The human heart works as a muscular pump that receives blood, generates pressure through coordinated contraction and directs blood through the pulmonary and systemic circulations. It has four chambers, four major valves and a specialised electrical conduction system that coordinates the timing of contraction. The right side receives deoxygenated blood from the body and pumps it to the lungs, while the left side receives oxygenated blood from the lungs and pumps it to the rest of the body.
The atria primarily receive blood and transfer it to the ventricles, while the ventricles generate the pressure required for pulmonary and systemic ejection. The heart therefore operates as two coordinated pumps connected in series. Its continuous activity is supported by coronary blood vessels that supply oxygen and nutrients to the cardiac muscle itself.
The right atrium receives deoxygenated blood mainly through the superior and inferior venae cavae and receives blood from the coronary circulation through the coronary sinus. Blood passes through the tricuspid valve into the right ventricle. When the right ventricle contracts, the tricuspid valve closes and the pulmonary valve opens, allowing blood to enter the pulmonary artery and travel to the lungs. After gas exchange, oxygenated blood returns through the pulmonary veins to the left atrium. It then passes through the mitral valve into the left ventricle. During left ventricular contraction, the mitral valve closes and the aortic valve opens, allowing blood to enter the aorta and travel throughout the systemic circulation. This sequence maintains one way flow.
The cardiac cycle is the sequence of events associated with one heartbeat. It includes periods of ventricular filling and contraction and relaxation. During ventricular diastole, the ventricles relax and fill with blood, first largely passively and then with additional blood supplied by atrial contraction. During ventricular systole, the ventricles contract and eject blood into the pulmonary artery and aorta. The valves open and close according to pressure differences rather than because the valves actively contract. When ventricular pressure rises above atrial pressure, the atrioventricular valves close. When ventricular pressure exceeds the pressure in the pulmonary artery or aorta, the semilunar valves open. As ventricular pressure falls below arterial pressure, the semilunar valves close.
The heartbeat is coordinated by the cardiac conduction system. The sinoatrial node, located in the right atrium, normally acts as the primary pacemaker and generates spontaneous electrical activity. The impulse spreads through the atria, producing atrial depolarisation and contraction. It then reaches the atrioventricular node, where conduction is briefly delayed. The signal subsequently travels through the atrioventricular bundle, bundle branches and Purkinje fibres, allowing rapid activation of the ventricles. This sequence ensures that the atria contract before the ventricles. The heart therefore contains specialised cells capable of generating and conducting electrical signals independently of ordinary skeletal muscle control, although the autonomic nervous system modifies heart rate and other aspects of cardiac function.
Cardiac muscle contraction depends on calcium and actin myosin interactions. Electrical excitation of cardiac muscle cells opens membrane calcium channels, allowing calcium to enter the cells. This calcium contributes to the release of additional calcium from the sarcoplasmic reticulum, a process called calcium induced calcium release. Calcium binds to regulatory proteins associated with the contractile apparatus and permits actin myosin interaction. ATP powers the cross bridge cycle and calcium reuptake. After the electrical signal ends, calcium concentration falls and the muscle relaxes. The repeated cycles of electrical excitation, calcium movement and contraction produce the mechanical pumping action of the heart. Cardiac cells are electrically coupled through gap junctions, allowing coordinated activity across the myocardium.
The amount of blood pumped by the heart is described by cardiac output, which is calculated as heart rate multiplied by stroke volume. Stroke volume is the amount of blood ejected by a ventricle during one contraction. Cardiac output changes according to the body's requirements. Exercise increases cardiac output so that active tissues receive more oxygen and nutrients and metabolic waste can be transported away more rapidly. Venous return, contractility, heart rate and vascular resistance influence cardiac performance.
The Frank Starling mechanism means that, within physiological limits, increased filling of the ventricles stretches cardiac muscle fibres and can increase the force of contraction. This helps the heart adjust output to changes in venous return and maintain balance between the amounts pumped by the two sides.
The coronary circulation supplies the heart muscle with oxygenated blood. The coronary arteries arise from the aorta near the aortic valve and branch through the myocardium. Cardiac muscle has a high metabolic demand because it contracts continuously, so interruption of coronary blood flow can rapidly damage cardiac tissue. The heart also requires efficient removal of carbon dioxide and metabolic waste products. During contraction, particularly in the left ventricular wall, blood flow through portions of the myocardium can be temporarily reduced by mechanical compression of vessels, making coronary perfusion during relaxation particularly important. The balance between myocardial oxygen demand and coronary oxygen supply is therefore central to healthy cardiac function.
Heart function is regulated by intrinsic mechanisms, the autonomic nervous system, hormones and local physiological conditions. Sympathetic stimulation generally increases heart rate and contractility, while parasympathetic activity through the vagus nerve generally slows heart rate. Hormones such as adrenaline can increase cardiac activity. Blood pressure and circulating volume influence cardiac filling and output, while changes in oxygen, carbon dioxide and metabolic demand affect the broader cardiovascular system. Disorders such as arrhythmias, coronary artery disease, valve disease and heart failure can interfere with different components of this coordinated process. Understanding how the heart works therefore requires consideration of electrical activity, muscle contraction, valves, pressure gradients, blood flow, metabolism and systemic regulation.
