Circulatory System is 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. The circulatory system increases the flow of blood to meet increased energy demands during exercise and regulates body temperature. Also, when foreign substances or organisms invade the body, the circulatory system quickly sends disease-fighting elements of the immune system, such as white blood cells and antibodies, to places under attack. 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.

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The heart, blood, and blood vessels are the three main 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 muscle called myocardium, which contracts to pump blood. The pumping action of the heart occurs in two stages for each heartbeat. 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. There are typically about 60 to 90 beats per minute. If the heart stops pumping, death usually occurs within four to five minutes.
Blood consists of three types of cells, red blood cells that carry oxygen, disease-fighting white blood cells, and blood-clotting platelets, which are all carried through plasma. Plasma is yellowish and consists of water, salts, proteins, vitamins, minerals, hormones, dissolved gases, and fats.
Three types of blood vessels make a network of tubes throughout the body. Arteries carry blood away from the heart, and veins carry it toward the heart. Capillaries are tiny links between the arteries and the veins where oxygen and nutrients spread to body tissues. The inner layer of blood vessels is lined with cells that create a smooth passage for the transfer of blood. This inner layer is surrounded by connective tissue and smooth muscle that help 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 so they can 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, which are the smallest of blood vessels, are only visible by microscope. 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 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.
The heart gets rid of oxygen-rich blood under high pressure out of the left ventricle, through 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 allow dissolved oxygen and nutrients from the blood to become a fluid, called the 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 to return 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, that brings blood from the lower half of the body, and the superior vena cava, that brings blood from the upper half. Both of these two large veins join at the right atrium of the heart.
The circulatory system plays an important role in controlling 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 pressure generated by the pumping action of the heart propels the blood to the arteries. Blood pressure, for instance, enables a person to rise quickly from a horizontal position without blood pooling in the legs, which would cause fainting from deprivation of blood to the brain. Normal blood pressure is regulated by a number of factors, such as the contraction of the heart, the elasticity of arterial walls, blood volume, and resistance of blood vessels to the passage of blood. Blood pressure is measured during systole, the active pumping phase of the heart, and diastole, the resting phase between heartbeats. Blood pressure varies between individuals and even during the normal course of a day in response to emotion, exertion, sleep, and other physical and mental changes. The average normal blood pressure is about 120/80 mm Hg. Higher blood pressures that are sustained over a long period of time may indicate hypertension, a damaging circulatory condition. Lower blood pressures could signal shock from heart failure, dehydration, internal bleeding, or blood loss.