Circulatory System, or cardiovascular system, in humans is the combined function of the heart, blood, and blood vessels to transport oxygen and nutrients to organs and tissues throughout the body and carry away waste products. Among its vital functions, the circulatory system increases the flow of blood to meet increased energy demands during exercise and regulates body temperature. In addition, when foreign substances or organisms invade the body, the circulatory system swiftly conveys disease-fighting elements of the immune system, such as white blood cells and antibodies, to regions under attack. Also, in the case of injury or bleeding, the circulatory system sends clotting cells and proteins to the affected site, which quickly stop bleeding and promote healing.
The heart, blood, and blood vessels are the three structural elements that make up the circulatory system. The heart is the engine of the circulatory system. It is divided into four chambers: the right atrium, the right ventricle, the left atrium, and the left ventricle. The walls of these chambers are made of a special muscle called myocardium, which contracts continuously and rhythmically to pump blood. The pumping action of the heart occurs in two stages for each heart beat: diastole, when the heart is at rest; and systole, when the heart contracts to pump deoxygenated blood toward the lungs and oxygenated blood to the body. During each heartbeat, typically about 60 to 90 ml (about 2 to 3 oz) of blood are pumped out of the heart. If the heart stops pumping, death usually occurs within four to five minutes.
Blood consists of three types of cells: oxygen-bearing red blood cells, disease-fighting white blood cells, and blood-clotting platelets, all of which are carried through blood vessels in a liquid called plasma. Plasma is yellowish and consists of water, salts, proteins, vitamins, minerals, hormones, dissolved gases, and fats.
Three types of blood vessels form a complex network of tubes throughout the body. Arteries carry blood away from the heart, and veins carry it toward the heart. Capillaries are the tiny links between the arteries and the veins where oxygen and nutrients diffuse to body tissues. The inner layer of blood vessels is lined with endothelial cells that create a smooth passage for the transit of blood. This inner layer is surrounded by connective tissue and smooth muscle that enable the blood vessel to expand or contract. Blood vessels expand during exercise to meet the increased demand for blood and to cool the body. Blood vessels contract after an injury to reduce bleeding and also to conserve body heat.
Arteries have thicker walls than veins to withstand the pressure of blood being pumped from the heart. Blood in the veins is at a lower pressure, so veins have one-way valves to prevent blood from flowing backwards away from the heart. Capillaries, the smallest of blood vessels, are only visible by microscope—ten capillaries lying side by side are barely as thick as a human hair. If all the arteries, veins, and capillaries in the human body were placed end to end, the total length would equal more than 100,000 km (more than 60,000 mi)—they could stretch around the earth nearly two and a half times.
The arteries, veins, and capillaries are divided into two systems of circulation: systemic and pulmonary. The systemic circulation carries oxygenated blood from the heart to all the tissues in the body except the lungs and returns deoxygenated blood carrying waste products, such as carbon dioxide, back to the heart. The pulmonary circulation carries this spent blood from the heart to the lungs. In the lungs, the blood releases its carbon dioxide and absorbs oxygen. The oxygenated blood then returns to the heart before transferring to the systemic circulation.
A. Systemic Circulation
The heart ejects oxygen-rich blood under high pressure out of the heart’s main pumping chamber, the left ventricle, through the largest artery, the aorta. Smaller arteries branch off from the aorta, leading to various parts of the body. These smaller arteries in turn branch out into even smaller arteries, called arterioles. Branches of arterioles become progressively smaller in diameter, eventually forming the capillaries. Once blood reaches the capillary level, blood pressure is greatly reduced.
Capillaries have extremely thin walls that permit dissolved oxygen and nutrients from the blood to diffuse across to a fluid, known as interstitial fluid, that fills the gaps between the cells of tissues or organs. The dissolved oxygen and nutrients then enter the cells from the interstitial fluid by diffusion across the cell membranes. Meanwhile, carbon dioxide and other wastes leave the cell, diffuse through the interstitial fluid, cross the capillary walls, and enter the blood. In this way, the blood delivers nutrients and removes wastes without leaving the capillary tube.
