Place your hand on the left side of the chest. You feel the vibrations. Keep your ear on the chest of your friend. You will hear sounds lub dub. These vibrations and sounds are made by the heart. The heart is situated inside the chest cavity. It is a muscular, and contractile organ. The contractions help the heart to pump the blood to various parts of the body. The heart is placed between the two lungs and above the diaphragm. It is hollow and conical in shape. The base is upwards and the apex or narrow portion is pointed downwards. The heart is about the size of one's fist.

The heart consists of four chambers . The two chambers present in the upper part are called auricles or atria. They are the right and left auricles. The right auricle is larger than the left auricle. The walls of the auricles are thin. Inside the heart, a muscular partition wall separates the right and left auricles and there is no connection between the two auricles.

The chambers present below the auricles are called ventricles. They are1be right and left ventricles. The walls of the ventricles are thicker than the walls of the auricles. The left ventricle is larger than the right ventricle.

The right auricle opens into the right ventricle. The left auricle opens into the left ventricle. Blood flows through the heart in the direction of auricles to ventricles. To regulate the flow of blood through the heart in one direction, there are valves between right auricle and right ventricle. Similarly valves are present between the left auricle and left ventricle. These valves act as 'traffic police'. They allow blood to flow from the auricles into ventricles. The backward flow of blood is prevented by these valves.

The contraction of the heart is called systole. The relaxation of the heart after contraction is called diastole. The two auricles of the heart contract together when blood from auricles is pumped in to ventricles. Next the two ventricles contract together, pumping out the blood to the various parts of the body from heart. So, actually the heart has two pumps, placed one below the other.

The contraction followed by the relaxation of the heart is called as heart-beat '. The contraction of the heart produces a sound generally called 'LUB '. Similarly, the relaxation the heart produces the sound 'DUB'. These two together constitute the ' LUB-DUB ' sound of the heart. There is no rest to the beating of the heart. It beats without stop, from birth to death.

The heart-beat can be felt on the wrist. When fingers are-placed on the outer side of the wrist, below the thumb, vibrations can be felt. These vibrations or pulsations are called as pulse. By counting the pulse, the rate and the character of the heart-beat can be judged.

The average pulse rate in an adult is 72 per minute. It increases in anger and fear. It is higher is children than in old people. Heart pumps the blood through separate tubes in the body These tubes are called blood vessels. There are three types of blood vessels in the body. They are arteries, veins and capillaries.

Arteries are the blood vessels that carry blood from the heart to the various parts of the body. The walls of the arteries are thick and elastic,' They are situated in the deeper parts of the body. Blood flow in arteries is not continuous but like waves. The arteries carry oxygenated blood to various body parts. But the artery that carries blood to the lungs, carriers deoxygenated blood. The largest artery in the body is called aorta. This arises from he heart. The aorta divides into smaller arteries and supplies blood to the various parts of the body. Thus, arteries are vessels that distribute blood.

Heart also requires oxygen, food and other substances for its working. So it is also supplied with blood vessels. The blood vessels that supply blood to the walls of the heart are called as coronary arteries.

The blood in large arteries moves at a speed of about 30 metres per minute.

The blood vessels that bring blood from the various parts of the body to the heart are called veins. The walls of the veins are thin. Valves are present in the veins to prevent the backward flow of blood. The flow of blood in the veins is continuous.

All the veins in the head, neck and hands unite into a single large vein. This is called as sound superior venacava. All the veins in the legs and trunk unite into a single large vein called as inferior venacava. The superior and inferior venacava pour blood into the heart. Thus veins act as vessels to collect blood from various body parts.

All the veins carry deoxygenated blood. But the vein from the lungs carries oxygenated blood. The capillaries are very small blood vessels. Their walls are very thin. They form a link between the blood vessels and tissues of the body. Arteries end in capillaries in the organs of the body. Veins start with capillaries in the organs of the body. Walls of capillaries freely allow the movement of nutrients, oxygen, carbon dioxide and waste materials.