The Heart: Structure, Function, and Blood Flow

Overview – The Heart (Activity time: 30 min)

• Central muscular pump that maintains systemic and pulmonary circulation.
• Located in the mediastinum, behind the sternum, roughly the size of a closed fist.
• Average adult mass ≈ 300\text{–}350\,\text{g}; beats ≈ 60\text{–}100\,\text{times/min} at rest.
• Protected by the pericardium (fibrous + serous layers with pericardial fluid ≈ 15\,\text{mL}).

Heart Structure & Anatomy (Activity)

• Four chambers
  • Right atrium (RA) – receives systemic venous blood via SVC & IVC.
  • Right ventricle (RV) – pumps blood to pulmonary trunk.
  • Left atrium (LA) – receives oxygenated blood from 4 pulmonary veins.
  • Left ventricle (LV) – ejects blood into aorta; wall ≈ 3× thicker than RV.
• Valves (one-way doors preventing back-flow)
  • Atrioventricular (AV): tricuspid (RA→RV), mitral/bicuspid (LA→LV).
  • Semilunar: pulmonary (RV→pulmonary artery), aortic (LV→aorta).
• Wall layers
  • Endocardium – inner endothelial lining.
  • Myocardium – muscular layer; thickness ∝ pressure it must generate.
  • Epicardium – visceral pericardium (outer surface).
• Conduction system
  • Sinoatrial (SA) node (pacemaker, \approx 70\,\text{impulses/min}).
  • Atrioventricular (AV) node → Bundle of His → Right/Left bundle branches → Purkinje fibres.
  • Ensures atria contract before ventricles (≈ 0.1\,\text{s} delay at AV node).
• Coronary circulation
  • Right coronary artery (RCA): supplies RA, RV, inferior LV.
  • Left coronary artery (LCA) → LAD & circumflex branches: supply LV anterior/lateral walls & septum.

How the Heart Works (Activity)

• Cardiac cycle = one heartbeat; duration \approx 0.8\,\text{s} at 75\,\text{bpm}.
  1. Atrial systole (≈ 0.1\,\text{s}) – tops up ventricular volume (end-diastolic volume, EDV ≈ 120\,\text{mL}).
  2. Ventricular systole (≈ 0.3\,\text{s})
  • Isovolumetric contraction – AV valves shut → S_1 heart sound.
  • Ventricular ejection once pressure > arterial pressure.
  1. Ventricular diastole (≈ 0.4\,\text{s})
  • Isovolumetric relaxation – semilunar valves close → S_2.
  • Passive ventricular filling (~80 % of volume).
• Key equations
  • Stroke volume: SV = EDV - ESV (end-systolic volume).
  • Cardiac output: CO = HR \times SV (normal ≈ 5\,\text{L/min} at rest).
  • Ejection fraction: EF = \frac{SV}{EDV} \times 100\% (normal ≥ 55\%).
• Autonomic control
  • Sympathetic ↑ HR & contractility via \beta_1 receptors (norepinephrine).
  • Parasympathetic (vagus) ↓ HR (acetylcholine acting on M_2 receptors).
• Frank-Starling law: Increased preload → stronger contraction (within physiological limits).

Blood Flow through the Heart (Activity)

1. Systemic venous blood → SVC/IVC → Right Atrium.
2. RA → tricuspid valve → Right Ventricle.
3. RV → pulmonary valve → Pulmonary trunk → Right & Left pulmonary arteries → Lungs.
4. Oxygenated blood → Four pulmonary veins → Left Atrium.
5. LA → mitral valve → Left Ventricle.
6. LV → aortic valve → Aorta → Systemic circulation.

• Pressure gradients drive one-way flow; valve incompetence causes murmurs.
• Pulmonary circulation: low-pressure (≈ 15\,\text{mmHg} mean), systemic: high-pressure (≈ 100\,\text{mmHg} mean).

Clinical, Ethical & Practical Notes

• Coronary artery disease ↓ myocardial oxygen → ischemia/infarction; leading global cause of death.
• Heart failure defined as inability to maintain CO; may require lifestyle changes, pharmacologic, or device therapy.
• Ethical triage in transplant allocation: prioritise based on severity, prognosis, and quality-of-life metrics.
• CPR guidelines: compression depth 5\text{–}6\,\text{cm}, rate 100\text{–}120\,\text{min}^{-1}.

End of compiled study notes derived from the provided transcript keywords.