The human heart is a conical, hollow, muscular organ which works continuously throughout the life of a person. It is about the size of a clenched fist and weighs 300 grams.
The heart lies in the chest just behind the breastbone and between the two lungs. It is enveloped in a two-layered tough membranous bag, the pericardium. The space between the two-layer is filled with fluid, This reduces the friction caused by the pumping movements of the heart, between the heart wall and the surrounding tissues.
when at rest the average human heart contracts and relaxes about 70 times per minute and pumps about 14 liters of blood a day. During exercise this rate goes up to over 100 thereby increasing the supply of oxygen and food to the body cells. To carry out such hard work the wall of the heart is made up of a special muscle, the cardiac muscle found only in the heart. It contracts and relaxes rhythmically, and can work continuously without becoming tired like the skeletal muscles. To work hard and continuously, the heart muscles need a good supply of blood to provide oxygen and nutrients and removes its wastes. This is supplied by the coronary arteries and veins which spread all over the heart walls.
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