The diastolic pressure measures the peripheral resistance. It measures the work of the heart, the potential energy, up to the moment of the opening of the aortic valves. It is the actual pressure in the aorta. The diastolic pressure is not very variable; it is not subject to the same influences which disturb the systolic pressure. It fluctuates as a rule, within a small range. It is not affected by diet, by mental excitement, by subconscious psychic influences, to anything like the extent to which the systolic pressure is affected by the action of these factors. The diastolic pressure is determined by the tone in the arterioles and is under the control of the vasomotor sympathetic system. Any agent which causes chronic irritation of the whole vasomotor system produces increase in the peripheral resistance with consequent rise in the diastolic pressure. Any agent which acts to produce thickening of the walls of the arterioles, narrowing their lumina, produces the same effect. Such states naturally result in increased work on the part of the heart, which as a result, hypertrophies in the left ventricle. The increase in size and strength is a compensatory process in order to keep the tissues supplied with their requisite quota of blood. Conversely, paralysis of the vasomotor system produces fall of diastolic pressure which, if long continued, results in death. The diastolic pressure then is of importance for the following reasons: 1. It measures peripheral resistance. 2. It is the measure of the tonus of the vasomotor system. 3. It is one of the points to determine pulse pressure. 4. Pulse pressure measures the actual driving force, the kinetic energy of the heart. 5. It enables us to judge of the volume output, for pulse pressure which is only determined by measuring both systolic and diastolic pressure, is such an index. 6. It is more stable than the systolic pressure, subject to fewer more or less unknown influences. 7. It is increased by exercise. 8. It is increased by conditions which increase peripheral resistance. 9. The gradual increase of diastolic pressure means harder work for the heart to supply the parts of the body with blood. 10. Increased diastolic pressure is always accompanied by increased pulse pressure, and increased size of the left ventricle, temporarily (exercise) or permanently. 11. Decreased diastolic pressure goes hand in hand with vasomotor relaxation, as in fevers, etc. 12. Low diastolic pressure is frequently pathognomonic of aortic insufficiency. 13. When the systolic and diastolic pressures approach, heart failure is imminent either when pressure picture is high or low. When all these factors are taken into consideration, it becomes apparent that the diastolic pressure is most important, if not the most important part of the pressure picture.