After delivering oxygen to tissues and absorbing wastes, the deoxygenated blood in the capillaries then starts the return trip to the heart. The capillaries merge to form tiny veins, called venules. These veins in turn join together to form progressively larger veins. Ultimately, the veins converge into two large veins: the inferior vena cava, bringing blood from the lower half of the body; and the superior vena cava, bringing blood from the upper half. Both of these two large veins join at the right atrium of the heart.
Because the pressure is dissipated in the arterioles and capillaries, blood in veins flows back to the heart at very low pressure, often running uphill when a person is standing. Flow against gravity is made possible by the one-way valves, located several centimeters apart, in the veins. When surrounding muscles contract, for example in the calf or arm, the muscles squeeze blood back toward the heart. If the one-way valves work properly, blood travels only toward the heart and cannot lapse backward. Veins with defective valves, which allow the blood to flow backward, become enlarged or dilated to form varicose veins.
B. Pulmonary Circulation
In pulmonary circulation, deoxygenated blood returning from the organs and tissues of the body travels from the right atrium of the heart to the right ventricle. From there it is pushed through the pulmonary artery to the lung. In the lung, the pulmonary artery divides, forming the pulmonary capillary region of the lung. At this site, microscopic vessels pass adjacent to the alveoli, or air sacs of the lung, and gases are exchanged across a thin membrane: oxygen crosses the membrane into the blood while carbon dioxide leaves the blood through this same membrane. Newly oxygenated blood then flows into the pulmonary veins, where it is collected by the left atrium of the heart, a chamber that serves as collecting pool for the left ventricle. The contraction of the left ventricle sends blood into the aorta, completing the circulatory loop. On average, a single blood cell takes roughly 30 seconds to complete a full circuit through both the pulmonary and systemic circulation.
ADDITIONAL FUNCTIONS OF CIRCULATORY SYSTEM
In addition to oxygen, the circulatory system also transports nutrients derived from digested food to the body. These nutrients enter the bloodstream by passing through the walls of the intestine. The nutrients are absorbed through a network of capillaries and veins that drain the intestines, called the hepatic portal circulation. The hepatic portal circulation carries the nutrients to the liver for further metabolic processing. The liver stores a variety of substances, such as sugars, fats, and vitamins, and releases these to the blood as needed. The liver also cleans the blood by removing waste products and toxins. After hepatic portal blood has crossed the liver cells, veins converge to form the large hepatic vein that joins the vena cava near the right atrium.
The circulatory system plays an important role in regulating body temperature. During exercise, working muscles generate heat. The blood supplying the muscles with oxygen and nutrients absorbs much of this heat and carries it away to other parts of the body. If the body gets too warm, blood vessels near the skin enlarge to disperse excess heat outward through the skin. In cold environments, these blood vessels constrict to retain heat.
The circulatory system works in tandem with the endocrine system, a collection of hormone-producing glands. These glands release chemical messengers, called hormones, directly into the bloodstream to be transported to specific organs and tissues. Once they reach their target destination, hormones regulate the body’s rate of metabolism, growth, sexual development, and other functions.
The circulatory system also works with the immune system and the coagulation system. The immune system is a complex system of many types of cells that work together to combat diseases and infections. Disease-fighting white blood cells and antibodies circulate in the blood and are transported to sites of infection by the circulatory system. The coagulation system is composed of special blood cells, called platelets, and special proteins, called clotting factors, that circulate in the blood. Whenever blood vessels are cut or torn, the coagulation system works rapidly to stop the bleeding by forming clots.
Other organs support the circulatory system. The brain and other parts of the nervous system constantly monitor blood circulation, sending signals to the heart or blood vessels to maintain constant blood pressure. New blood cells are manufactured in the bone marrow. Old blood cells are broken down in the spleen, where valuable constituents, such as iron, are recycled. Metabolic waste products are removed from the blood by the kidneys, which also screen the blood for excess salt and maintain blood pressure and the body’s balance of minerals and fluids.
DISORDERS OF CIRCULATORY SYSTEM
Disorders of the circulatory system include any injury or disease that damages the heart, the blood, or the blood vessels. The three most important circulatory diseases are hypertension, arteriosclerosis, and atherosclerosis.
