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Circulatory System

Circulatory system

The human circulatory system is responsible for delivering food, oxygen, and other needed substances to all cells in all parts of the body while taking away waste products. The circulatory system is also known as the cardiovascular system, from the Greek word kardia, meaning "heart," and the Latin vasculum, meaning "small vessel." The basic components of the cardiovascular system are the heart, the blood vessels, and the blood. As blood circulates around the body, it picks up oxygen from the lungs, nutrients from the small intestine, and hormones from the endocrine glands, and delivers these to the cells. Blood then picks up carbon dioxide and cellular wastes from cells and delivers these to the lungs and kidneys, where they are excreted.

The human heart

The adult heart is a hollow cone-shaped muscular organ located in the center of the chest cavity. The lower tip of the heart tilts toward the left. The heart is about the size of a clenched fist and weighs approximately 10.5 ounces (300 grams). A heart beats more than 100,000 times a day and close to 2.5 billion times in an average lifetime. The pericardiuma triple-layered sacsurrounds, protects, and anchors the heart. Pericardial fluid located in the space between two of the layers reduces friction when the heart moves.

The heart is divided into four chambers. A septum or partition divides it into a left and right side. Each side is further divided into an upper and lower chamber. The upper chambers, the atria (singular atrium), are thin-walled. They receive blood entering the heart and pump it to the ventricles, the lower heart chambers. The walls of the ventricles are thicker and contain more cardiac muscle than the walls of the atria. This enables the ventricles to pump blood out to the lungs and the rest of the body.

The left and right sides of the heart function as two separate pumps. The right atrium receives blood carrying carbon dioxide from the body through a major vein, the vena cava, and delivers it to the right ventricle. The right ventricle, in turn, pumps the blood to the lungs via the pulmonary artery. The left atrium receives the oxygen-rich blood from the lungs from the pulmonary veins, and delivers it to the left ventricle. The left ventricle then pumps it into the aorta, the major artery that leads to all parts of the body. The wall of the left ventricle is thicker than the wall of the right ventricle, making it a more powerful pump, able to push blood through its longer trip around the body.

One-way valves in the heart keep blood flowing in the right direction and prevent backflow. The valves open and close in response to pressure changes in the heart. Atrioventricular valves are located between the atria and ventricles. Semilunar valves lie between the ventricles and the major arteries into which they pump blood. People with a heart murmur have a defective heart valve that allows the backflow of blood.

Words to Know

Artery: Vessel that transports blood away from the heart.

Atherosclerosis: Condition in which fatty material such as cholesterol accumulates on artery walls forming plaque that obstructs blood flow.

Atrium: Receiving chamber of the heart.

Capillary: Vessel that connects artery to vein.

Diastole: Period of relaxation and expansion of the heart when its chambers fill with blood.

Hormones: Chemical messengers that regulate body functions.

Hypertension: High blood pressure.

Sphygmomanometer: Instrument that measures blood pressure in millimeters of mercury.

Systole: Rhythmic contraction of the heat.

Vein: Vessel that transports blood to the heart.

Ventricle: Pumping chamber of the heart.

The heart cycle refers to the events that occur during a single heartbeat. The cycle involves systole (the contraction phase) and diastole (the relaxation phase). In the heart, the two atria contract while the two ventricles relax. Then, the two ventricles contract while the two atria relax. The heart cycle consists of a systole and diastole of both the atria and ventricles. At the end of a heartbeat all four chambers rest. The average heart beats about 75 times per minute, and each heart cycle takes about 0.8 seconds.

Blood vessels

The blood vessels of the body (arteries, capillaries, and veins) make up a closed system of tubes that carry blood from the heart to tissues all over the body and then back to the heart. Arteries carry blood away from the heart, while veins carry blood toward the heart. Large arteries leave

the heart and branch into smaller ones that reach out to various parts of the body. These divide still further into smaller vessels called arterioles that penetrate the body tissues. Within the tissues, the arterioles branch into a network of microscopic capillaries. Substances move in and out of the capillary walls as the blood exchanges materials with the cells. Before leaving the tissues, capillaries unite into venules, which are small veins. The venules merge to form larger and larger veins that eventually return blood to the heart.

