The heart’s modest size, that of a human fist with a weight of around 250–300 g, seems inconsistent with its strength and endurance. This four-chamber muscular pump is enclosed in the mediastinum of the thorax, with about two-thirds lying left of the mid sternal line (Figure 22.1). The atria act as the receiving chambers for blood flowing into the heart assisting ventricular filling, whilst the ventricles perform the work of pumping the blood round the pulmonary and systemic circulations. It is largely self-regulating, its beating initiated by its own pacemaker, but it is also responsive to the changing needs of the body, influenced by the autonomic nervous system (Chapter 24). The heart pumps blood simultaneously around both the pulmonary and systemic circulations. Valves, placed between the atria and ventricles (atrioventricular valves) and in the outflow tracts (pulmonary artery and aorta) are essential to ensure blood continues to flow in the right direction. They open and close in response to pressure changes generated by the heart muscle contracting (systole) and then relaxing (diastole) whilst the chambers refill with blood.

The right side of the heart receives venous blood from the superior and inferior vena cava draining from the body into the right atrium. Blood then flows through the tricuspid (right atrioventricular) valve