In arteriosclerosis, commonly known as hardening of the arteries, the walls of the arteries thicken, harden, and lose their elasticity. The heart must work harder than normal to deliver blood, and in advanced cases, it becomes impossible for the heart to supply sufficient blood to all parts of the body. Nobody knows what causes arteriosclerosis, but heredity, obesity, smoking, and a high-fat diet all appear to play roles.
Atherosclerosis, a form of arteriosclerosis, is the reduction in blood flow through the arteries caused by greasy deposits called plaque that form on the insides of arteries and partially restrict the flow of blood. Plaque deposits are associated with high concentrations of cholesterol in the blood. Recent studies have also shown an association between inflammation and plaque deposits. Blood flow is often further reduced by the formation of blood clots (see Thrombosis), which are most likely to form where the artery walls have been roughened by plaque. These blood clots can also break free and travel through the circulatory system until they become lodged somewhere else and reduce blood flow there (see Embolism). Reduction in blood flow can cause organ damage. When brain arteries become blocked and brain function is impaired, the result is a stroke. A heart attack occurs when a coronary artery becomes blocked and heart muscle is destroyed.
Risk factors that contribute to atherosclerosis include physical inactivity, smoking, a diet high in fat, high blood pressure, and diabetes. Some cases of atherosclerosis can be corrected with healthy lifestyle changes, aspirin to reduce blood clotting or inflammation, or drugs to lower the blood cholesterol concentration. For more serious cases, surgery to dilate narrowed blood vessels with a balloon, known as angioplasty, or to remove plaque with a high-speed cutting drill, known as atherectomy, may be effective. Surgical bypass, in which spare arteries are used to construct a new path for blood flow, is also an option.
Hypertention: condition that arises when the blood pressure is abnormally high. Hypertension occurs when the body's smaller blood vessels (the arterioles) narrow, causing the blood to exert excessive pressure against the vessel walls and forcing the heart to work harder to maintain the pressure. Although the heart and blood vessels can tolerate increased blood pressure for months and even years, eventually the heart may enlarge (a condition called hypertrophy) and be weakened to the point of failure. Injury to blood vessels in the kidneys, brain, and eyes also may occur.
Blood pressure is actually a measure of two pressures, the systolic and the diastolic. The systolic pressure (the higher pressure and the first number recorded) is the force that blood exerts on the artery walls as the heart contracts to pump the blood to the peripheral organs and tissues. The diastolic pressure (the lower pressure and the second number recorded) is residual pressure exerted on the arteries as the heart relaxes between beats. A diagnosis of hypertension is made when blood pressure reaches or exceeds 140/90 mmHg (read as “140 over 90 millimetres of mercury”).
When there is no demonstrable underlying cause of hypertension, the condition is classified as essential hypertension. (Essential hypertension is also called primary or idiopathic hypertension.) This is by far the most common type of high blood pressure, occurring in up to 90 percent of patients. Genetic factors appear to play a major role in the occurrence of essential hypertension. Secondary hypertension is associated with an underlying disease, which may be renal, neurologic, or endocrine in origin; examples of such diseases include Bright disease (glomerulonephritis; inflammation of the urine-producing structures in the kidney), atherosclerosis of blood vessels in the brain, and Cushing syndrome (hyperactivity of the adrenal glands). In cases of secondary hypertension, correction of the underlying cause may cure the hypertension. Various external agents also can raise blood pressure. These include cocaine, amphetamines, cold remedies, thyroid supplements, corticosteroids, nonsteroidal anti-inflammatory drugs (NSAIDs), and oral contraceptives.
Malignant hypertension is present when there is a sustained or sudden rise in diastolic blood pressure exceeding 120 mmHg, with accompanying evidence of damage to organs such as the eyes, brain, heart, and kidneys. Malignant hypertension is a medical emergency and requires immediate therapy and hospitalization.
Elevated arterial pressure is one of the most important public health problems in developed countries. In the United States, for instance, it is generally accepted that 15 to 20 percent of the adult population is hypertensive and often undiagnosed. High blood pressure is significantly more prevalent and serious among African Americans. Age, race, sex, smoking, alcohol intake, elevated serum cholesterol, salt intake, glucose intolerance, obesity, and stress all may contribute to the degree and prognosis of this disease. In both men and women, the risk of developing high blood pressure increases with age. Until age 55 more men than women have hypertension; after that the ratio reverses.
