It has been shown that the mean pressure is quite constant throughout the whole arterial system. The maximum pressure necessarily falls as the periphery of the vascular system is approached. In general it may be said that the minimal pressure is quite constant. Too little attention is paid to minimal and pulse pressure. The minimal pressure is important, for it gives us valuable data as to the actual propulsive force driving the blood forward to the periphery at the end of diastole.
It is readily understood how the maximum pressure falls as the periphery is approached, until in the arterioles the maximum and minimum pressures are about equal. The pressure then in these arterioles is practically the same as the diastolic pressure. Actually it is a few millimeters less. The diastolic blood pressure would, therefore, measure the peripheral resistance and, as the maximum for systolic pressure represents approximately the intraventricular pressure, the difference between the two, the pulse pressure, actually represents the force which is driving the blood onward from the heart to the periphery. It is hence very evident that the mere estimation of the systolic pressure gives us but a portion of the information we are seeking.
The pulse pressure is subject to wide fluctuations but as a rule for any one normal heart it remains fairly constant as the rate varies. In a rapidly beating heart the diastole is short and the diastolic pressure rises. If the systolic pressure does not also rise, as in a normal heart following exercise, we will say, the pulse pressure falls. We know that when the pulse rate is constant, vasodilatation causes a fall in diastolic pressure and a rise in pulse pressure. On the contrary, vasoconstriction causes a rise in diastolic pressure and a fall in pulse pressure.
It is very probably the case that with two individuals of equal age and equal pulse rate, and equal systolic pressure of 160 mm., the one with a diastolic pressure of 110 mm. and, therefore, a pulse pressure of 50 mm. is much worse off than the other with a diastolic pressure of 90 mm. and a pulse pressure of 70 mm. The latter may be normal for the age of the person especially when certain forms of fibrous arteriosclerosis accompanied by enlarged heart are present.
The former is not normal for any age. Low pulse pressure usually means a weak vasomotor control and is only found in failing circulation or in markedly run down states, such as after serious illness or in tuberculosis. Therefore, it is most important to estimate accurately the diastolic pressure as well as the systolic pressure, for only in this way can we obtain any data of value regarding the driving power of the heart and the condition of the vasomotor system. A high systolic pressure does not necessarily mean that a great deal of blood is forced into the capillaries. Actually it may mean that very little blood enters the periphery. The heart wastes its strength in dilating constricted vessels without actually carrying on the circulation adequately.
Normal Pressure Variations
The systolic pressure varies considerably under conditions which are by no means abnormal. Thus, the average for men at all ages is about 127 mm. Hg. (All measurements are taken from the brachial artery, with the individuals in the sitting posture.) For women the average is somewhat lower, 120 mm. Hg. The pressure is lowest in children. In children from 6-12 years the average systolic pressure is 112 mm. Normally, there is a gradual increase as age comes on, due, as will be shown in the succeeding chapter, to physiologic changes which take place in the arteries from birth to old age. In the chart here appended is graphically shown the normal variations in the blood pressure at different ages compiled from observations made on one thousand presumably normal persons. (Fig. 28.)
Fig. 28.—Chart showing the normal limits of variation in systolic blood pressure. (After Woley.)