CHAPTER XXIV 



THE RATE OF MOVEMENT OF THE BLOOD IN THE 

 BLQOD VESSELS 



Since the object of the circulation is to maintain an adequate move- 

 ment of blood in the tissues and capillaries, it is evident that besides 

 measuring the pressure of bloodflow, we should also measure the rate 

 of its movement, or, as it is often called, the mean velocity. This measure- 

 ment may be undertaken either for a given vessel or for a complete 

 vascular area, such, for example, as that of one of the viscera or one 

 of the extremities the mass movement of the blood. Or instead of 

 measuring the mean velocity we may desire to know how long it takes 

 for a particle of blood to traverse a given vascular area. Such a meas- 

 urement is called the circulation time; it does not at all tell us IIOAV long 

 it takes for all the blood to pass through the given area, but only, as 

 stated, the time required for the circulation of a fraction of the blood 

 through a particular field. 



VELOCITY OF FLOW IN A VESSEL 



Special methods have been devised for the measurement of each of 

 these three velocities. For the measurement of the velocity of flow 

 through a main artery or vein, methods similar to those employed by 

 hydraulic engineers are employed; that is to say, the volume of blood, 

 in cubic centimeters, which passes a given point is measured for r 

 given time, and the result divided by the cross section of the vessel at 

 the point of observation. The result gives us the mean lineal velocity. 

 To measure the outflow of blood in a given time, the simplest method 

 would be to cut across the vessel and collect the blood in a graduate, 

 but obviously in this method an error would be introduced, because 

 cutting the vessel would lower the peripheral resistance and remove the 

 natural obstruction to the flow present in the intact animal. Moreover, 

 the hemorrhage would in itself introduce a disturbing factor on account 

 of the loss of circulating fluid. 



To make such measurements of any value, it is obviously necessary to 

 retain the peripheral resistance. For smaller vessels this can be done 

 by introducing in the course of the artery a long glass tube bent in the 



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