Hypertension has been called the “silent killer” because it usually produces no symptoms. It is important, therefore, for anyone with risk factors to have their blood pressure checked regularly and to make appropriate lifestyle changes.
The most common immediate cause of hypertension-related death is heart disease, but death from stroke or renal (kidney) failure is also frequent. Complications result directly from the increased pressure (cerebral hemorrhage, retinopathy, left ventricular hypertrophy, congestive heart failure, arterial aneurysm, and vascular rupture), from atherosclerosis (increased coronary, cerebral, and renal vascular resistance), and from decreased blood flow and ischemia (myocardial infarction, cerebral thrombosis and infarction, and renal nephrosclerosis).
Effective treatment will reduce overall cardiovascular morbidity and mortality. Nondrug therapy consists of: (1) relief of stress, (2) dietary management (restricted intake of salt, calories, cholesterol, and saturated fats; sufficient intake of potassium, magnesium, calcium, and vitamin C), (3) regular aerobic exercise, (4) weight reduction, (5) smoking cessation, and (6) reduced intake of alcohol and caffeine.
Mild to moderate hypertension may be controlled by a single-drug regimen, although more severe cases often require a combination of two or more drugs. Diuretics are a common medication; these agents lower blood pressure primarily by reducing body fluids and thereby reducing peripheral resistance to blood flow. However, they deplete the body's supply of potassium, so it is recommended that potassium supplements be added or that potassium-sparing diuretics be used. Beta-adrenergic blockers (beta-blockers) block the effects of epinephrine (adrenaline), thus easing the heart's pumping action and widening blood vessels. Vasodilators act by relaxing smooth muscle in the walls of blood vessels, allowing small arteries to dilate and thereby decreasing total peripheral resistance. Calcium channel blockers promote peripheral vasodilation and reduce vascular resistance. Angiotensin-converting enzyme (ACE) inhibitors inhibit the generation of a potent vasoconstriction agent (angiotensin II), and they also may retard the degradation of a potent vasodilator (bradykinin) and involve the synthesis of vasodilatory prostaglandins. Angiotensin receptor antagonists are similar to ACE inhibitors in utility and tolerability, but instead of blocking the production of angiotensin II, they completely inhibit its binding to the angiotensin II receptor.
The heart, blood, and blood vessels are the three structural elements that make up the circulatory system. The heart is the engine of the circulatory system. It is divided into four chambers: the right atrium, the right ventricle, the left atrium, and the left ventricle. The walls of these chambers are made of a special muscle called myocardium, which contracts continuously and rhythmically to pump blood. The pumping action of the heart occurs in two stages for each heart beat: diastole, when the heart is at rest; and systole, when the heart contracts to pump deoxygenated blood toward the lungs and oxygenated blood to the body. During each heartbeat, typically about 60 to 90 ml (about 2 to 3 oz) of blood are pumped out of the heart. If the heart stops pumping, death usually occurs within four to five minutes.
Blood consists of three types of cells: oxygen-bearing red blood cells, disease-fighting white blood cells, and blood-clotting platelets, all of which are carried through blood vessels in a liquid called plasma. Plasma is yellowish and consists of water, salts, proteins, vitamins, minerals, hormones, dissolved gases, and fats.
Three types of blood vessels form a complex network of tubes throughout the body. Arteries carry blood away from the heart, and veins carry it toward the heart. Capillaries are the tiny links between the arteries and the veins where oxygen and nutrients diffuse to body tissues. The inner layer of blood vessels is lined with endothelial cells that create a smooth passage for the transit of blood. This inner layer is surrounded by connective tissue and smooth muscle that enable the blood vessel to expand or contract. Blood vessels expand during exercise to meet the increased demand for blood and to cool the body. Blood vessels contract after an injury to reduce bleeding and also to conserve body heat.
