circulatory system definition of circulatory system in the Free Online Encyclopedia.

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 heart heart, 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.
..... Click the link for more information. , that drives the blood.

Of the vessels, the arteries artery, 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.
..... Click the link for more information. carry blood away from the heart; the main arterial vessel, the aorta aorta (āôr`tə), primary artery of the circulatory system in mammals, delivering oxygenated blood to all other arteries except
..... Click the link for more information. , 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 capillaries capillary (kăp`əlĕr'ē), microscopic blood vessel, smallest unit of the circulatory system .
..... Click the link for more information. through which gas and nutrient exchange occurs (see respiration respiration, 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 (CO
..... Click the link for more information. ). Blood leaving the tissue capillaries enters converging vessels, the veins vein, 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).
..... Click the link for more information. , 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 system lymphatic system (lĭmfăt`ĭk)
..... Click the link for more information. , 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 pressure blood 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
..... Click the link for more information. 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 spleen spleen, 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
..... Click the link for more information. , which removes worn red blood cells, or lymphocytes; and the liver liver, 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.
..... Click the link for more information. , 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 system urinary 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
..... Click the link for more information. ).

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 arteriosclerosis arteriosclerosis (ärtĭr'ēōsklərō`sis)
..... Click the link for more information. and high blood pressure (see hypertension hypertension 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).
..... Click the link for more information. ); low blood pressure resulting from injury (shock shock, 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.
..... Click the link for more information. ) 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 infarction myocardial infarction or heart attack is usually caused by a blockage in one of the coronary arteries that supply blood to the heart muscle. The blockage typically occurs when a blood clot (see thrombosis ) lodges in an area already narrowed by arteriosclerosis ; other
..... Click the link for more information. , or to the brain (stroke stroke, 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
..... Click the link for more information. ) are most dangerous. Structural defects of the heart affecting blood distribution may be congenital or caused by many diseases, e.g., rheumatic fever rheumatic fever (r
..... Click the link for more information. , coronary artery disease coronary artery disease, condition that results when the coronary arteries are narrowed or occluded, most commonly by atherosclerotic deposits of fibrous and fatty tissue.
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See also heart disease; angina pectoris angina pectoris (ănjī`nə pĕk`tərĭs)
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