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.

Atherosclerosis can occur in any artery. Most commonly it occurs in the aorta, the artery that receives blood directly from the heart. Since the aorta is the largest artery in the body, it is rarely critically narrowed by atheromas. Nonetheless, atherosclerosis can contribute to aneurysms (ballooning of an artery, responsible for only one-fortieth the mortality rate of heart attack -- an aortic aneurysm killed Albert Einstein, who refused to be operated-upon). The most frequent life-threatening problems, however, are caused by the arteries supplying the heart, the brain and the kidneys, in that order. An estimated 70% of North Americans have coronary atherosclerosis (narrowing of the arteries of the heart), although the overt symptoms may not be evident until the arteries are three-quarters occluded. Coronary artery atherosclerosis leads to angina pectoris, heart attack and cardiac arrest. Brain artery atherosclerosis leads to transient ischemic attack (TIA) and stroke.

In angina pectoris (Latin: "strangling in the chest") the coronary vessels narrow temporarily causing the heart muscle to suffer ischemia (lack of oxygen). This is often accompanied by symptoms of crushing, diffuse pain in the chest (directly over the heart), a shortness of breath (air hunger) that leads to gasping, weakness, anxiety, light-headedness, nausea and sweating. The exact same set of symptoms occur in myocardial infarction (heart attack), in which a narrowed coronary artery becomes completely blocked, usually by a blood clot. Heart muscle which formerly received blood from the blocked artery dies if deprived of oxygen for over 40-60 minutes. (An area of dead tissue resulting from a blocked artery is called an infarct). If enough heart tissue dies, the heart may stop altogether (cardiac arrest, also known as sudden cardiac death). Although atherosclerosis is by far the most common cause of heart attack & cardiac arrest, other causes include coronary artery spasm, electrical irregularities and congenital defects.

Transient Ischemic Attack (TIA) is the brain's version of angina. A cerebral artery temporarily narrowed deprives a region of the brain of oxygen. Since only one half of the brain may be affected, paralysis, weakness or lack of sensation on one side of the body is a frequent symptom. Other symptoms include defects of speech, comprehension or vision. Whereas TIA is due to temporary narrowing of a brain artery, stroke results from complete blockage of the artery. In 60% of cases stroke is due to a blood clot formed at the site of blockage, in 20% of cases stroke is due to a clot (embolus) flowing in the bloodstream being wedged in an area of atherosclerotic narrowing, and in 20% of cases stroke is due to bursting of the artery (hemorrhagic stroke, the most dangerous form of stroke).

Everything else being equal, the risk of heart attack is doubled for a diabetic man and increased 5 times for a diabetic woman. Diabetes also doubles the risk of stroke independently from effects of heart disease and high blood pressure -- although good control of blood glucose can reduce the risk.

Arteriosclerosis (hardening of the arteries) occurs "naturally" with aging as a result of cross-linking of macromolecules like proteins and polysaccharides. Atherosclerosis refers to the formation and hardening of fatty plaques (atheromas) on the inner surface of the arteries. In atherosclerosis, the arteries not only harden, they narrow, sometimes narrowing so much that hardly any blood can get through. Such narrowed vessels are easily blocked by constriction or objects in the bloodstream.

The internal surface of an artery is covered with a single layer of endothelial cells that are pressed against each other like flagstones on a terrace. Atherosclerosis begins with injury to endothelial cells, exposing portions of the artery surface below the endothelium. Free radicals, chemicals in cigarette smoke or other irritants could be responsible for the injury, as could turbulence and mechanical force due to high blood pressure. Platelets (round cells half as large as red blood cells) clump around the injured endothelial cells and release prostaglandins, which cause the endothelial cells to proliferate like cancer. LDL-cholesterol particles release their fat into the areas made porous by prostaglandins. Macrophages (scavenger white blood cells) engorge themselves on oxidized LDL-cholesterol until they become unrecognizable "foam cells" that invade atheromas. Then the atheromas are hardened by fibrin (which forms scar tissue) and finally by calcium patches. A vicious circle often arises with scar tissue attracting more platelets and LDL-engorged macrophages.