Arteries have thicker walls than veins to withstand the pressure of blood being pumped from the heart. Blood in the veins is at a lower pressure, so veins have one-way valves to prevent blood from flowing backwards away from the heart. Capillaries, the smallest of blood vessels, are only visible by microscope—ten capillaries lying side by side are barely as thick as a human hair. If all the arteries, veins, and capillaries in the human body were placed end to end, the total length would equal more than 100,000 km (more than 60,000 mi)—they could stretch around the earth nearly two and a half times.
The arteries, veins, and capillaries are divided into two systems of circulation: systemic and pulmonary. The systemic circulation carries oxygenated blood from the heart to all the tissues in the body except the lungs and returns deoxygenated blood carrying waste products, such as carbon dioxide, back to the heart. The pulmonary circulation carries this spent blood from the heart to the lungs. In the lungs, the blood releases its carbon dioxide and absorbs oxygen. The oxygenated blood then returns to the heart before transferring to the systemic circulation.
A. Systemic Circulation
The heart ejects oxygen-rich blood under high pressure out of the heart’s main pumping chamber, the left ventricle, through the largest artery, the aorta. Smaller arteries branch off from the aorta, leading to various parts of the body. These smaller arteries in turn branch out into even smaller arteries, called arterioles. Branches of arterioles become progressively smaller in diameter, eventually forming the capillaries. Once blood reaches the capillary level, blood pressure is greatly reduced.
Capillaries have extremely thin walls that permit dissolved oxygen and nutrients from the blood to diffuse across to a fluid, known as interstitial fluid, that fills the gaps between the cells of tissues or organs. The dissolved oxygen and nutrients then enter the cells from the interstitial fluid by diffusion across the cell membranes. Meanwhile, carbon dioxide and other wastes leave the cell, diffuse through the interstitial fluid, cross the capillary walls, and enter the blood. In this way, the blood delivers nutrients and removes wastes without leaving the capillary tube.
After delivering oxygen to tissues and absorbing wastes, the deoxygenated blood in the capillaries then starts the return trip to the heart. The capillaries merge to form tiny veins, called venules. These veins in turn join together to form progressively larger veins. Ultimately, the veins converge into two large veins: the inferior vena cava, bringing blood from the lower half of the body; and the superior vena cava, bringing blood from the upper half. Both of these two large veins join at the right atrium of the heart.
Because the pressure is dissipated in the arterioles and capillaries, blood in veins flows back to the heart at very low pressure, often running uphill when a person is standing. Flow against gravity is made possible by the one-way valves, located several centimeters apart, in the veins. When surrounding muscles contract, for example in the calf or arm, the muscles squeeze blood back toward the heart. If the one-way valves work properly, blood travels only toward the heart and cannot lapse backward. Veins with defective valves, which allow the blood to flow backward, become enlarged or dilated to form varicose veins.
B. Pulmonary Circulation
In pulmonary circulation, deoxygenated blood returning from the organs and tissues of the body travels from the right atrium of the heart to the right ventricle. From there it is pushed through the pulmonary artery to the lung. In the lung, the pulmonary artery divides, forming the pulmonary capillary region of the lung. At this site, microscopic vessels pass adjacent to the alveoli, or air sacs of the lung, and gases are exchanged across a thin membrane: oxygen crosses the membrane into the blood while carbon dioxide leaves the blood through this same membrane. Newly oxygenated blood then flows into the pulmonary veins, where it is collected by the left atrium of the heart, a chamber that serves as collecting pool for the left ventricle. The contraction of the left ventricle sends blood into the aorta, completing the circulatory loop. On average, a single blood cell takes roughly 30 seconds to complete a full circuit through both the pulmonary and systemic circulation.
ADDITIONAL FUNCTIONS OF CIRCULATORY SYSTEM
In addition to oxygen, the circulatory system also transports nutrients derived from digested food to the body. These nutrients enter the bloodstream by passing through the walls of the intestine. The nutrients are absorbed through a network of capillaries and veins that drain the intestines, called the hepatic portal circulation. The hepatic portal circulation carries the nutrients to the liver for further metabolic processing. The liver stores a variety of substances, such as sugars, fats, and vitamins, and releases these to the blood as needed. The liver also cleans the blood by removing waste products and toxins. After hepatic portal blood has crossed the liver cells, veins converge to form the large hepatic vein that joins the vena cava near the right atrium.
