The human heart is a remarkable muscular organ that serves as the central pump of the circulatory system. Located slightly left of the centre of the chest, it is roughly the size of a clenched fist and weighs between 250 and 350 grams. Despite its modest size, the heart beats approximately 100,000 times each day, pumping about 7,500 litres of blood through the body’s extensive network of blood vessels. The heart is divided into four chambers: two upper atria and two lower ventricles. The right atrium and right ventricle handle deoxygenated blood returning from the body, while the left atrium and left ventricle manage oxygenated blood coming from the lungs.
Valves between these chambers ensure that blood flows in only one direction, preventing any backflow. This intricate design allows the heart to function as a dual pump, simultaneously sending blood to the lungs for oxygenation and to the rest of the body to deliver oxygen and nutrients. The right side of the heart receives deoxygenated blood from the body via the superior and inferior vena cavae. This blood, which is low in oxygen and high in carbon dioxide, enters the right atrium. When the right atrium contracts, blood passes through the tricuspid valve into the right ventricle.
The right ventricle then contracts, pumping blood through the pulmonary valve into the pulmonary artery, which carries it to the lungs. In the lungs, carbon dioxide is exchanged for oxygen in the tiny air sacs called alveoli. The now oxygenated blood returns to the heart via the pulmonary veins, entering the left atrium. This process is known as pulmonary circulation, and it is essential for replenishing the blood’s oxygen supply and removing waste gases. The left side of the heart receives oxygenated blood from the lungs and pumps it out to the rest of the body.
This intricate design allows the heart to function as a dual pump, simultaneously sending blood to the lungs for oxygenation and to the rest of the body to deliver oxygen and nutrients.
Blood enters the left atrium from the pulmonary veins. When the left atrium contracts, it pushes blood through the mitral valve into the left ventricle, the most muscular chamber. The left ventricle has a thick wall because it must generate enough force to propel blood through the entire systemic circulation, from the heart to the extremities and back. When the left ventricle contracts, it sends blood through the aortic valve into the aorta, the largest artery in the body. From the aorta, blood branches off into smaller arteries and eventually into capillaries, where oxygen and nutrients are exchanged for carbon dioxide and waste products.
This systemic circulation supplies every organ and tissue with the resources they need to function. The cardiac cycle consists of alternating periods of contraction and relaxation. Systole refers to the phase when the ventricles contract, pumping blood out of the heart. Diastole is the relaxation phase when the ventricles fill with blood. The cycle begins with diastole: the atria contract slightly to push residual blood into the ventricles, then the ventricles relax. As the atria fill with blood, pressure increases, causing the atrioventricular valves to open. Ventricular systole follows: the ventricles contract, closing the atrioventricular valves and opening the semilunar valves (pulmonary and aortic) to eject blood.
The sounds of the heartbeat, often described as "lub-dub," are caused by the closing of these valves. The "lub" is from the atrioventricular valves closing, and the "dub" is from the semilunar valves closing at the end of systole. The heart’s rhythmic contractions are controlled by its own intrinsic electrical conduction system. The sinoatrial (SA) node, located in the right atrium, acts as the natural pacemaker. It generates electrical impulses that spread across the atria, causing them to contract. The impulse then reaches the atrioventricular (AV) node, which delays the signal slightly to allow the atria to fully empty into the ventricles.
From the AV node, the signal travels down the bundle of His and into the Purkinje fibres, which stimulate the ventricles to contract from the apex upward. This coordinated sequence ensures efficient pumping. The SA node fires at a rate of about 60 to 100 times per minute at rest, but this can be modified by the autonomic nervous system and hormones to meet the body’s changing demands. Heart rate is regulated by a combination of neural and hormonal signals. The medulla oblongata in the brainstem contains the cardiac centre, which sends sympathetic and parasympathetic impulses to the heart via the autonomic nervous system.
Sympathetic stimulation (via the cardiac accelerator nerves) increases heart rate and contractility during stress or exercise, while parasympathetic stimulation (via the vagus nerve) slows the heart rate during rest. Hormones such as adrenaline and noradrenaline, released from the adrenal medulla, also increase heart rate. Additionally, changes in blood pressure, body temperature, and blood chemistry (such as oxygen and carbon dioxide levels) can influence heart rate through reflexes such as the baroreceptor reflex. This precise regulation ensures that the heart pumps enough blood to meet the body’s metabolic needs at all times.
Understanding the heart’s function is crucial for recognising common cardiovascular disorders. Coronary artery disease occurs when plaque builds up in the arteries that supply the heart muscle, reducing blood flow and potentially causing chest pain (angina) or a heart attack. Arrhythmias are abnormalities in the heart’s rhythm, such as atrial fibrillation, where the atria quiver instead of contracting effectively. Heart failure is a condition where the heart cannot pump enough blood to meet the body’s needs, often due to damage from heart attacks or high blood pressure. Lifestyle factors such as a balanced diet, regular exercise, avoiding smoking, and managing stress can significantly reduce the risk of these conditions. The heart’s resilience and adaptability are extraordinary, but maintaining cardiovascular health is essential for a long and active life.