CH XXII.] THE PRESSURE GRADIENT 285 



or ^ gramme-centimetres) ; and (2) the difference between the two 



lateral pressures at the two points in question. The important 

 point to remember with respect to the part the pressure plays, is 

 that the actual value of the lateral pressure does not matter, but 

 that the resulting velocity, so far as pressure is concerned, depends 

 only upon the fall of pressure between the two points. Therefore, 

 the measurement to be determined is the rate of fall of pressure, 

 or, as it is usually expressed, the pressure gradient. The steeper 

 this gradient is, the more rapid is the flow. Thus, if an artery 

 is suddenly cut across, the blood will spurt out at a far greater 

 velocity than it possessed when flowing along the intact artery, 

 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 



