What is high blood pressure?
High blood pressure is a common disorder in which the force of circulating blood against artery walls is constantly too high. By repeatedly damaging the body's tissues, it reduces life expectancy and increases the risk of heart attack, stroke, and kidney failure. The risk of high blood pressure usually increases with age; more than one-third of adults aged over 60 already suffer from high blood pressure.
High blood pressure is known by several names, including hypertension, arterial tension or just plain 'pressure'. To understand how it comes about, you first need to know that the force pressing outwards on the inside of your artery walls is provided by tiny muscular pumps called heart ventricles. The greater the amount of blood pumped out by the left ventricle each time it contracts, and the faster this happens, the higher your blood pressure.
When your heart beats (contracts), it pumps blood into a large artery called the aorta; from there, this life-giving fluid is carried into smaller and smaller vessels called arterioles. Blood flows into the arterioles because their muscular walls are constricted. The smaller the vessel, the more intense is this squeezing - so as blood passes through successive sections of arteriole, its pressure increases enormously.
The highest blood pressure occurs in the arterioles that supply your muscles and other active tissues with food and oxygen. This is why an increase in blood pressure is called 'vascular resistance'.
As the heart continues to pump, eventually the arterioles become so narrow that the column of blood flowing through them exerts enough force on their walls for this constriction to be released. Beyond a certain point, if a muscle cell needs more oxygen and nutrients, its membrane's sensors will detect this increased need and signal the arterioles to constrict more tightly. Once they do, more blood will flow into the muscle cell. At rest, when there is no demand for extra oxygen and nutrients, these arterioles are wide open; in this state of 'relaxation, blood pressure falls to its lowest point.
The pressure of the outflowing blood falls steeply as it moves through increasingly larger artery branches towards your hand, foot, leg, or whatever part is currently furthest away from your heart. The arterioles supplying these parts are at their narrowest when they branch off to supply an organ; beyond this point, their walls are close enough together for blood to flow more easily between them. The decrease in pressure continues until it reaches your capillaries, where this life-giving fluid passes through the fine membrane that separates each tiny vessel from its neighbour.
Your little finger does not need a constant supply of nutrients and oxygen; at rest, its arterioles follow the same pattern as other parts of your body. Blood flows into them effortlessly, and they are wide open when your muscles are not in use. The further blood travels from these fine vessels before reaching an organ cell where it can be used, the lower is its pressure.
Even at rest, however, the arterioles supplying your heart muscle are continuously contracting and relaxing to keep your blood pressure at a fairly steady level. This process is referred to as the 'autonomic nervous system' because it does not require conscious control by your brain.
In addition, other physiological mechanisms produce changes in arterial resistance, among them hormones such as adrenalin and cortisol. These 'fight or flight hormones are released when you feel angry or afraid. Adrenalin quickens your heart rate and makes the muscles of your arms and legs tremble so that they can be put to use if you want to fight or run away.
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