Blood must reach every cell in the body to provide it with food and oxygen, and to remove waste products. The group of organs which circulate blood through the body is called the circulatory system.
In the human beings, the circulatory system has a muscular pump called heart. It pumps the blood through long, tube-like blood vessels. Blood vessels carry the blood throughout the body to the cells. In our circulatory system there are five types of blood vessels: arteries, arterioles, capillaries, venules and veins. An artery is a large vessel that carries blood away from the heart to the cells. It branches off into the smaller arterioles which further divide into very small capillaries. The capillaries carry blood to the cells. These reunite to form larger tubes called venules. These venules merge to form large tubes called veins. Veins carry impure blood back to the heart. According to a rough estimate if all the blood vessels are joined together in a single-line they would stretch to 60,000 miles.
Blood is pumped from the right ventricle of the heart into the pulmonary artery. The pulmonary artery carries it to the lungs where it absorbs oxygen and releases carbon-dioxide. The blood returns to the left auricle of the heart through the pulmonary vein. The left auricle pumps the oxygenated blood into the left ventricle of the heart which pumps it into aorta. The aorta is the largest artery in the body. It carries blood to other arteries and arterioles. The blood absorbs food when it passes near the small intestines. The wastes from the cells are removed from the blood when it passes through the kidneys. After the blood passes through the cells of the body, delivering food and oxygen and removing wastes, it returns to the heart through the vena cava. The vena cava is the largest vein in the body. The unoxygenated blood enters the right auricle which returns it to the right ventricle. It is then pumped to the lungs to receive more oxygen.
The muscular walls of the arteries are thick and elastic. They carry bright red, oxygen-rich blood. As the heart pumps, a wave of pressure travels along the walls of arteries and can be felt as pulse. Their walls can contract and release and can regulate the amount of blood flowing to the body tissues.
The walls of veins have three layers: elastic, muscular and lining. The veins are thinner and less muscular than the arteries. In the arms and legs the veins have valves that prevent the back flow and pooling of blood due to gravity. The veins that are swollen, stretched or coiled on themselves are called varicose veins. These can sometimes be found on the legs of older people as well as those whose jobs involve a lot of walking. Blood in the veins is under low pressure and flows slowly. Since it contains less oxygen, it turns purplish red in colour.
