Heart Anatomy and Function

Heart Anatomy

• The heart is a cone-shaped, muscular organ that pumps blood through the body.
• It is approximately the size of a fist, weighing around 300 grams and measuring 11cm long.
• Located within the mediastinum, it is protected by the ribs and sternum.
• The heart lies between ribs 2-5 to the left of the midline, with the apex (bottom) positioned in the 5th intercostal space at the midclavicular line.

Heart Chambers

• The heart is divided into four chambers:
  • Two atria (upper chambers)
  • Two ventricles (lower chambers)
• The atria and ventricles are separated by the interatrial septum (between atria) and the interventricular septum (between ventricles).

Heart Valves

• Valves are essential for ensuring unidirectional blood flow within the heart.
• There are four valves:
  • Two atrioventricular (AV) valves:
    • Tricuspid valve (right AV valve): located between the right atrium and right ventricle; three cusps.
    • Bicuspid/mitral valve (left AV valve): located between the left atrium and left ventricle; two cusps
  • Two semilunar (SL) valves:
    • Pulmonary valve: three cusps; located between the right ventricle and pulmonary trunk.
    • Aortic valve: three cusps; located between the left ventricle and aorta.

Atrioventricular (AV) Valves

• Open when blood flows from the atria to the ventricles.
• Close during ventricular contraction (systole).
• Chordae tendinae, attached to papillary muscles, prevent the valves from everting (turning inside out) and ensure blood does not flow back into the atria.
• The closing of AV valves (tricuspid and mitral) produces S1 ("Lub") heart sound.

Semilunar (SL) Valves

• Open during ventricular contraction (systole) as blood is ejected from the ventricles.
• Close during ventricular relaxation (diastole) to prevent blood from flowing back into the ventricles.
• The closing of SL valves (pulmonic and aortic) produces the S2 ("Dub") heart sound.

Heart Circulation

• Pulmonary Circulation (blue):
  • Involves the right side of the heart and the lungs.
  • Deoxygenated blood is pumped from the right ventricle to the lungs for oxygenation.
• Systemic Circulation (red):
  • Involves the left side of the heart and the rest of the body.
  • Oxygenated blood is pumped from the left ventricle to the body's organs and tissues to deliver oxygen and nutrients.

Coronary Circulation

• Supplies the heart muscle with oxygen and nutrients.
• Consists of coronary arteries that deliver blood and cardiac veins that remove waste.

Coronary Arteries

• Main Branches:
  • Right Coronary Artery (RCA):
    • Marginal artery
    • Posterior interventricular artery
  • Left Coronary Artery (LCA):
    • Anterior interventricular artery (LAD)
    • Circumflex artery

Cardiac Veins

• Drain into the coronary sinus, which empties into the right atrium.
• Anterior cardiac veins drain directly into the right atrium.

Fibrous Skeleton of the Heart

• A meshwork of dense connective tissue.
• Functions as an insulator for electrical impulses, preventing direct conduction from the atria to the ventricles.
• Supports and anchors the heart valves.

Regulation of Cardiac Activity

• Conduction system:
  • Sinoatrial (SA) node (pacemaker): initiates electrical impulses.
  • Atrioventricular (AV) node: delays impulse conduction.
  • AV bundle (bundle of His): conducts impulses to the ventricles.
  • Right and left bundle branches: conduct impulses to specific ventricular regions.
  • Purkinje fibers: conduct impulses throughout the ventricular myocardium.
• Autonomic nervous system (ANS):
  • Sympathetic: increases heart rate, force of contraction, and dilates coronary vessels.
  • Parasympathetic: decreases heart rate and force of contraction.

Foetal Circulation

• Before birth, the circulatory system undergoes significant adjustments to adapt to the absence of respiratory function in the womb.
• Key features of foetal circulation:
  • Foramen ovale: an opening between the right and left atria, allowing blood to bypass the lungs.
  • Ductus arteriosus: a vessel connecting the pulmonary trunk to the aorta, allowing blood to bypass the pulmonary circulation.
  • Umbilical cord: supplies the fetus with oxygen and nutrients from the placenta, and removes waste products.

Changes at Birth

• When the newborn takes its first breath, the lungs expand, increasing pulmonary blood flow.
• The foramen ovale closes, preventing blood from bypassing the lungs.
• The ductus arteriosus constricts and eventually becomes the ligamentum arteriosum.
• The umbilical cord is tied and severed, leading to closure of the umbilical vessels.

Clinical Correlations

• Valves: Mitral valve prolapse, Tricuspid valve stenosis, Pulmonary valve stenosis, Aortic valve stenosis
• Myocardium: Heart failure, Myocarditis, Myocardial infarction, Hypertrophic cardiomyopathy, Dilated cardiomyopathy
• Pericardium: Pericarditis, Pericardial effusion, Cardiac tamponade, Constrictive pericarditis
• Conduction System: Arrhythmias, Heart block, Congenital conduction defects, Atrial fibrillation, Ventricular fibrillation
• Coronary Arteries: Coronary artery disease (CAD), Myocardial infarction (MI), Angina, Coronary artery aneurysm
• Aorta and Great Vessels: Aortic aneurysm, Aortic dissection, Coarctation of the aorta, Atherosclerosis
• Pulmonary Arteries: Pulmonary embolism, Pulmonary hypertension
• Pulmonary Veins: Pulmonary venous congestion, Pulmonary venous obstruction, Pulmonary hypertension
• Chambers of the Heart (Atria and Ventricles): Atrial fibrillation, Atrial septal defect, Ventricular septal defect, Congestive heart failure (CHF)
• Ventricular Septum: Ventricular septal defect (VSD), Myocardial infarction involving the septum, Hypertrophic cardiomyopathy
• Veins (e.g., coronary veins): Coronary sinus thrombosis, Venous insufficiency, Varicose veins, Deep vein thrombosis (DVT)
• Pericardial Cavity: Pericardial effusion, Cardiac tamponade
• Coronary Circulation (Arteries and Veins): Coronary artery disease (CAD), Angina, Coronary vein thrombosis, Coronary artery bypass grafting (CABG)
• Fibrous Skeleton of the Heart: Valve regurgitation, Valve stenosis, Heart block (due to electrical disruption)
• Autonomic Nervous System (ANS) Influence on Heart: Tachycardia, Bradycardia, Vasovagal syncope, Autonomic dysfunction
• Foetal Circulation: Patent ductus arteriosus (PDA), Patent foramen ovale (PFO), Congenital heart defects