circulatory system

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circulatory system,

group of organs that transport blood and the substances it carries to and from all parts of the body. The circulatory system can be considered as composed of two parts: the systemic circulation, which serves the body as a whole except for the lungs, and the pulmonary circulation, which carries the blood to and from the lungs. The organs of circulatory system consist of vessels that carry the blood and a muscular pump, the heartheart,
muscular organ that pumps blood to all parts of the body. The rhythmic beating of the heart is a ceaseless activity, lasting from before birth to the end of life. Anatomy and Function

The human heart is a pear-shaped structure about the size of a fist.
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, that drives the blood.

Of the vessels, the arteriesartery,
blood vessel that conveys blood away from the heart. Except for the pulmonary artery, which carries deoxygenated blood from the heart to the lungs, arteries carry oxygenated blood from the heart to the tissues.
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 carry blood away from the heart; the main arterial vessel, the aortaaorta
, primary artery of the circulatory system in mammals, delivering oxygenated blood to all other arteries except those of the lungs. The human aorta, c.1 in. (2.54 cm) in diameter, originates at the left ventricle of the heart.
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, branches into smaller arteries, which in turn branch repeatedly into still smaller vessels and reach all parts of the body. Within the body tissues, the vessels are microscopic capillariescapillary
, microscopic blood vessel, smallest unit of the circulatory system. Capillaries form a network of tiny tubes throughout the body, connecting arterioles (smallest arteries) and venules (smallest veins).
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 through which gas and nutrient exchange occurs (see respirationrespiration,
process by which an organism exchanges gases with its environment. The term now refers to the overall process by which oxygen is abstracted from air and is transported to the cells for the oxidation of organic molecules while carbon dioxide (CO2
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). Blood leaving the tissue capillaries enters converging vessels, the veinsvein,
blood vessel that returns blood to the heart. Except for the pulmonary vein, which carries oxygenated blood from the lungs to the heart, veins carry deoxygenated blood. The oxygen-depleted blood passes from the capillaries to the venules (small veins).
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, to return to the heart and lungs. The human heart is a four-chambered organ with a dividing wall, or septum, that separates it into a right heart for pumping blood from the returning veins into the lungs and a left heart for pumping blood from the lungs to the body via the aorta.

An auxiliary system, the lymphatic systemlymphatic system
, network of vessels carrying lymph, or tissue-cleansing fluid, from the tissues into the veins of the circulatory system. The lymphatic system functions along with the circulatory system in absorbing nutrients from the small intestines.
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, is composed of vessels that collect lymph from body tissues. Carried to converging vessels of increasing size, the lymph enters the thoracic duct and is emptied into a large vein near the heart.

Systemic Circulation

In the systemic circulation, which serves the body except for the lungs, oxygenated blood from the lungs returns to the heart from two pairs of pulmonary veins, a pair from each lung. It enters the left atrium, which contracts when filled, sending blood into the left ventricle (a large percentage of blood also enters the ventricle passively, without atrial contraction). The bicuspid, or mitral, valve controls blood flow into the ventricle. Contraction of the powerful ventricle forces the blood under great pressure into the aortic arch and on into the aorta. The coronary arteries stem from the aortic root and nourish the heart muscle itself. Three major arteries originate from the aortic arch, supplying blood to the head, neck, and arms. The other major arteries originating from the aorta are the renal arteries, which supply the kidneys; the celiac axis and superior and inferior mesenteric arteries, which supply the intestines, spleen, and liver; and the iliac arteries, which branch out to the lower trunk and become the femoral and popliteal arteries of the thighs and legs, respectively. The arterial walls are partially composed of fibromuscular tissue, which help to regulate blood pressureblood pressure,
force exerted by the blood upon the walls of the arteries. The pressure in the arteries originates in the pumping action of the heart, and pressure waves can be felt at the wrist and at other points where arteries lie near the surface of the body (see pulse).
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 and flow. In addition, a system of shunts allows blood to bypass the capillary beds and helps to regulate body temperature.

At the far end of the network, the capillaries converge to form venules, which in turn form veins. The inferior vena cava returns blood to the heart from the legs and trunk; it is supplied by the iliac veins from the legs, the hepatic veins from the liver, and the renal veins from the kidneys. The subclavian veins, draining the arms, and the jugular veins, draining the head and neck, join to form the superior vena cava. The two vena cavae, together with the coronary veins, return blood low in oxygen and high in carbon dioxide to the right atrium of the heart.

Pulmonary Circulation

The pulmonary circulation carries the blood to and from the lungs. In the heart, the blood flows from the right atrium into the right ventricle; the tricuspid valve prevents backflow from ventricles to atria. The right ventricle contracts to force blood into the lungs through the pulmonary arteries. In the lungs oxygen is picked up and carbon dioxide eliminated, and the oxygenated blood returns to the heart via the pulmonary veins, thus completing the circuit. In pulmonary circulation, the arteries carry oxygen-poor blood, and the veins bear oxygen-rich blood.

