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Circulatory System.

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Circulatory System.
I

INTRODUCTION

Circulatory System, or cardiovascular system, in humans, 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.

II

COMPONENTS OF THE CIRCULATORY SYSTEM

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.

III

OPERATION AND FUNCTION

Only in the past 400 years have scientists recognized that blood moves in a cycle through the heart and body. Before the 17th century, scientists believed that the liver
creates new blood, and then the blood passes through the heart to gain warmth and finally is soaked up and consumed in the tissues. In 1628 English physician William
Harvey first proposed that blood circulates continuously. Using modern methods of observation and experimentation, Harvey noted that veins have one-way valves that
lead blood back to the heart from all parts of the body. He noted that the heart works as a pump, and he estimated correctly that the daily output of fresh blood is
more than seven tons. He pointed out the absurdity of the old doctrine, which would require the liver to produce this much fresh blood daily. Harvey's theory was soon
proven correct and became the cornerstone of modern medical science.

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.

C

Additional Functions

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.

D

Blood Pressure

The pressure generated by the pumping action of the heart propels the blood to the arteries. In order to maintain an adequate flow of blood to all parts of the body, a
certain level of blood pressure is needed. Blood pressure, for instance, enables a person to rise quickly from a horizontal position without blood pooling in the legs, which
would cause fainting from deprivation of blood to the brain. Normal blood pressure is regulated by a number of factors, such as the contraction of the heart, the
elasticity of arterial walls, blood volume, and resistance of blood vessels to the passage of blood.
Blood pressure is measured using an inflatable device with a gauge called a that is wrapped around the upper arm. Blood pressure is measured during systole, the
active pumping phase of the heart, and diastole, the resting phase between heartbeats. Systolic and diastolic pressures are measured in units of millimeters of mercury
(abbreviated mm Hg) and displayed as a ratio. Blood pressure varies between individuals and even during the normal course of a day in response to emotion, exertion,
sleep, and other physical and mental changes. Normal blood pressure is less than 120/80 mm Hg, in which 120 describes systolic pressure and 80 describes diastolic
pressure. Higher blood pressures that are sustained over a long period of time may indicate hypertension, a damaging circulatory condition. Lower blood pressures
could signal shock from heart failure, dehydration, internal bleeding, or blood loss.

IV

CIRCULATORY SYSTEM DISORDERS

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.
Hypertension, or elevated blood pressure, develops when the body's blood vessels narrow, causing the heart to pump harder than normal to push blood through the
narrowed openings. Hypertension that remains untreated may cause heart enlargement and thickening of the heart muscle. Eventually the heart needs more oxygen to
function, which can lead to heart failure, brain stroke, or kidney impairment. Some cases of hypertension can be treated by lifestyle changes such as a low-salt diet,
maintenance of ideal weight, aerobic exercise, and a diet rich in fruits, vegetables, plant fiber, and the mineral potassium. If blood pressure remains high despite these
lifestyle adjustments, medications may be effective in lowering the pressure by relaxing blood vessels and reducing the output of blood.
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.

V

CIRCULATORY SYSTEMS IN NONHUMANS

One-celled organisms and many simple multicelled animals, such as sponges, jellyfishes, sea anemones, flatworms, and roundworms, do not have a circulatory system.
All of their cells are able to absorb nutrients, exchange gases, and expel wastes through direct contact with either the outside or with a central cavity that serves as a
digestive tract.
More complex invertebrates have a wide range of circulatory system designs. These invertebrate circulatory systems are classified as either open or closed. Open
systems--found in starfishes, clams, oysters, snails, crabs, insects, spiders, and centipedes--lack capillaries, and the blood bathes the tissues directly. In closed
systems, the blood is confined to a system of blood vessels. Invertebrates with closed systems include segmented worms, squids, and octopuses.

All vertebrate animals have closed circulatory systems. These systems are classified by the number of chambers in the heart, which determines the basic configuration
of blood flow. Fish have two-chambered hearts with one atrium and one ventricle. Blood pumped from the ventricle travels through arteries to the gills, where it
diverges into capillaries and exchanges gases. Leaving the gills, the capillaries reconvene into blood vessels that carry the oxygenated blood to the rest of the body,
where the vessels again diverge into capillaries before reconvening into veins that return to the heart. In this way, the blood passes through first the respiratory organs
(the gills) and then the systemic circulation between each pass through the heart.
Frogs and amphibians have three-chambered hearts, with two atriums and one ventricle. Blood pumped from the ventricle enters a forked artery. One fork, the
pulmonary circulation, leads to the lung. The other fork, the systemic circulation, leads to the rest of the body. Blood returning from the pulmonary circulation enters
the left atrium, while blood from the systemic circulation enters the right atrium. Although there is some mixing of oxygenated and deoxygenated blood in the ventricle,
a ridge within the ventricle assures that most of the oxygenated blood is diverted to the systemic circulation and most of the deoxygenated blood goes to the
pulmonary circulation. In reptiles, this ridge is more developed, forming a partial wall. In crocodiles, the wall is complete, forming a four-chambered heart like that found
in mammals and birds.

