Heart Anatomy and Physiology

The Pulmonary and Systemic Circuits

  • Pulmonary Circuit:

    • Blood flows from the right heart to the lungs.
    • Facilitates gas exchange in the lungs:
    • Carbon dioxide is unloaded.
    • Oxygen is loaded.

  • Systemic Circuit:

    • Blood flows from the left heart to all body organs.
    • Facilitates gas exchange in organs:
    • Oxygen is unloaded.
    • Carbon dioxide is loaded.

Position, Size, and Shape of the Heart

  • The heart is located in the mediastinum (central compartment of the thoracic cavity).
  • Approximate size of a human fist.
  • Base: Superior part of the heart.
  • Apex: Points inferiorly and to the left.

Overview of Surface Anatomy of the Heart

  • Major structures:
    • Aortic Arch
    • Superior Vena Cava
    • Pulmonary Arteries and Veins
    • Atria and Auricles:
    • Right Atrium
    • Left Atrium
    • Right and Left Auricles (ear-like projections).
    • Ventricles:
    • Right Ventricle
    • Left Ventricle
    • Coronary Sulcus: Groove encircling the heart.

The Pericardium

  • Definition: A double-walled membrane surrounding the heart.
  • Components:
    • Fibrous Pericardium: Tough outer layer composed of fibrous connective tissue.
    • Serous Pericardium:
    • Parietal layer lies below fibrous pericardium.
    • Visceral layer (epicardium): Outer layer of the heart wall.
    • Pericardial Cavity: Space containing serous fluid to reduce friction.

Layers of the Heart Wall

  • Three primary layers:
    1. Epicardium: Visceral layer and part of the serous pericardium.
    2. Myocardium: Thick muscular layer responsible for heart contractions (cardiac muscle).
    3. Endocardium: Inner layer lining the heart chambers, made of simple squamous epithelium.

Chambers of the Heart

  • Atria (Right and Left):

    • Thin-walled superior chambers (receivers of blood).
    • Separated by Interatrial Septum.
    • Features:
    • Auricles: Ear-like extensions.
    • Pectinate Muscles: Internal ridges of the atrial wall.

  • Ventricles (Right and Left):

    • Thick-walled inferior chambers (pumping blood).
    • Separated by Interventricular Septum.
    • Features:
    • Trabeculae Carneae: Internal ridges of ventricles.
    • Chordae Tendineae: Tendinous cords connecting valves to papillary muscles.

The Valves of the Heart

  • Atrioventricular (AV) Valves: Close when ventricles contract.
    • Right AV Valve: Tricuspid valve.
    • Left AV Valve: Bicuspid (mitral) valve.
    • Function of Chordae Tendineae: Holds valves shut during contraction.
  • Semilunar Valves: Close when ventricles relax.
    • Pulmonary Valve: At the exit of the right ventricle.
    • Aortic Valve: At the exit of the left ventricle.

Pathway of Blood Flow Through the Heart

  1. Blood enters the right atrium from the superior and inferior vena cavae.
  2. Flows through the right AV valve into the right ventricle.
  3. Right ventricle contraction forces the pulmonary valve open.
  4. Blood flows through the pulmonary valve into the pulmonary trunk.
  5. Distributed to lungs via pulmonary arteries for gas exchange.
  6. Blood returns from lungs via pulmonary veins to left atrium.
  7. Blood in left atrium moves through left AV valve into left ventricle.
  8. Contraction of left ventricle forces the aortic valve open.
  9. Blood flows through the aortic valve into the ascending aorta.
  10. Distributed via aorta to the body organs for gas exchange.
  11. Blood returns to right atrium via vena cavae.

Coronary Circulation: Arterial Supply

  • Essential for supplying the heart with oxygen.
  • Left Coronary Artery (LCA):
    • First branch from the aorta; supplies left side of the heart.
    • Branches:
    • Anterior Interventricular (LAD)
    • Circumflex Branch (includes left marginal).
  • Right Coronary Artery (RCA):
    • Supplies right side of the heart.
    • Branches:
    • Right Marginal Branch
    • Posterior Interventricular Branch.
  • Blockages in these arteries can cause myocardial infarction.

Venous Drainage of Cardiac Muscle

  • About 20% of blood is drained directly through small cardiac veins back to heart chambers.
  • 80% of blood is returned to the right atrium via the coronary sinus, which collects blood from:
    • Great cardiac vein
    • Small cardiac vein
    • Middle cardiac vein
    • Left marginal vein

The Conduction System

  • Specialized non-contractile cardiac cells generate and conduct electrical signals for coordinated contraction:
    • Sinoatrial (SA) Node: Pacemaker of the heart.
    • Atrioventricular (AV) Node: Relays impulses.
    • AV Bundle: Splits into bundle branches.
    • Purkinje Fibers: Distribute excitation throughout the ventricles.

Changes to the Heart at Birth

  • In the fetal heart, blood circulation bypasses the pulmonary circuit due to:
    • Foramen Ovale: Closes after birth, eventually forms fossa ovalis.
    • Ductus Arteriosus: Closes after birth, becomes the ligamentum arteriosum.

Heart Disease

  • Leading cause of death in the U.S.
  • Most common forms:
    • Coronary Atherosclerosis: Narrowing of coronary arteries due to plaque buildup.
    • Can lead to myocardial infarction and heart failure.
    • Congenital defects (e.g., Tetralogy of Fallot).
    • Myocardial hypertrophy due to hypertension.
    • Inflammation of heart wall (pericarditis, myocarditis).
    • Valvular defects (e.g., valve prolapse, regurgitation, stenosis).