Circulatory system



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Circulatory System; MS Encarta


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.


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.


The Circle Of Blood            

The Circulatory System
On average, your body has about 5 liters of blood continually traveling through it by way of the circulatory system. The heart, the lungs, and the blood vessels work together to form the circle part of the circulatory system. The pumping of the heart forces the blood on its journey.

The body's circulatory system really has three distinct parts: pulmonary circulation, coronary circulation, and systemic circulation. Or, the lungs (pulmonary), the heart (coronary), and the rest of the system (systemic). Each part must be working independently in order for them to all work together.

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The Circulatory System


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