Blood pressure is regulated by the sympathetic and parasympathetic branches of the autonomic nervous system. In the sympathetic branch, pressure receptors in the aortic arch and the carotid arteries send signals to the vasomotor center of the brain stem. When blood pressure drops, sympathetic nerves to the heart release norepinephrine, which promotes calcium uptake by heart muscle and acts on beta receptors of the heart to increase the strength and rate of heartbeat. Sympathetic nerves also release norepinephrine to the alpha receptors of the arterioles, causing them to contract. All of these effects increase blood pressure. The parasympathetic branch sends the vagus nerve to the heart, where it releases acetylcholine, causing the heart to beat more slowly, and with less force.
The kidneys can influence blood pressure by (1) excreting more sodium and water (lowering blood volume, and hence, lowering blood pressure) and (2) secreting the enzyme renin which produces angiotension I in the bloodstream. When angiotension I reaches the lungs, Angiotension Converting Enzyme (ACE) converts angiotensin I to angiotension II, which rapidly and powerfully constricts the arterioles, raising blood pressure.
Blood pressure is measured by two values: the highest pressure, when the heart is pumping or Squeezing (Systolic), and the lowest pressure, when the heart is relaxing
or Dilating (Diastolic). A pressure of 140 mmHg is the amount of force necessary to raise a column of Mercury (Hg) 140 millimetres. The same amount of pressure would raise a column of water nearly 2 metres -- the heaviness of mercury makes it a more convenient measuring standard. In North America, Systolic/Diastolic pressure of 120/80 is considered "normal", although natives of New Guinea typically have a diastolic blood pressure of 60 mmHg -- and an extremely low incidence of stroke.
The following table indicates the hazards of high blood pressure:

YEARS OF LIFE LOST DUE TO HIGH BLOOD PRESSURE

Sys/Dia |        MEN (age)        |       WOMEN (age)
|    35            55     |    35            55
-------------------------------------------------------------
130/90   |  4 years       1 year   |  2 years       1 year

150/100  | 17 years       6 year   |  9 years       4 years

People with hypertension are 2-3 times more likely to have a heart attack than those with normal blood pressure. 45% of people with high blood pressure are unaware that they have it. Of those who are aware, fewer than half are being treated. People with uncontrolled high blood pressure are 7 times more likely to have a stroke than people with controlled high blood pressure. Hypertension is the major risk factor for stroke. 25% of people currently undergoing kidney dialysis can blame high blood pressure for their kidney damage. High blood pressure also damages many other organs.
75% of overweight (overfat) people with hypertension who lost 20 pounds or more achieved normal blood pressure -- besides reducing their blood LDL and elevating their blood HDL-cholesterol. Will-power to resist the temptations of hunger is at least as important in losing weight as the will-power to exercise. Dieting should only be undertaken as a permanent life-style change because "yo-yo dieting" is associated with an increased incidence of heart attacks.