ABSTRACT
The heart is a hollow muscular organ that pumps blood around the body. With each beat, it pumps, at rest, about 70 millilitres of blood and considerably more during exercise. The heart consists of four main chambers left and right atria, and left and right ventricles and four valves aortic, mitral, pulmonary and tricuspid. However, should the sinoatrial (SA) node fail, or an impulse not reach the ventricles, cardiac contraction may be initiated by these secondary sites. The SA node is susceptible to influence from: Cardiac drugs can also affect cardiac rate, some acting through the SA node, others through the atrioventricular node or directly on ventricular myocytes: Each normal heartbeat begins with the discharge of the SA node. The events that occur during each heartbeat are termed the cardiac cycle. The cardiac cycle has four phases; isovolumic contraction, ventricular ejection, isovolumic relaxation and ventricular filling.
