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.

Systolic blood pressure

Systolic blood pressure is related to the strength of the heart muscle as it pushes blood into the arterial system, and more importantly, to the elasticity of the arteries -- their ability to expand and contract with changes in pressure. Thus, if the arteries are flexible and give easily as the heart beats, systolic pressure will not be excessively high. This is characteristic of young persons who often have a systolic pressure of 110, or even less. Such low pressure is not clinically significant. A medical school teacher may remark that the only thing to do for such individuals is to offer congratulations.

As we grow older, the blood vessel walls tend to stiffen, and arteriosclerosis (hardening of the arteries) may aggravate the normal loss of elasticity. Loss of elasticity causes a higher systolic pressure as the heart tries to force a normal amount of blood into the unyielding arteries. After the age of 55, systolic pressure tends to be higher, and a pressure of 150 to 160 is not unusual due solely to the normal loss of elasticity in aging tissues.