Human Anatomy and Physiology: The Heart

Heart Location and Anatomy

  • Located in the subcavity of the mediastinum known as the Pericardial Cavity, slightly left in the chest.
  • Cone-shaped with the Apex pointing toward the left hip and the Base facing the posterior rib cage.
  • Measures approximately the size of a fist and weighs between 250−350 grams250-350\,grams.
  • Contains four hollow chambers: Superior Right and Left Atria, and inferior Right and Left Ventricles.
  • External markings include the Atrioventricular Sulcus and the Interventricular Sulcus.

Circulation Circuits and Hormone Function

  • Pulmonary Circuit: Right side of the heart pumps deoxygenated blood to lungs through pulmonary arteries (low-pressure circuit).
  • Systemic Circuit: Left side of the heart pumps oxygenated blood to the body through the Aorta (high-pressure circuit).
  • Endocrine Function: Atria produce Atrial Natriuretic Peptide (ANP), which decreases sodium retention and lowers blood pressure.

Heart Wall and Pericardium

  • Pericardium: Consists of a tough outer Fibrous Pericardium and a thin inner Serous Pericardium (subdivided into Parietal and Visceral/Epicardium layers).
  • Pericardial Cavity: Space between layers filled with Pericardial Fluid for lubrication.
  • Heart Wall Layers:
    • Myocardium: Thickest layer composed of cardiac myocytes and a Fibrous Skeleton.
    • Endocardium: Deepest layer composed of Endothelial Cells creating a blood-heart barrier.
  • Fibrous Skeleton: Provides structural support and electrical insulation between atria and ventricles.

Heart Valves and Great Vessels

  • Great Vessels: Superior and Inferior Venae Cavae, Pulmonary Trunk, Pulmonary Veins, and the Aorta.
  • Atrioventricular (AV) Valves: Prevent backflow into atria; includes the Tricuspid (right) and Bicuspid/Mitral (left) valves.
  • AV Support: Chordae Tendineae and Papillary Muscles prevent valve eversion.
  • Semilunar (SL) Valves: Prevent backflow into ventricles; includes the Pulmonary and Aortic valves (no chordae tendineae).

Coronary Circulation

  • Right Coronary Artery: Supplies right atrium and ventricle via Marginal and Posterior Interventricular Arteries.
  • Left Coronary Artery: Branches into the Anterior Interventricular Artery (LAD) and the Circumflex Artery.
  • Coronary Veins: Great, Small, and Middle cardiac veins drain into the Coronary Sinus, which empties into the right atrium.
  • Anastomoses: Provide Collateral Circulation or alternate routes for blood flow.

Cardiac Electrophysiology and ECG

  • Pacemaker Cells: Spontaneously generate action potentials (Autorhythmicity).
    • Sinoatrial (SA) Node: Primary pacemaker (60−70 bpm60-70\,bpm).
    • Atrioventricular (AV) Node: Slower rate (40−50 bpm40-50\,bpm) with a 0.13 second0.13\,second delay to allow ventricular filling.
    • Purkinje Fiber System: Includes AV Bundle and bundle branches (20 bpm20\,bpm).
  • Contractile Cells: Feature a Plateau Phase due to slow Ca2+Ca^{2+} influx, resulting in an Effective Refractory Period to prevent tetany.
  • Electrocardiogram (ECG):
    • P Wave: Atrial depolarization.
    • QRS Complex: Ventricular depolarization (atrial repolarization is obscured).
    • T Wave: Ventricular repolarization.

The Cardiac Cycle and Heart Sounds

  • Mechanical Events: Systole (contraction) and Diastole (relaxation).
  • Phases: Ventricular filling, Isovolumetric contraction, Ventricular ejection, and Isovolumetric relaxation.
  • Heart Sounds: S1 ("Lub") from AV valve closure; S2 ("Dub") from SL valve closure.
  • Pressures: Left ventricle generates up to 118 mm Hg118\,mm\,Hg, while right ventricle generates up to 28 mm Hg28\,mm\,Hg.

Cardiac Output and Regulation

  • Cardiac Output (CO): Calculated as HR×SVHR \times SV; average is 5 liters/minute5\,liters/minute.
  • Stroke Volume (SV): Determined by Preload (Frank-Starling Law regarding stretch), Contractility (inotropic agents), and Afterload (back pressure).
  • Neural Regulation: Sympathetic nerves increase CO (norepinephrine); Parasympathetic nerves decrease HR (acetylcholine via vagus nerves).
  • Endocrine Regulation: Epinephrine, thyroid hormone, and glucagon increase CO; ANP decreases it.

Clinical Pathologies

  • Cardiac Tamponade: Excess fluid in the pericardial cavity compressing the heart.
  • Myocarditis: Inflammation of the heart muscle, associated with SARS-CoV-2.
  • Valvular Disease: Insufficiency (leakage) or Stenosis (stiffening) causing heart murmurs.
  • Coronary Artery Disease (CAD): Plaque buildup leading to Myocardial Ischemia or Myocardial Infarction (MI).
  • Heart Failure: Pulmonary Oedema (left failure) or Peripheral Oedema (right failure).