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But there are a lot of things that can throw off the calibration of the whole fragile system, starting with the vessel walls. When we're young, our vessels are healthy and springy, easily stretching and contracting to accommodate blood pressure as it rises and falls. The arteries, which are a type of muscle, even pulse to help keep blood moving along. But the fibers that make up the scaffolding of the vessel walls can take only so much flexing. As we age, the rubbery tissue slowly gets replaced with stiffer collagen. The vessels don't expand as well anymore, but the blood keeps rushing through at the same rate, increasing the pressure. The higher the pressure climbs, the more punishment the walls take, and the more collagen is added. "The vessel wall becomes almost like concrete," says Dr. Michael Weber, a past president of the American Society of Hypertension.
The kidneys too play a big role. The urinary and renal systems govern not only the quantity of water that is kept or dumped by the body but also its composition. Drain or retain too much sugar, potassium or countless other essential components, and the chemistry of the whole body goes awry. One of the things the kidneys keep an especially close watch on is salt. The more sodium you hold, the more water your body retains, storing it first in the bloodstream and then off-loading it into tissues. When your system gets waterlogged, overfilled vessels feel the strain.
The kidneys work hard to keep that from happening. If salt content is too high, the body's water content will be elevated too. The system responds by slowing down the manufacture of renin, an enzyme that increases water retention. Dialing back the renin also dials back the production of angiotensin, a protein that constricts blood vessels. Should the salt level fall too far, the body reverses the procedure, cranking up renin to hold on to water and releasing angiotensin to tighten vessels. There are a lot of things that can throw that system off, including kidney disease and tumors on the thyroid gland. In most cases, however, it's simply too little exercise, too much food and too much salt.
"The great majority of people with high blood pressure have what is called essential hypertension," says Labarthe. "That is high blood pressure that is a reflection of lifestyle."
Whatever the causes of hypertension, doctors have been pretty clear about what its yard markers are. A reading of 120/80 or below is considered normal; 140/90 marks the onset of hypertension; 160/100 is Stage 2 hypertension; 220/120 is the onset of what is known as malignant hypertension, pressure so high that fluid is squeezed from vessels into the brain and blood leaks out of capillaries into the liquid that fills the eyeballs. "Malignant hypertension is a medical emergency," says cardiologist Richard Devereux of Cornell University Medical College.
THE DAMAGE DONE