The walls of arteries, veins, and capillaries differ in structure. In all three, the vessel wall surrounds a hollow center through which the blood flows. The walls of both arteries and veins are composed of three coats, but they differ in thickness. The inner and middle coats of arteries are thicker than those of veins. This makes arteries more elastic and capable of expanding when blood surges through them from the beating heart. The walls of veins are more flexible than artery walls. This allows skeletal muscles to contract against them, squeezing the blood along as it returns to the heart. One-way valves in the walls of veins keep blood flowing in one direction. The walls of capillaries are only one cell thick. Of all the blood vessels, only capillaries have walls thin enough to allow the exchange of materials between cells and the blood.

Blood pressure is the pressure of blood against the wall of an artery. Blood pressure originates when the ventricles contract during the heartbeat. It is strongest in the aorta and decreases as blood moves through progressively smaller arteries. A sphygmomanometer (pronounced sfigmoe-ma-NOM-i-ter) is an instrument that measures blood pressure in millimeters (mm) of mercury. Average young adults have a normal blood pressure reading of about 120 mm for systolic pressure and 80 mm for diastolic pressure. Blood pressure normally increases with age.

Blood

Blood is liquid connective tissue. It transports oxygen from the lungs and delivers it to cells. It picks up carbon dioxide from the cells and brings it to the lungs. It carries nutrients from the digestive system and hormones from the endocrine glands to the cells. It takes heat and waste products away from cells. It protects the body by clotting and by fighting disease through the immune system.

Blood is heavier and stickier than water, and has a temperature in the body of about 100.4°F (38°C). Blood makes up approximately 8 percent of an individual's total body weight. A male of average weight has about 1.5 gallons (5.5 liters) of blood in his body, while a female has about 1.2 gallons (4.5 liters).

Blood is composed of plasma (liquid portion) and blood cells. Plasma, which is about 91.5 percent water, carries blood cells and helps conduct heat. The three types of cells in blood are red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). More than 99 percent of all the blood cells are red blood cells. They contain hemoglobin, a red pigment that carries oxygen, and each red cell has about 280 million hemoglobin molecules. White blood cells fight disease organisms by destroying them or by producing antibodies. Platelets bring about clotting of the blood.

Circulatory diseases. Two disorders that involve blood vessels are hypertension and atherosclerosis. Hypertension, or high blood pressure, is the most common circulatory disease. In about 90 percent of hypertension sufferers, blood pressure stays high without any known physical cause. Limiting salt and alcohol intake, stopping smoking, losing weight, increasing exercise, and managing stress all help reduce blood pressure. Medications also help control hypertension.

In atherosclerosis, fatty material such as cholesterol accumulates on the artery wall forming plaque that obstructs blood flow. The plaque can form a clot that breaks off, travels in the blood, and can block a smaller vessel. A stroke may occur when a clot obstructs an artery or capillary in the brain. Treatment for atherosclerosis includes medication, surgery, a high-fiber diet low in fat, and exercise.

[See also Blood; Heart; Lymphatic system ]

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Circulatory System

Circulatory System

Every living organism on Earth, from amoebas to redwoods to whales, has a circulatory systema means of gathering and transporting nutrients and collecting and removing waste products.

Plants have an elegant system of strawlike tubes called phloem and xylem, which stretch from the roots to the topmost leaves. Stomata, tiny evaporative holes in the leaves, create suction that steadily draws water up the xylem from the roots, allowing plants hundreds of feet tall to circulate nutrients without a pump.

All cells of the simplest animals, such as single-celled amoebas and multicellular flatworms, are close to the surface. In these cells, nutrients wash through the cell fluid, and wastes pass out through a porous outer membrane between the cell and its environment. The cells of larger animals are buried many layers deep, so these animals require a system that connects each cell to the outer world. This system, which consists of the fluid that carries nutrients through vessels that reach every part of the body and the mechanism that powers the flow of nutrients, is called the circulatory system.

The simplest form of circulatory system is an open circulatory system. In an open circulatory system, blood flows through a network of open tubes and hollow spaces, and the movement of the animal itself keeps the blood flowing. In more complex systems, blood is pumped through the body by contractions of the blood vessels. Invertebrates, such as insects and other arthropods , have a central blood vessel that runs down the length of the back. A series of bulbous pumping centers slowly squeeze the blood through a maze of hollow spaces around the body past all the organs.

