Q.What does a pacemaker do?

The human heart is a fist-sized, roughly 300 g muscular pump made of cardiac muscle, sitting in the thoracic cavity with its apex tilted slightly left. It has four chambers - two smaller, thinner-walled upper atria and two larger, thicker-walled lower ventricles (the ventricle walls are thickened partly by internal papillary muscles) - and its wall is built of three layers: an outer epicardium, a muscular middle myocardium, and an inner endocardium, with a fluid-filled pericardial space surrounding the whole organ. An inter-atrial septum and an inter-ventricular septum keep the two sides of the heart, and therefore oxygenated and deoxygenated blood, completely separate. Each atrium opens into the ventricle below it through an atrioventricular aperture guarded by a one-way valve: the three-flapped tricuspid valve on the right, and the two-flapped bicuspid (mitral) valve on the left; both allow blood to flow only from atria to ventricles. Where the ventricles empty into the great arteries, three-cusped semilunar valves (pulmonary and aortic) permit outward flow only, held open and shut by chordae tendineae anchored to the ventricular wall's papillary muscles v …