The Human Heart: Anatomy, Function, and the Pump That Sustains Life

The Heart as a Biological Pump

The human heart is a four-chambered muscular organ that functions as a dual pump, circulating ~5 L/min of blood at rest through two serial circuits:

Pulmonary Circuit (Right Heart)

• Right atrium receives systemic venous return (SVC/IVC) → tricuspid valve → right ventricle.
• Right ventricle contracts → pulmonary valve → pulmonary arteries → lungs (gas exchange).

Systemic Circuit (Left Heart)

• Pulmonary veins → left atrium → mitral valve → left ventricle.
• Left ventricle contracts → aortic valve → aorta → systemic arteries → capillaries → veins.

Key Structural Features Enabling Pump Function

• Myocardium: Cardiac muscle with intercalated discs for synchronized contraction.
• Valves (4): Ensure unidirectional flow (AV: tricuspid, mitral; Semilunar: pulmonary, aortic).
• Conduction System: SA node → AV node → Bundle of His → bundle branches → Purkinje fibers.
• Coronary Circulation: Heart’s own blood supply (RCA, LAD, LCx) — perfused mainly during diastole.

Cardiac Cycle Phases

1. Atrial systole (atrial kick = 20–30% ventricular filling).
2. Ventricular systole: Isovolumetric contraction → ejection.
3. Early diastole: Isovolumetric relaxation → rapid filling → diastasis → atrial systole.

Regulation

• Intrinsic: Frank-Starling mechanism (preload), autoregulation.
• Extrinsic: Autonomic (sympathetic ↑HR/contractility; parasympathetic ↓HR), hormones (catecholamines, ANP, ADH).

Vital Importance

Cessation of cardiac output → loss of consciousness in ~10 seconds; irreversible brain injury in 4–6 minutes. CPR and defibrillation are time-critical interventions.