The Body's Filtering System

The organs most intimately related to the substances carried by the blood are the kidneys, which filter out nitrogenous wastes and regulate concentration of salts; the spleenspleen,
soft, purplish-red organ that lies under the diaphragm on the left side of the abdominal cavity. The spleen acts as a filter against foreign organisms that infect the bloodstream, and also filters out old red blood cells from the bloodstream and decomposes them.
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, which removes worn red blood cells, or lymphocytes; and the liverliver,
largest glandular organ of the body, weighing about 3 lb (1.36 kg). It is reddish brown in color and is divided into four lobes of unequal size and shape. The liver lies on the right side of the abdominal cavity beneath the diaphragm.
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, which contributes clotting factors to the blood, helps to control blood sugar levels, also removes old red blood cells and, receiving all the veins from the intestines and stomach, detoxifies the blood before it returns to the vena cava (see urinary systemurinary system,
group of organs of the body concerned with excretion of urine, that is, water and the waste products of metabolism. In humans, the kidneys are two small organs situated near the vertebral column at the small of the back, the left lying somewhat higher than the
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Circulatory Disorders

Disorders of the circulatory system generally result in diminished flow of blood and diminished oxygen exchange to the tissues. Blood supply is also impeded in such conditions as arteriosclerosisarteriosclerosis
, general term for a condition characterized by thickening, hardening, and loss of elasticity of the walls of the blood vessels. These changes are frequently accompanied by accumulations inside the vessel walls of lipids, e.g.
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 and high blood pressure (see hypertensionhypertension
or high blood pressure,
elevated blood pressure resulting from an increase in the amount of blood pumped by the heart or from increased resistance to the flow of blood through the small arterial blood vessels (arterioles).
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); low blood pressure resulting from injury (shockshock,
any condition in which the circulatory system is unable to provide adequate circulation to the body tissues, also called circulatory failure or circulatory collapse. Shock results in the slowing of vital functions and in severe cases, if untreated, in death.
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) is manifested by inadequate blood flow. Acute impairment of blood flow to the heart muscle itself with resulting damage to the heart, known as a heart attack or myocardial infarctioninfarction,
blockage of blood circulation to a localized area or organ of the body resulting in tissue death. Infarctions commonly occur in the spleen, kidney, lungs, brain, and heart.
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, or to the brain (strokestroke,
destruction of brain tissue as a result of intracerebral hemorrhage or infarction caused by thrombosis (clotting) or embolus (obstruction in a blood vessel caused by clotted blood or other foreign matter circulating in the bloodstream); formerly called apoplexy.
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) are most dangerous. Structural defects of the heart affecting blood distribution may be congenital or caused by many diseases, e.g., rheumatic feverrheumatic fever
, systemic inflammatory disease, extremely variable in its manifestation, severity, duration, and aftereffects. It is frequently followed by serious heart disease, especially when there are repeated attacks. Rheumatic fever usually affects children.
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, coronary artery diseasecoronary artery disease,
condition that results when the coronary arteries are narrowed or occluded, most commonly by atherosclerotic deposits of fibrous and fatty tissue. Coronary artery disease is the most common underlying cause of cardiovascular disability and death.
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See also heart diseaseheart disease,
any of several abnormalities of the heart and its function in maintaining blood circulation. Heart disease is the cause of approximately half the deaths in the United States each year.
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; angina pectorisangina pectoris
, condition characterized by chest pain that occurs when the muscles of the heart receive an insufficient supply of oxygen. This results when the arteries that supply the heart muscle with oxygenated blood are narrowed by arteriosclerosis.
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Circulatory System


in man and animals, the system of blood vessels and cavities through which blood or hemolymph circulates. By the circulatory system, cells and tissues are supplied with nutrients and oxygen, and the products of metabolism are eliminated. For this reason it is sometimes called a transport or distributing system.

A distinction is made between two types of circulatory systems; the open (lacunary) system is peculiar to most invertebrates (arthropods and mollusks) and lower chordates (hemichordates and tunicates); the closed system is characteristic of some invertebrates (nemertines and segmented worms), all vertebrates, and man. In animals with an open circulatory system the blood vessels are interrupted by slitlike spaces (lacunae and sinuses) that do not have actual walls. The blood (in this case called hemolymph) comes in direct contact with all the body tissues. In animals with a closed circulatory system the blood flows through vessels, and substances are exchanged between the blood and various tissues through the walls of the vessels. The lymphatic system became separate from the closed circulatory system (from its venous part) of vertebrates in the course of evolution.

