VELOCITY AND PRESSURE OF BLOOD-FLOW. 



491 



capillaries and in the veins there is no pulse wave, and no difference 

 between systolic and diastolic pressure. In speaking of the pressure 

 in the blood-vessels we refer usually to what is called the mean 

 pressure. It is obvious that, so far as the larger arteries are con- 

 cerned, the mean pressure is only a convenient expression for the 

 average pressure during a certain period. If, by the methods 

 described below, we determine the systolic and diastolic pressures 

 in the artery of a man, and assume that there has been no general 

 variation between the two observations, we can estimate the mean 

 pressure with approximate accuracy by taking the arithmetical 

 mean of the two figures, or by adding to the diastolic pressure one- 

 half of the pulse pressure. 



The arithmetical mean of systolic and diastolic pressures during any given 

 heart-beat does not give the true mean pressure, owing to the form of the 

 pulse wave (see Fig. 213). If the rise from diastolic to systolic pressure and 

 ithe succeeding fall took place uniformly, so that the pulse curve constituted 



Fig. 195. — Schema to indicate the general relations of systolic, mean, and diastolic 

 pressures throughout the arterial system: s, Systolic: m, mean: rf, diastolic; c, pressure 

 at beginning of the capillaries. The distance from s to d represents the pulse pressure at 

 different parts of the arterial system. 



a true triangle, the true mean pressure would be given by the arithmetical 

 mean of the two pressures. As a matter of fact, the descending limb 

 of the pulse curve is not a straight but a curved line, and it is broken, 

 moreover, by secondary waves. The position of the mean pressure during 

 any given heart-beat v/ill vary, therefore, with the form of the pulse curve. 

 Generally speaking, it lies nearer to the diastolic than to the systolic level.* 



In physiological observations, as a rule, no attempt is made to 

 estimate the mean pressure for any given time with mathematical 

 accuracy. In the ordinary tracing as given by the mercury man- 

 ometer (Fig. 193) the mean pressure for any given period during 

 which the variations have been symmetrical and not extreme is 

 estimated as the arithmetical mean of the highest and lowest 

 points reached. When desirable, the mean pressure may be 

 recorded by introducing a resistance (narrowing the tube by means 

 ■of a stopcock) between the artery and the manometer. The latter 

 * See Dawson, " British Medical Journal," 1906, 996. 