The circulatory system plays an important role in regulating body temperature. During exercise, working muscles generate heat. The blood supplying the muscles with oxygen and nutrients absorbs much of this heat and carries it away to other parts of the body. If the body gets too warm, blood vessels near the skin enlarge to disperse excess heat outward through the skin. In cold environments, these blood vessels constrict to retain heat.
The circulatory system works in tandem with the endocrine system, a collection of hormone-producing glands. These glands release chemical messengers, called hormones, directly into the bloodstream to be transported to specific organs and tissues. Once they reach their target destination, hormones regulate the body’s rate of metabolism, growth, sexual development, and other functions.
The circulatory system also works with the immune system and the coagulation system. The immune system is a complex system of many types of cells that work together to combat diseases and infections. Disease-fighting white blood cells and antibodies circulate in the blood and are transported to sites of infection by the circulatory system. The coagulation system is composed of special blood cells, called platelets, and special proteins, called clotting factors, that circulate in the blood. Whenever blood vessels are cut or torn, the coagulation system works rapidly to stop the bleeding by forming clots.
Other organs support the circulatory system. The brain and other parts of the nervous system constantly monitor blood circulation, sending signals to the heart or blood vessels to maintain constant blood pressure. New blood cells are manufactured in the bone marrow. Old blood cells are broken down in the spleen, where valuable constituents, such as iron, are recycled. Metabolic waste products are removed from the blood by the kidneys, which also screen the blood for excess salt and maintain blood pressure and the body’s balance of minerals and fluids.
DISORDERS OF CIRCULATORY SYSTEM
Disorders of the circulatory system include any injury or disease that damages the heart, the blood, or the blood vessels. The three most important circulatory diseases are hypertension, arteriosclerosis, and atherosclerosis.
In arteriosclerosis, commonly known as hardening of the arteries, the walls of the arteries thicken, harden, and lose their elasticity. The heart must work harder than normal to deliver blood, and in advanced cases, it becomes impossible for the heart to supply sufficient blood to all parts of the body. Nobody knows what causes arteriosclerosis, but heredity, obesity, smoking, and a high-fat diet all appear to play roles.
Atherosclerosis, a form of arteriosclerosis, is the reduction in blood flow through the arteries caused by greasy deposits called plaque that form on the insides of arteries and partially restrict the flow of blood. Plaque deposits are associated with high concentrations of cholesterol in the blood. Recent studies have also shown an association between inflammation and plaque deposits. Blood flow is often further reduced by the formation of blood clots (see Thrombosis), which are most likely to form where the artery walls have been roughened by plaque. These blood clots can also break free and travel through the circulatory system until they become lodged somewhere else and reduce blood flow there (see Embolism). Reduction in blood flow can cause organ damage. When brain arteries become blocked and brain function is impaired, the result is a stroke. A heart attack occurs when a coronary artery becomes blocked and heart muscle is destroyed.
Risk factors that contribute to atherosclerosis include physical inactivity, smoking, a diet high in fat, high blood pressure, and diabetes. Some cases of atherosclerosis can be corrected with healthy lifestyle changes, aspirin to reduce blood clotting or inflammation, or drugs to lower the blood cholesterol concentration. For more serious cases, surgery to dilate narrowed blood vessels with a balloon, known as angioplasty, or to remove plaque with a high-speed cutting drill, known as atherectomy, may be effective. Surgical bypass, in which spare arteries are used to construct a new path for blood flow, is also an option.
Hypertention: condition that arises when the blood pressure is abnormally high. Hypertension occurs when the body's smaller blood vessels (the arterioles) narrow, causing the blood to exert excessive pressure against the vessel walls and forcing the heart to work harder to maintain the pressure. Although the heart and blood vessels can tolerate increased blood pressure for months and even years, eventually the heart may enlarge (a condition called hypertrophy) and be weakened to the point of failure. Injury to blood vessels in the kidneys, brain, and eyes also may occur.
Blood pressure is actually a measure of two pressures, the systolic and the diastolic. The systolic pressure (the higher pressure and the first number recorded) is the force that blood exerts on the artery walls as the heart contracts to pump the blood to the peripheral organs and tissues. The diastolic pressure (the lower pressure and the second number recorded) is residual pressure exerted on the arteries as the heart relaxes between beats. A diagnosis of hypertension is made when blood pressure reaches or exceeds 140/90 mmHg (read as “140 over 90 millimetres of mercury”).