In man, vertebrates, and some invertebrates (arthropods and mollusks) the heart is the main organ of the circulatory system. The vessels that carry blood from the heart are called arteries, and the vessels that carry blood to the heart are called veins. In a closed circulatory system blood from the arteries flows into vessels of increasingly smaller caliber and finally into arterioles, from which the blood enters the capillaries. The capillaries combine to form a complex network from which the blood enters first small veins (venules) and then increasingly larger veins. The inner layer of the venous walls forms peculiar pocketlike valves that direct the flow of blood in one direction. The middle layer of the arterial walls contains an unusually large number of smooth muscular and elastic fibers, which enable the arteries to pulsate.

The structure of the circulatory system in nemertines is very simple. It consists of three longitudinal vessels—one dorsal and two lateral. Blood flows into the anterior and posterior parts of the body through the dorsal and lateral vessels respectively. Segmented worms have, in addition to the main longitudinal vessels (dorsal and ventral), transverse vessels that branch off to the intestine, parapodia, and excretory organs. The circulatory system is even more complicated in arthropods, brachiopods, and mollusks because they have a heart situated on the dorsal side of the body. In some arthropods, especially those with tracheal respiration, the open circulatory system is simplified because the respiratory function has largely shifted from the circulatory system to the tracheae. Mollusks exhibit all the transitions from an open circulatory system to an almost closed one (cephalopods). Among the invertebrates, only in the mollusks is the heart divided into a ventricle and atria. The blood, enriched with oxygen in the gills, enters the atria; thus, the blood contained in the heart is arterial. In echinoderms the weakly developed circulatory system is due to the system of lacunae and sinuses. Sea urchins and holothurians have well-developed blood vessels.

The structure of the circulatory system is most complex in vertebrate animals and man. Their heart has a powerful muscular wall. The blood flows in one or two circulations, depending on whether the vertebrate animal breathes through gills or lungs. With the gill type of respiration (in cyclostomes and fish except lungfish), there is a single circulatory system. The heart consists of two main parts—an atrium and a ventricle (bicameral); in addition there is a venous sinus and, in most fish, an arterial cone as well. The heart is filled with venous blood. From it emerges the abdominal aorta, through which the venous blood enters the afferent gill arteries. The blood is oxygenated in the gills, becomes arterial, and flows through the efferent gill arteries to the dorsal aorta, from which it is carried to all the organs of the body. Venous blood enters the heart through the anterior and posterior cardinal veins, which in cyclostomes empty directly into the venous sinus and in fish through the ducts of Cuvier.

With the lung type of respiration (in all terrestrial vertebrate animals and man as well as in lungfish), there are two circulatory systems, the greater and the lesser. In the greater circuit, arterial blood flows from the heart through the arteries to all the organs and tissues. Passing through the capillary network of the individual organs, the blood moves into the venous system and enters the heart through the large veins. In the lesser circuit, venous blood from the heart passes through the pulmonary arteries into the lungs. Passing through the capillary network of the lungs, the oxygenated (arterial) blood returns to the heart via the pulmonary veins. Because of the existence of a second (lesser) circulatory system, the structure of the heart of terrestrial vertebrates became complex; instead of being bicameral, it is tricameral (two atria and one ventricle) in amphibians and quadricameral (two atria and two ventricles) in certain reptiles (crocodiles), birds, mammals, and man.

In most reptiles the ventricle is separated by an incomplete septum; for this reason, their heart has a structure intermediate between that of the tricameral and quadricameral heart. In the quadricameral heart the arterial blood is completely separated from the venous blood, and as a result the tissues and organs are supplied only with arterial blood. In the tricameral heart arterial blood mixes with venous blood in the ventricle, and the organs are supplied with mixed blood. In all terrestrial vertebrate animals and man the blood vessels that branch off the abdominal aorta (which correspond to gill vessels in fish) change during embryonic development. Adult amphibians and reptiles have two aortic arches—right and left. Birds have only a right aortic arch, and mammals and man have only a left aortic arch. The venous system of all terrestrial vertebrates and man has a vena cava inferior, which performs the function of the posterior cardinal veins, and two (less commonly, one) venae cavae superiores, which are formed from the ducts of Cuvier. All vertebrates have a liver portal system. The kidney portal system is well developed in fish, amphibians, and reptiles. It is poorly developed in mammals and absent in man.


Shmal’gauzen, I. I. Osnovy sravnitel’noi anatomii pozvonochnykh zhivotnykh, 4th ed. Moscow, 1947.
Beklemishev, V. N. Osnovy sravnitel’noi anatomii bespozvonochnykh, 3rd ed., vol. 2. Moscow, 1964.


circulatory system

[′sər·kyə·lə‚tȯr·ē ‚sis·təm]
The vessels and organs composing the lymphatic and cardiovascular systems.
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