The heart is placed behind the breast bone and within the ribs (thoracic cage) with the lungs on either side. It is a hollow muscular organ made up of smooth muscles (myocardium). It is enclosed in a sac known as the pericardium (outside layer), and shaped like an egg, is about the size of a person’s clenched fist and weighs around 300 g. in a man and 250 g. in a woman.
The heart has four chambers, two atria (upper) and ventricles (lower). Valves connect the upper and lower chambers. A valve is like a one-way door that allows blood to flow only in one direction. The right and left side of the heart are totally separated by a muscular wall and there is no communication between them. The right side of the heart receives deoxygenated (impure) blood collected from different parts of the body through small and big veins which enter the lungs. In the lungs blood is oxygenated and carbon dioxide and metabolic waste are removed. The left side of the heart gets oxygenated (pure) blood from the lungs and supplies it to the entire body through the major blood vessel (aorta) and its innumerable branches (arteries and capillaries). The left ventricle generates considerably grater pressure than the right ventricle, to enable the blood to be pumped throughout the body. Hence the left ventricle is thicker and muscular, it is the largest of the four chambers and needs considerable blood and oxygen supply. Blood carries nourishment and oxygen to each and every cell and tissue of the body. Like any other tissue, the heart muscle also needs a good supply of oxygenated blood. This is done through two major coronary arteries which are placed as a crown on the heart. They pursue a wavy course to adapt themselves to the beating heart. The coronary arteries branch out (left and right coronary arteries) from the root of the aorta near its origin from the left ventricle. The left coronary artery further gets divided into two main branches near its origin itself. Both the coronary arteries branch off into smaller vessels which are distributed all over the surface of the heart. The left ventricle, the principal and largest chamber of the heart receives the maximum blood supply.
The coronary blood flow in a normal adult averages 200-250 ml/minutes(4-5% of cardiac output). In a healthy adult at rest, the heart pumps approximately 5 liters of blood every minute. Each heart beat is a act of blood being pumped out of the heart. For efficient pumping, it is necessary for the heart to beat at a reasonable rate of 60-90 beats/minute which is achieved through controlled electrical impulses (conduction system)
The normal pressure of blood against the walls of the arteries is called blood pressure. The normal pressure, when the heart contracts, is called systolic pressure and is between 100-140 mm of mercury. Then the heart relaxes the pressure is known as diastolic pressure and lies between 70-90 mm of mercury. Blood pressure is commonly expressed as systolic/diastolic, for example 100/70, or 140/90 mm of mercury. In its strenuous daily routine the heart pauses for rest for a split second between beats. The heart and the blood vessels together constitute the cardiovascular system.
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