282 THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXI. 



because the pressure gradient has been enormously increased in 

 steepness. If, on the other hand, we suddenly cut across a vein 

 along which the blood had been flowing at the same pace as in the 

 intact artery first investigated, the flow will not be markedly 

 accelerated, because the change in pressure gradient has not been 

 increased to nearly so great an extent. 



Again, the flow along a vein is just as rapid as along an artery 

 of the same size, for although the actual pressure in the vein is much 

 less, the pressure gradient is just as steep. 



The influence of the kinetic factor is also of great importance in 

 the consideration of the flow of blood along the arteries and veins. 

 In the first place, it is obviously possible for the blood to flow from 

 one point to another at a higher pressure if the kinetic energy at the 

 first point is more than enough to compensate for the pressure 

 increase. Under such circumstances the velocity at the second 

 point must of course be less than that at the first. This implies, 

 therefore, that the bed of the stream has widened, and under such 

 circumstances the blood could actually flow uphill. In the case of 

 the veins, as we have previously seen, the bed continuously narrows, 

 so that this cannot take place ; still it is possible to conceive such 

 a condition to occur as that in which the blood from a well-filled 

 vein empties into a more collapsed larger vein situated at a higher 

 level. The one instance in which this effect is produced and is of 

 great importance is in the filling of the auricles and ventricles. As 

 these cavities fill, the blood comes to rest and so loses all kinetic 

 energy; consequently the whole of the kinetic energy possessed 

 by the blood flowing in the veins is converted into static energy, 

 that is, into a pressure-head ; in this way the cavities are distended 

 to a much higher pressure than that in the great veins. The 

 acute distension of the right auricle which follows any sudden 

 failure of the right ventricle is brought about chiefly in this 

 way. 



It is usual to speak of the lateral pressure of the blood on the 

 vessel wall as the pressure-head, and of the kinetic energy measured 

 in terms of a pressure as the velocity -head. We could then say that 

 the velocity between any two points is determined by the difference 

 between the two pressure-heads plus the velocity-head at the first 

 point. One method of recording the velocity-head is by the use of 

 a tube (Pitot's tube) shaped as in the accompanying figure (fig. 282). 

 The blood is made to enter at A, and leave through B; in the same 

 straight line as A is a tube C, and a second tube D is placed at right 

 angles to the tube B. If the tubes and D are placed vertically 

 and were sufficiently long, the blood would flow up C until it 

 reached a height which would balance the pressure-head plus the 

 velocity-head ; in D it would only reach a height sufficient to balance 