Vertebrates, including amphibians, reptiles, birds, and mammals, have increasingly complex, closed circulatory systems. Closed circulatory systems consist of an intricate network of vessels filled with blood that delivers nutrients, regulates internal temperature, and takes away waste products. The system is powered by the heart, a muscular pump that never stops working, which continually circulates the blood through the body.

In all vertebrates, the heart is made of involuntary muscle tissue, but the structure is very different in each group. Fish have a two-chambered, single-pump heart. Amphibians have a three-chambered heart that also acts as a single pump. Birds and mammals have a more sophisticated four-chambered, double-pump heart design. One chamber sends blood to the lungs to be purified and reoxygenated, the other sends the enriched blood out into the body. Interestingly, the human embryo goes through every stage of circulatory development, from a passive single-celled heart to a two-, three-, and four-chambered heart.

In adult humans, the circulatory system consists of blood, the heart, and a network of vessels through which the blood travels. Blood is plasma (a watery liquid) that contains billions of molecules of sugars, proteins, hormones , antibodies , and gases. The heart is a strong, muscular, double pump that pushes the blood continuously and automatically around the body through roughly 100,000 kilometers (62,000 miles) of arteries, veins, and capillaries. It takes the blood about one minute to complete a circuit around the body, and this happens about 1,000 times a day.

The human circulatory system has two loops. The shorter pulmonary circulation goes from the lower-right chamber of the heart (the ventricle) through the pulmonary artery to the lungs and back to the upper-left chamber (the atrium) through the pulmonary vein. From there, the newly oxygenated blood descends into the left ventricle through a one-way valve and is pumped into the longer systemic circulation through the main artery of the body, the aorta. The spent blood travels back to the right atrium in two main veins. The superior vena cava drains the upper body, the head, neck, and arms. The inferior vena cava handles the lower body. From the right atrium, the blood flows through the relaxed one-way valve into the right ventricle. Then another pulse of the powerful heart muscle closes the valve and spurts the blood into the pulmonary artery, beginning the cycle again. The sound known as the heart "beat" is the sound of the valves between the atria and ventricles and between the ventricles and arteries as they snap shut to keep the blood from flowing backward.

A healthy, relaxed, adult heart beats about seventy times a minute, pumping blood under high pressure into the thick-walled arteries. The elastic walls of the arteries stretch open to allow the blood to flow in, then squeeze back together to force it along. Arteries branch into narrower and more muscular arterioles. Arterioles branch into finer and finer capillaries, thin-walled, hairlike vessels that interact with surrounding body cells to exchange nutrients and wastes. Capillaries then enlarge into venules, which merge into veins, and carry the spent blood back to the heart. After the blood has traveled through the capillary network the pressure is greatly reduced, and the veins can afford to be much thinner than arteries with weaker muscle fiber. Small, one-way valves inside the veins keep the blood moving against gravity toward the heart.

As well as delivering the supplies that keep cells functioning, the bloodstream regulates body temperature by dissipating heat that builds up in the organs. The contraction or dilation of surface capillaries allows more or less heat to escape the system, depending on whether the body is too hot or too cold. The bloodstream also contains disease-and infection-fighting antibodies.

The lymphatic system is a one-way independent drainage network of fine capillaries primarily involved in fighting disease and infection. White blood cells used in neutralizing bacteria collect in lymph glands. Then normal muscle movement and one-way valves keep the lymph flowing toward the chest, where it drains into two large veins and reenters the blood stream.

see also Blood.

Nancy Weaver

Bibliography

Ballard, Carol. Heart and Circulatory System. Austin, TX: Steck Vaughn, 1997.

Silverstein, Alvin, Virginia, and Robert Alvin. The Circulatory System. New York: Henry Holt & Co., 1994.

Unschuld, Paul U., trans. Nan Ching (Chinese Medicine). Berkeley: University of California Press, 1985.

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circulatory system

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, that drives the blood.

Of the vessels, the arteries carry blood away from the heart; the main arterial vessel, the aorta, 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 through which gas and nutrient exchange occurs (see respiration). Blood leaving the tissue capillaries enters converging vessels, the veins, 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, 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 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, which removes worn red blood cells, or lymphocytes; and the liver, 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).

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 and high blood pressure (see hypertension); low blood pressure resulting from injury (shock) 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, or to the brain (stroke) are most dangerous. Structural defects of the heart affecting blood distribution may be congenital or caused by many diseases, e.g., rheumatic fever, coronary artery disease.

See also heart disease; angina pectoris.

