pressure ; a tap connected this with the artificial arterial system, 

 this tap (representing the contraction of the heart) being opened at the 

 rate of 30 to 40 times a minute by hand, the time being judged by a 

 pendulum. The arterial system consisted of thin-walled rubber tubing, 

 ending in a resistance created by capillary glass tubes, which could be 

 changed. The water flowing through was collected and measured. It was 

 found that the output per minute (MV) was proportional to the product of 

 the pulse rate and pulse pressure. 



MVa PKxPP. 



It was also found that the resistance (K) of the artificial schema arteriole 

 was indicated by the following formula : — 



P MP 

 ^^'(PKxPPf ' 



where MP is the mean pressure and a high figure for R indicates a high 

 resistance — but the proportion is not an arithmetical one. By the use of 

 this formula it was easy to recognise, by the examination of the record, wliich 

 of a series of capillary tube resistances had been used. Fig. A gives some 

 examples of records obtained. In applying this formula, however, to blood 

 pressure changes it did not appear to give reasonable results in cases where 

 there were big changes of blood pressure. 



However, on examination of the average pulse pressures found in man with 

 different systolic pressures, as quoted for instance in Oliver's ' Studies 

 in Blood Pressure,' it is found that the pulse pressure is a function of the 

 square of the systolic pressure, if the blood circulation is otherwise normal. 

 Thus, taking the figures from Oliver's book, a child of 15 has a systolic 

 pressure of 107, and a pulse pressure of 33, a ratio of, PP/(SP)2 of 0-00288. 

 On the other hand, an arterio-sclerotic with a blood pressure of 165 

 has a pulse pressure usually of 75, a ratio PP/(SP)2 of 0-00275. The 

 ratio is therefore remarkably constant. These results are charted in 

 fig. B. 



In considering the actual circulation, the viscosity of the blood must also 

 be allowed for. Fig. C shows the probable changes in the viscosity of the 

 blood for man with different percentages of haemoglobin, this curve being 



2 L 2 