When there is no demonstrable underlying cause of hypertension, the condition is classified as essential hypertension. (Essential hypertension is also called primary or idiopathic hypertension.) This is by far the most common type of high blood pressure, occurring in up to 90 percent of patients. Genetic factors appear to play a major role in the occurrence of essential hypertension. Secondary hypertension is associated with an underlying disease, which may be renal, neurologic, or endocrine in origin; examples of such diseases include Bright disease (glomerulonephritis; inflammation of the urine-producing structures in the kidney), atherosclerosis of blood vessels in the brain, and Cushing syndrome (hyperactivity of the adrenal glands). In cases of secondary hypertension, correction of the underlying cause may cure the hypertension. Various external agents also can raise blood pressure. These include cocaine, amphetamines, cold remedies, thyroid supplements, corticosteroids, nonsteroidal anti-inflammatory drugs (NSAIDs), and oral contraceptives.
Malignant hypertension is present when there is a sustained or sudden rise in diastolic blood pressure exceeding 120 mmHg, with accompanying evidence of damage to organs such as the eyes, brain, heart, and kidneys. Malignant hypertension is a medical emergency and requires immediate therapy and hospitalization.
Elevated arterial pressure is one of the most important public health problems in developed countries. In the United States, for instance, it is generally accepted that 15 to 20 percent of the adult population is hypertensive and often undiagnosed. High blood pressure is significantly more prevalent and serious among African Americans. Age, race, sex, smoking, alcohol intake, elevated serum cholesterol, salt intake, glucose intolerance, obesity, and stress all may contribute to the degree and prognosis of this disease. In both men and women, the risk of developing high blood pressure increases with age. Until age 55 more men than women have hypertension; after that the ratio reverses.
Hypertension has been called the “silent killer” because it usually produces no symptoms. It is important, therefore, for anyone with risk factors to have their blood pressure checked regularly and to make appropriate lifestyle changes.
The most common immediate cause of hypertension-related death is heart disease, but death from stroke or renal (kidney) failure is also frequent. Complications result directly from the increased pressure (cerebral hemorrhage, retinopathy, left ventricular hypertrophy, congestive heart failure, arterial aneurysm, and vascular rupture), from atherosclerosis (increased coronary, cerebral, and renal vascular resistance), and from decreased blood flow and ischemia (myocardial infarction, cerebral thrombosis and infarction, and renal nephrosclerosis).
Effective treatment will reduce overall cardiovascular morbidity and mortality. Nondrug therapy consists of: (1) relief of stress, (2) dietary management (restricted intake of salt, calories, cholesterol, and saturated fats; sufficient intake of potassium, magnesium, calcium, and vitamin C), (3) regular aerobic exercise, (4) weight reduction, (5) smoking cessation, and (6) reduced intake of alcohol and caffeine.
Mild to moderate hypertension may be controlled by a single-drug regimen, although more severe cases often require a combination of two or more drugs. Diuretics are a common medication; these agents lower blood pressure primarily by reducing body fluids and thereby reducing peripheral resistance to blood flow. However, they deplete the body's supply of potassium, so it is recommended that potassium supplements be added or that potassium-sparing diuretics be used. Beta-adrenergic blockers (beta-blockers) block the effects of epinephrine (adrenaline), thus easing the heart's pumping action and widening blood vessels. Vasodilators act by relaxing smooth muscle in the walls of blood vessels, allowing small arteries to dilate and thereby decreasing total peripheral resistance. Calcium channel blockers promote peripheral vasodilation and reduce vascular resistance. Angiotensin-converting enzyme (ACE) inhibitors inhibit the generation of a potent vasoconstriction agent (angiotensin II), and they also may retard the degradation of a potent vasodilator (bradykinin) and involve the synthesis of vasodilatory prostaglandins. Angiotensin receptor antagonists are similar to ACE inhibitors in utility and tolerability, but instead of blocking the production of angiotensin II, they completely inhibit its binding to the angiotensin II receptor.