eart is kept from flowing backwards by a system of four one-way valves, each closing off the hearts chambers at the appropriate time in the cardiac cycle. The valves let the blood through when the chambers contract, and then snap shut to prevent it from flowing backward as the chambers relax. The valve system helps to maintain different pressures on the right and left sides of the heart.
             The two valves separating the circulatory system's ventricles are called semilunar due to their crescent shaped cusps. The ventricles are separated from the atria by valves that in addition to cusps have thin but strong cords of fibrous tissue.
             These are called chordae tendineae which tether the valves to the ventricular walls.
             The valve located between the left ventricle and left atrium is a cone shaped funnel that resembles a miter ( a triangular head dress worn by bishops and abbots) and is therefore called the mitral valve. The corresponding valve between the right ventricle and right atrium is called the tricuspid valve. As it's name suggests it has three cusps, that are thinner than the mitral valve but just as mobile.
             On the inside the heart is lined with a protective layer of cells that form a smooth membrane called the endocardium. On the outside the heart is encased in a fibrous double layered sac called the pericardium. The pericardium's inner layer is attached to the heart muscle while the outer layer holds the heart firmly in place. The two layers are separated by a thin film of lubricating fluid that allows the heart to move more freely within the outer pericardium.
             Electrical currents that regulate the heart rhythm originate in cells of the heart muscle and travel through a network of specialised fibres referred to as the hearts conduction system. Its major elements include the sinus or sinoatrial node, the atrial ventricular node (AV node), the bundle of His and the Purkinje fibres.
             The Sinus node known, as the hearts pacema...