Contributed By:
John F. Setaro
Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.

« C Additional Functions In addition to oxygen, the circulatory system also transports nutrients derived from digested food to the body. These nutrients enter the bloodstream by passingthrough 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, andvitamins, 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 theliver 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 withoxygen 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 todisperse 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, calledhormones, directly into the bloodstream to be transported to specific organs and tissues. Once they reach their target destination, hormones regulate the body’s rate ofmetabolism, 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 worktogether to combat diseases and infections. Disease-fighting white blood cells and antibodies circulate in the blood and are transported to sites of infection by thecirculatory 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 bloodvessels to maintain constant blood pressure. New blood cells are manufactured in the bone marrow. Old blood cells are broken down in the spleen, where valuableconstituents, 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 maintainblood pressure and the body’s balance of minerals and fluids. D Blood Pressure The pressure generated by the pumping action of the heart propels the blood to the arteries. In order to maintain an adequate flow of blood to all parts of the body, acertain level of blood pressure is needed. Blood pressure, for instance, enables a person to rise quickly from a horizontal position without blood pooling in the legs, whichwould cause fainting from deprivation of blood to the brain. Normal blood pressure is regulated by a number of factors, such as the contraction of the heart, theelasticity of arterial walls, blood volume, and resistance of blood vessels to the passage of blood. Blood pressure is measured using an inflatable device with a gauge called a that is wrapped around the upper arm. Blood pressure is measured during systole, theactive pumping phase of the heart, and diastole, the resting phase between heartbeats. Systolic and diastolic pressures are measured in units of millimeters of mercury(abbreviated mm Hg) and displayed as a ratio. Blood pressure varies between individuals and even during the normal course of a day in response to emotion, exertion,sleep, and other physical and mental changes. Normal blood pressure is less than 120/80 mm Hg, in which 120 describes systolic pressure and 80 describes diastolicpressure. Higher blood pressures that are sustained over a long period of time may indicate hypertension, a damaging circulatory condition. Lower blood pressurescould signal shock from heart failure, dehydration, internal bleeding, or blood loss. IV CIRCULATORY SYSTEM DISORDERS 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 diseasesare hypertension, arteriosclerosis, and atherosclerosis. Hypertension, or elevated blood pressure, develops when the body’s blood vessels narrow, causing the heart to pump harder than normal to push blood through thenarrowed openings. Hypertension that remains untreated may cause heart enlargement and thickening of the heart muscle. Eventually the heart needs more oxygen tofunction, which can lead to heart failure, brain stroke, or kidney impairment. Some cases of hypertension can be treated by lifestyle changes such as a low-salt diet,maintenance of ideal weight, aerobic exercise, and a diet rich in fruits, vegetables, plant fiber, and the mineral potassium. If blood pressure remains high despite theselifestyle adjustments, medications may be effective in lowering the pressure by relaxing blood vessels and reducing the output of blood. 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 thannormal 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 causesarteriosclerosis, 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 arteriesand partially restrict the flow of blood. Plaque deposits are associated with high concentrations of cholesterol in the blood. Recent studies have also shown an associationbetween 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 somewhereelse 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 atherosclerosiscan be corrected with healthy lifestyle changes, aspirin to reduce blood clotting or inflammation, or drugs to lower the blood cholesterol concentration. For more seriouscases, 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, maybe effective. Surgical bypass, in which spare arteries are used to construct a new path for blood flow, is also an option. V CIRCULATORY SYSTEMS IN NONHUMANS One-celled organisms and many simple multicelled animals, such as sponges, jellyfishes, sea anemones, flatworms, and roundworms, do not have a circulatory system.All of their cells are able to absorb nutrients, exchange gases, and expel wastes through direct contact with either the outside or with a central cavity that serves as adigestive tract. More complex invertebrates have a wide range of circulatory system designs. These invertebrate circulatory systems are classified as either open or closed. Opensystems—found in starfishes, clams, oysters, snails, crabs, insects, spiders, and centipedes—lack capillaries, and the blood bathes the tissues directly. In closedsystems, the blood is confined to a system of blood vessels. Invertebrates with closed systems include segmented worms, squids, and octopuses. »

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