=The Relative Importance of the Systolic and Diastolic Pressures= The systolic pressure represents the maximum force of the heart. It is measured by noting the first sound audible over the brachial artery using the auscultatory method. It is the summation of two factors largely; the force expended in opening the aortic valves (potential) and the force expended from that point to the end of systole, the force which is actually driving the blood to the periphery (kinetic). To start the blood in motion, the heart must overcome a dead weight equal to the sum of all the forces holding the aortic valves closed. This sum of factors, called the peripheral resistance, must be reached and passed by the force of the ventricular beat before one drop of blood is set in motion along the aorta. This factor of resistance assumes a great importance. The systolic pressure is always fluctuating as it depends upon so many conditions, and the calls of the body except during sleep are many and various. In a study of diurnal variations in arterial blood pressure it has been found that--(1) A rise of maximum pressure averaging 8 mm. of Hg. occurs immediately on the ingestion of food. A gradual fall then takes place until the beginning of the next meal. There is also a slight general rise of the maximum pressure during the day. (2) The range of maximum pressure varies considerably in different individuals, but the highest and lowest maximum pressures are practically equidistant from the average pressure of any one individual.[4] [4] Weyse, A. W., and Lutz, B. R.: Diurnal Variations in Arterial Blood Pressure, Am. Jour. Physiol., 1915, xxxvii, 330. The pressure is lowest during sleep and gradually rises near the end of sleep, so that on awakening the pressure was the same as before sleep. Physiologically there are many conditions which modify the systolic pressure. Sleep, position, meals, exercise, emotional states cause often wide fluctuations which may be very sudden. It should be constantly borne in mind, that the systolic pressure reading which is made, is the maximum effort of the heart at that moment only.