Diastolic blood pressure

Diastolic blood pressure is controlled primarily by the tiny arteries at the end of the innumerable branches of the arterial tree in the muscles, skin, brain, and other parts of the body. These tiny arteries, or arterioles, contract and expand in response to muscle activity in the arterial wall. The muscles are controlled in turn by the autonomic nervous system, influenced by the amount of salt in the muscle. When the arterial muscles are relaxed and the arterioles are open, blood rushes into the arterial system as the heart pumps and out through the arterioles into the veins. Between heart beats, the pressure falls to the lower diastolic pressure. If systolic pressure is low, diastolic pressure will be correspondingly lower.

The inelastic large arteries of the older person with high systolic pressure tend to contribute to high diastolic pressure even if the small arterioles are clear because the pressure doesn't have enough time to fall to a normal low level before the next heart beat raises it again. Thus, an older individual may have a systolic pressure of 160 and a diastolic pressure of 90, yet have completely normal arterioles.