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Circulatory Systems

Circulatory Systems

Animal circulatory systems consist of a blood or a bloodlike fluid, a system of tubular blood vessels, and one or more pulsating hearts that pump the blood through the vessels. Animals that are only a few cell layers thick do not need or possess circulatory systems, because they can rely on diffusion through the body surface to exchange materials with the environment. Larger animals, however, require a circulatory system to transport nutrients and oxygen to their tissues, remove wastes, transport hormones , equalize body temperature, and maintain homeostasis.

Circulatory systems are classified as open or closed. In an open circulatory system, the heart pumps a fluid through arteries that empty into a large space, the hemocoel. The fluid bathes the organs in the hemocoel, and returns through veins to the heart. Since there is no distinction between blood and tissue fluid in such a system, the fluid is called hemolymph. Open circulatory systems are found in most mollusks and arthropods .

In a closed circulatory system, blood never leaves the blood vessels, and is thus separated from the tissue fluid. Blood flows away from the heart by way of arteries and returns to the heart by way of veins. Arteries are connected to veins by tiny, thin-walled capillaries. Arteries and veins have a wall made of elastic and muscular tissue, and an inner lining of thin epithelium called endothelium. Capillaries are made of endothelium only. This thin wall allows for exchange of substances between the blood and tissue fluid.

Closed systems have a relatively high blood pressure. This enables nutrients and oxygen to be delivered quickly to their tissues and supports the high metabolic rate associated with the relatively high mobility of some animals. Squids, for example, have closed circulatory systems with three hearts, one to serve each gill and one for the rest of the body. Earthworms, although not highly mobile, have a closed circulatory system with five pairs of hearts.

Vertebrates independently evolved closed circulatory systems in close association with the respiratory systems. In fish, blood flows from the heart to the gills for gas exchange, then to the rest of the body, and finally back to the heart. This is called a single circulation since the blood flows through the heart only once during each complete trip around the body. Amphibians evolved a double circulation; blood flows from the heart to the gills or lungs for gas exchange, then back to the heart to be repressurized before flowing to the rest of the body. The vessels that serve the respiratory organs are called the branchial circuit (for gills) or pulmonary circuit (for lungs). Vessels that serve the rest of the body are called the systemic circuit.

The amphibian heart and most reptilian hearts have only three chamberstwo atria and one ventricle and there is some mixing of oxygen-rich and oxygen-poor blood in the single ventricle. Endothermic vertebrates, the birds and mammals, have higher metabolic rates and require stricter separation of the pulmonary and systemic blood. Thus, they have four-chambered hearts. Oxygen-rich blood flows through the other ventricle to the systemic circuit.

see also Arthropods; Blood; Blood Vessels; Heart and Circulation; Tissue

Barbara Cocanour

Bibliography

Raven, Peter H., and George B. Johnson. Biology, 5th ed. Boston: McGraw-Hill, 1999.

Walker, Warren F., Jr., and Karel F. Liem. Functional Anatomy of the Vertebrates: An Evolutionary Perspective, 2nd ed. Orlando, FL: Saunders College Publishing, 1994.

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circulatory system

circulatory system Means by which oxygen and nutrients are carried to the body's tissues, and carbon dioxide and other waste products are removed. It consists of blood vessels that carry the blood, propelled by the pumping action of the heart. In humans and other mammals, blood travels to the lungs, where it picks up oxygen and loses carbon dioxide. It then flows to the heart, from where it is pumped out into the aorta, which branches into smaller arteries, arterioles and capillaries. Oxygen and other nutrients diffuse out of the blood, and carbon dioxide and other tissue wastes pass into the capillaries, which join to form veins leading back to the heart. Blood then returns to the lungs and the entire cycle is repeated. This is known as a double circulatory system, as the blood is pumped first to the lungs, and then to the rest of the body. In fish and many other animals, there is a single circulatory system, with blood passing through the gills and on to the rest of the body without an extra boost from the heart. Both these circulatory systems are closed; the blood remains confined within the blood vessels. Insects and many other invertebrates have an open circulatory system, where the blood flows freely within the body cavity, but passes through a series of open blood vessels and heart(s), whose pumping maintains a directional flow.

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circulatory system

circulatory system The heart, blood vessels, blood, lymphatic vessels, and lymph, which together serve to transport materials throughout the body. See also double circulation; single circulation; vascular system.

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