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
Human Circulatory System
The human circulatory system is composed of the muscular heart and an intricate network of
elastic blood vessels known as arteries, veins, and capillaries. These structures work together to
circulate blood throughout the body, in the process delivering life-preserving oxygen and
nutrients to tissue cells while also removing waste products.
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.
Artery, one of the tubular vessels that conveys blood from the heart to the tissues of the body.
Two arteries have direct connection with the heart: (1) the aorta, which, with its branches,
conveys oxygenated blood from the left ventricle to every part of the body; and (2) the
pulmonary artery, which conveys blood from the right ventricle to the lungs, whence it is
returned bearing oxygen to the left side of the heart (see Heart: Structure and Function).
Arteries in their ultimate minute branchings are connected with the veins by capillaries. They
are named usually from the part of the body where they are found, as the brachial (arm) or the
metacarpal (wrist) artery; or from the organ which they supply, as the hepatic (liver) or the
ovarian artery. The facial artery is the branch of the external carotid artery that passes up over
the lower jaw and supplies the superficial portion of the face; the hemorrhoidal arteries are
three vessels that supply the lower end of the rectum; the intercostal arteries are the arteries
that supply the space between the ribs; the lingual artery is the branch of the external carotid
artery that supplies the tongue. The arteries expand and then constrict with each beat of the
heart, a rhythmic movement that may be felt as the pulse.
Disorders of the arteries may involve inflammation, infection, or degeneration of the walls of
the arterial blood vessels. The most common arterial disease, and the one which is most often a
contributory cause of death, particularly in old people, is arteriosclerosis, known popularly as
hardening of the arteries. The hardening usually is preceded by atherosclerosis, an
accumulation of fatty deposits on the inner lining of the arterial wall. The deposits reduce the
normal flow of the blood through the artery. One of the substances associated with
atherosclerosis is cholesterol. As arteriosclerosis progresses, calcium is deposited and scar
tissue develops, causing the wall to lose its elasticity. Localized dilatation of the arterial wall,
called an aneurysm, may also develop. Arteriosclerosis may affect any or all of the arteries of
the body. If the blood vessels supplying the heart muscle are affected, the disease may lead to a
painful condition known as angina pectoris. See Heart: Heart Diseases.
The presence of arteriosclerosis in the wall of an artery can precipitate formation of a clot, or
thrombus (see Thrombosis). Treatment consists of clot-dissolving enzymes called urokinase and
streptokinase, which were approved for medical use in 1979. Studies indicate that compounds
such as aspirin and sulfinpyrazone, which inhibit platelet reactivity, may act to prevent
formation of a thrombus, but whether they can or should be taken in tolerable quantities over
a long period of time for this purpose has not yet been determined.
Embolism is the name given to the obstruction of an artery by a clot carried to it from another
part of the body. Such floating clots may be caused by arteriosclerosis, but are most commonly
a consequence of the detachment of a mass of fibrin from a diseased heart. Any artery may be
obstructed by embolism; the consequences are most serious in the brain, the retina, and the
limbs. In the larger arteries of the brain, embolism causes stroke.
Vein (anatomy), in anatomy, blood vessel that conducts the deoxygenated blood from the
capillaries back to the heart. Three exceptions to this description exist: the pulmonary veins
return blood from the lungs, where it has been oxygenated, to the heart; the portal veins
receive blood from the pyloric, gastric, cystic, superior mesenteric, and splenic veins and,
entering the liver, break up into small branches that pass through all parts of that organ; and
the umbilical veins convey blood from the fetus to the mother's placenta. Veins enlarge as they
proceed, gathering blood from their tributaries. They finally pour the blood through the
superior and inferior venae cavae into the right atrium of the heart. Their coats are similar to
those of the arteries, but thinner, and often transparent.
The human heart is a hollow, pear-shaped organ about the size of a fist. The heart is made of
muscle that rhythmically contracts, or beats, pumping blood throughout the body. Oxygen-poor
blood from the body enters the heart from two large blood vessels, the inferior vena cava and
the superior vena cava, and collects in the right atrium. When the atrium fills, it contracts, and
blood passes through the tricuspid valve into the right ventricle. When the ventricle becomes
full, it starts to contract, and the tricuspid valve closes to prevent blood from moving back into
the atrium. As the right ventricle contracts, it forces blood into the pulmonary artery, which
carries blood to the lungs to pick up fresh oxygen. When blood exits the right ventricle, the
ventricle relaxes and the pulmonary valve shuts, preventing blood from passing back into the
ventricle. Blood returning from the lungs to the heart collects in the left atrium. When this
chamber contracts, blood flows through the mitral valve into the left ventricle. The left ventricle
fills and begins to contract, and the mitral valve between the two chambers closes. In the final
phase of blood flow through the heart, the left ventricle contracts and forces blood into the
aorta. After the blood in the left ventricle has been forced out, the ventricle begins to relax, and
the aortic valve at the opening of the aorta closes.
Thin, fibrous flaps called valves lie at the opening of the heart's pulmonary artery and aorta.
Valves are also present between each atrium and ventricle of the heart. Valves prevent blood
from flowing backward in the heart. In this illustration of the pulmonary valve, as the heart
contracts, blood pressure builds and pushes blood up against the pulmonary valve, forcing it to
open. As the heart relaxes between one beat and the next, blood pressure falls. Blood flows
back from the pulmonary artery, forcing the pulmonary valve to close, and preventing backflow
Heart and Blood Circulation
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.
Constituents of Blood
In an average healthy person, approximately 45 percent of the blood volume is cells, among
them red cells (the majority), white cells, and platelets. A clear, yellowish fluid called plasma
makes up the rest of blood. Plasma, 95 percent of which is water, also contains nutrients such
as glucose, fats, proteins, and the amino acids needed for protein synthesis, vitamins, and
minerals. The level of salt in plasma is about equal to that of sea water. The test tube on the
right has been centrifuged to separate plasma and packed cells by density.
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 bac