TH-E VELOCITY OF BLOOD-FLOW 



205 



ance in the peripheral arterioles. During the operative procedure, 

 the blood-flow must for a time be cut off, and this causes a temporary 

 paralysis of the arterioles, which, passing off as the circulation is 

 restored, causes variations in resistance. 



In another instrument, the ha3modromometer (Fig. 108), a T-tube 

 is inserted into the artery, in which hangs a small pendulum, the stem 

 of the pendulum pasing through a rubber dam, which closes the vertical 

 limb of the tube. The pendulum is deflected by the flow, and the 

 greater the velocity, the greater the deflection. The deflection can 

 be recorded by connecting the free end of the pendulum to a tambour 

 arrangement. By this instrument the variations of velocity during 

 systole and diastole of the heart can be recorded and 

 measured, but it can only be used in the vessels of large 

 animals. 



If in a schema similar to that given in Fig. 85 two 1 1 - 

 shaped tubes, a and b (Pitot's tubes), be inserted, one with 

 the elbow opposing the stream, the other with the elbow 

 in the direction of the stream, the fluid will rise higher 

 in a than in an ordinary side tube, and lower than this 

 in 6. This is because the flowing stream exerts a push 

 on a and a pull on 6. The amount of 'this push and 

 pull varies with the velocity of the stream, so that from 

 the difference in the height of the two tubes the velocity 

 can be calculated. In an instrument known as the 

 photohsematochometer (Fig. 109) the difference in height 

 is recorded by photography. 



The velocity may also be calculated by the electrical 

 method, estimating the time taken for the blood to pass 

 between two points of an artery when salt solution is 

 injected into the circulation (see Circulation-time, p. 209). 



In man, the quantity of blood which passes through 

 the hand or foot has been measured by plethysmographic 

 means (Fig. 110) and also deduced from the quantity 

 of heat which the part gives off to a water calorimeter 

 in which it is immersed. The flow in grammes 



[G. 109. 

 CYBULSKI'S 

 PHOTOH.E- 

 MAT o c H o- 



METER. 



per minute is obtained from the formula, Q= 



H 



1 



where Q 



m(\. 1 ) 8 



is the quantity of blood, H the number of small calories given off in 

 ra minutes, T the temperature of the blood entering the hand, T 1 that 

 of the blood leaving the hand, and s the specific heat of the blood 

 (0-9). T may be taken as 0-5 lower than the rectal temperature, 

 and T 1 the same as that of the tepid water in the calorimeter. 



The general relations of the velocity of the blood in the arteries, 

 capillaries, and veins, is expressed by the curve shown in Fig. 111. 

 The velocity in the large arteries may reach 500 millimetres per second 

 in systole, and fall to 150 millimetres in diastole. The smaller the 

 artery the less is this difference, and the more uniform the rate of 

 flow. 



The velocity and pressure of the blood in the aorta are dependent 



