Human heart

The human heart is a hollow, muscular organ that functions as the central pump of the circulatory system, driving oxygenated blood to the body's tissues and returning deoxygenated blood to the lungs for re-oxygenation. In adult humans, the heart beats approximately 60 to 100 times per minute at rest, completing roughly 2.5 billion beats over an average lifetime. It sits within the mediastinum, the central compartment of the thoracic cavity, slightly left of the body's midline, and is enclosed in a protective double-walled sac called the pericardium.

Anatomy

Overall structure

The adult human heart is roughly cone-shaped, weighing between 250 and 350 grams in most individuals, though size varies with body mass and fitness level. Its outer surface is covered by the epicardium, beneath which lies the thick muscular wall called the myocardium, and its interior is lined by the endocardium. The organ is divided into a right side and a left side by the interventricular septum, with each side containing one upper chamber (atrium) and one lower chamber (ventricle).

The four chambers

The heart comprises four chambers:

• Right atrium – receives deoxygenated blood returning from the body via the superior and inferior venae cavae.
• Right ventricle – pumps deoxygenated blood through the pulmonary valve into the pulmonary arteries toward the lungs.
• Left atrium – receives oxygenated blood from the lungs via the four pulmonary veins.
• Left ventricle – pumps oxygenated blood through the aortic valve into the aorta for systemic distribution. Its wall is significantly thicker than that of the right ventricle, reflecting the greater pressure it must generate.

Valves

Four valves ensure unidirectional blood flow through the heart:

1. Tricuspid valve – between the right atrium and right ventricle.
2. Pulmonary valve – between the right ventricle and the pulmonary artery.
3. Mitral (bicuspid) valve – between the left atrium and left ventricle.
4. Aortic valve – between the left ventricle and the aorta.

Each valve opens and closes in response to pressure differences on either side, producing the characteristic lub-dub sounds audible through a stethoscope.

Coronary circulation

The heart muscle itself is supplied with oxygen and nutrients by the coronary arteries, which branch from the aorta just above the aortic valve. The two principal vessels are the left coronary artery and the right coronary artery. Venous blood from the myocardium drains primarily into the coronary sinus and then into the right atrium. Obstruction of a coronary artery is the leading cause of myocardial infarction.

Physiology and electrical conduction

The cardiac cycle

Each heartbeat constitutes one cardiac cycle, divided into systole (contraction) and diastole (relaxation). During systole, the ventricles contract and eject blood into the pulmonary and systemic circulations. During diastole, the ventricles relax and refill with blood from the atria. Cardiac output — the volume of blood pumped per minute — is the product of heart rate and stroke volume, typically around 5 litres per minute at rest in a healthy adult.

Electrical conduction system

The heartbeat is initiated and coordinated by the heart's intrinsic electrical conduction system:

1. The sinoatrial (SA) node, located in the right atrium, acts as the natural pacemaker, generating electrical impulses that spread across both atria.
2. The atrioventricular (AV) node introduces a brief delay before transmitting the impulse to the ventricles.
3. The bundle of His and its left and right bundle branches carry the impulse into the ventricular walls.
4. Purkinje fibres distribute the impulse rapidly throughout the ventricular myocardium, triggering synchronised contraction.

This electrical activity is recorded non-invasively by an electrocardiogram (ECG or EKG).

Regulation of heart rate

Heart rate is modulated by the autonomic nervous system. The sympathetic division increases heart rate and contractility (as during exercise or stress), while the parasympathetic division — primarily via the vagus nerve — slows the heart during rest. Hormones such as adrenaline (epinephrine) also influence cardiac output.

Development and embryology

The human heart is one of the first organs to develop in the embryo, beginning to beat as early as three to four weeks after fertilisation. It forms from the cardiogenic mesoderm and initially takes the shape of a simple tube that loops and septates into the mature four-chambered structure by approximately the eighth week of gestation. Errors in this developmental process can result in congenital heart disease, a category of structural abnormalities present at birth.

Common diseases and clinical conditions

Coronary artery disease

Coronary artery disease (CAD) is the most prevalent form of heart disease worldwide and results from the build-up of atherosclerotic plaques within the coronary arteries, narrowing them and reducing blood flow to the myocardium. Acute complete blockage causes myocardial infarction (heart attack).

Heart failure

Heart failure occurs when the heart cannot pump sufficient blood to meet the body's demands. It may arise from damage caused by myocardial infarction, chronic hypertension, cardiomyopathy, or valvular disease, and is classified as systolic (reduced ejection fraction) or diastolic (preserved ejection fraction).

Arrhythmias

Abnormal heart rhythms, or arrhythmias, range from clinically benign extra beats to life-threatening conditions such as ventricular fibrillation. Atrial fibrillation is one of the most common sustained arrhythmias and significantly raises the risk of stroke.

Valvular heart disease

Disease of any of the four heart valves — through stenosis (narrowing) or regurgitation (leaking) — can impair cardiac function and may require surgical repair or replacement with mechanical or biological prosthetic valves.

Diagnosis and treatment

Diagnostic tools

Clinicians use a range of techniques to evaluate the heart:

• Electrocardiogram (ECG) – records electrical activity.
• Echocardiography – uses ultrasound to image cardiac structure and function.
• Cardiac MRI and CT – provide detailed anatomical and functional imaging.
• Coronary angiography – visualises coronary artery patency via contrast dye injection.
• Blood biomarkers – such as troponin levels, which rise after myocardial injury.

Interventional and surgical treatments

Treatments depend on the condition and may include:

• Medications (antihypertensives, statins, anticoagulants, antiarrhythmics).
• Percutaneous coronary intervention (PCI) – angioplasty and stenting to open blocked coronary arteries.
• Coronary artery bypass grafting (CABG) – surgical rerouting of blood flow around blocked arteries.
• Valve repair or replacement surgery.
• Implantable devices – pacemakers and implantable cardioverter-defibrillators (ICDs).
• Heart transplantation – reserved for end-stage heart failure refractory to other treatments.

Frequently asked questions

Is the human heart located entirely on the left side of the chest?

The heart is not entirely on the left side; it is positioned centrally within the chest but tilted so that its apex points toward the lower left. The bulk of the organ sits slightly left of the midline, which is why heartbeats are most easily felt on the left side of the chest.

Is the heart rate the same in all healthy adults?

Normal resting heart rate ranges from 60 to 100 beats per minute, but individual variation is substantial. Highly trained athletes frequently have resting rates below 60 beats per minute (bradycardia), which in their case reflects efficient cardiac adaptation rather than disease.

Is the heart capable of beating independently of the brain?

Yes. The heart's sinoatrial node generates its own electrical impulses, allowing the organ to contract rhythmically without input from the brain or spinal cord. This intrinsic automaticity is what enables heart transplantation, since the transplanted organ functions without its original nerve connections.

Is heart disease preventable?

Many forms of heart disease, particularly coronary artery disease, are substantially influenced by modifiable risk factors including diet, physical activity, smoking, blood pressure, and blood cholesterol levels. Public health evidence strongly supports lifestyle modification and, where appropriate, medical therapy in reducing incidence, though genetic predisposition also plays a role.

Is an electrocardiogram the same as an echocardiogram?

No. An electrocardiogram (ECG) records the heart's electrical activity as a waveform trace and is used to detect arrhythmias, conduction abnormalities, and signs of myocardial injury. An echocardiogram uses ultrasound waves to produce real-time images of the heart's structures and assesses mechanical function, valve integrity, and chamber dimensions.