heart and blood vessel disorders
High Blood Pressure
The Body's Control of Blood Pressure
The body has many mechanisms to control blood pressure. The body can change the Amount of blood the heart pumps Diameter of arteries Volume of blood in the bloodstream To increase blood pressure,the heart can pump more blood by pumping more forcefully or more rapidly. Small arteries (arterioles) can narrow (constrict), forcing the blood from each heartbeat through a narrower space than normal. Because the space in the arteries is narrower, the same amount of blood passing through them increases the blood pressure. Veins can constrict to reduce their capacity to hold blood, forcing more blood into the arteries. As a result, blood pressure increases. Fluid can be added to the bloodstream to increase blood volume and thus increase blood pressure. To decrease blood pressure,the heart can pump less forcefully or rapidly, arterioles and veins can widen (dilate), and fluid can be removed from the bloodstream. These mechanisms are controlled by the sympathetic division of the Autonomic nervous system (the part of the nervous system that regulates internal body processes requiring no conscious effort) and by the kidneys. The sympathetic division uses several means to temporarily increase blood pressure during the fight-or-flight response (the body's physical reaction to a threat). The sympathetic division stimulates the adrenal glands to release the hormonesepinephrine(adrenaline) andnorepinephrine(noradrenaline). These hormones stimulate the heart to beat faster and more forcefully, most arterioles to constrict, and some arterioles to dilate. The arterioles that dilate are those in areas where an increased blood supply is needed (such as in skeletal muscle—the muscles controlled by conscious effort). The sympathetic division also stimulates the kidneys to decrease their excretion of sodium and water, thereby increasing blood volume. The body controls the Overview of Sodium's Role in the Body , to prevent an excess of sodium inside cells. Excessive amounts of sodium inside cells can cause the body to become overly sensitive to stimulation by the sympathetic division. The kidneys also respond directly to changes in blood pressure. If blood pressure increases, the kidneys increase their excretion of sodium and water, so that blood volume decreases and blood pressure returns to normal. Conversely, if blood pressure decreases, the kidneys decrease their excretion of sodium and water, so that blood volume increases and blood pressure returns to normal. The kidneys can increase blood pressure by secreting the enzyme renin, which eventually results in the production of the hormoneangiotensin II. Angiotensin IIhelps increase blood pressure by Causing the arterioles to constrict Triggering the sympathetic division of the autonomic nervous system Triggering the release of two other hormones,aldosteroneandvasopressin(also called antidiuretic hormone), which cause the kidneys to increase the retention of sodium and water The kidneys normally produce substances that cause arterioles within the kidney to dilate. This helps balance the effects of hormones that cause constriction of arterioles. Blood pressure varies naturally over a person’s life. Infants and children normally have much lower blood pressure than adults. For almost everyone living in industrialized countries such as the United States, blood pressure increases with aging. Systolic pressure increases until at least age 80 years, and diastolic pressure increases until age 55 to 60 years, then levels off or even decreases. However, for people living in some regions where sodium intake is low and the physical activity level is higher than in the United States, neither systolic nor diastolic pressure increases with aging, and high blood pressure is practically nonexistent. The renin-angiotensin-aldosterone system is a series of reactions designed to help regulate blood pressure. When blood pressure falls (for systolic, to 100 mm Hg or lower), the kidneys release the enzyme renin into the bloodstream. Renin splits angiotensinogen, a large protein that circulates in the bloodstream, into pieces. One piece is angiotensin I. Angiotensin I, which is relatively inactive, is split into pieces by angiotensin-converting enzyme (ACE). One piece isangiotensin II, a hormone, which is very active. Angiotensin IIcauses the muscular walls of small arteries (arterioles) to constrict, increasing blood pressure.Angiotensin IIalso triggers the release of the hormone aldosterone from the adrenal glands andvasopressin(antidiuretic hormone) from the pituitary gland. Aldosterone andvasopressincause the kidneys to retain sodium (salt). Aldosterone also causes the kidneys to excrete potassium. The increased sodium causes water to be retained, thus increasing blood volume and blood pressure. Activity temporarily affects blood pressure, which is higher when a person is active and lower when a person rests. Blood pressure also varies with the time of day: It is highest in the morning and lowest at night during sleep. These variations are normal. Whenever a change causes a transient increase in blood pressure, one of the body's compensatory mechanisms is triggered to counteract the change and keep blood pressure at normal levels. For example, an increase in the amount of blood pumped out by the heart—which tends to increase blood pressure—causes dilation of blood vessels and an increase in the kidneys' excretion of sodium and water—which tend to reduce blood pressure. 