CIRCULATORY SYSTEM
Circulatory system or cardiovascular system is a system that combines the functions of the heart, blood and blood vessels to supply the organs and tissues of the body with oxygen and nutrients and carry away waste products. Basically, the circulatory system is an organ system that moves substances to and from cells. The circulatory system increases the flow of blood to meet the increased energy demands during exercise. It also helps in maintaining a constant body temperature and pH. It is responsible for maintaining homeostasis. The circulatory system also fights against foreign bodies which cause infection. When foreign substances and disease causing organisms invade the body, the circulatory system swiftly transports white blood cells and antibodies to the region of infection. White blood cells and antibodies are the disease fighting elements of the immune system. In case of injury or bleeding, the circulatory system sends clotting cells and proteins to the injured site. Clotting cells quickly stop the bleeding and promote healing.

The heart is the most vital component of the circulatory system. It has four chambers namely: the right atrium or right auricle, the right ventricle, the left atrium or the left auricle and the left ventricle. The walls of these chambers are made up of myocardium which contacts continuously in a rhythmic manner to pump blood to all parts of the body. The pumping action occurs in two stages for each heart beat. When in resting stage, it is called diastole and when the heart contracts to pump deoxygenated blood towards the lungs and oxygenated blood to the body, it is called systole. About 60-90 ml of blood is pumped out of the heart during each heart beat.

Blood is made of three types of cells: red blood cells that transport oxygen, white blood cells that fight against disease causing agents, and the platelets which are responsible for clotting of blood. All the three types of cells are carried through blood vessels in liquid called plasma. Plasma is yellowish in color and is made up of water, salts, proteins, vitamins, minerals, hormones, dissolved gases, and fats. There are three types of blood vessels in the body. These form a complicated network of tubes throughout the whole of the human body. The arteries and veins are joined together by means of capillaries. Capillaries are the tiny links between the arteries and the veins where oxygen and nutrients diffuse to the cells and tissues of the body. Endothelial cells line the inner surfaces of the blood vessels. They help in creating a smooth passage for the flow of blood through the blood vessels. Blood vessels can expand and contract in accordance with the passage of the blood because they are lined by elastic muscular tissue. During rigorous exercise, blood vessels expand to meet the increased demand for blood. Expansion of blood vessels also helps in cooling the body. Blood vessels also contract as and when required. For example, blood vessels contract after an injury to reduce blood loss and also to conserve body heat.

The walls of arteries are thicker than those of veins. The reason behind this is that arteries have to resist the pressure of blood pumped through them by the heart. Veins have one-way valves to prevent back flow of blood. Capillaries are the smallest blood vessels. Being of microscopic dimensions, they are only visible by microscope.

There are two systems of circulation namely systemic and pulmonary circulation systems. The systemic circulation transports oxygenated blood from the heart to all parts of the body except the lungs and returns deoxygenated blood 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 gets oxygenated. This oxygenated blood then returns to the heart before being transferred to the systemic circulation