The Pulmonary and Systemic Circuits

The Pulmonary and Systemic Circuits

  • Heart Function
    • Functions as a transport system with two side-by-side pumps:
    • Right Side: Receives oxygen-poor blood from tissues.
      • Pumps this blood to the lungs via the pulmonary circuit for oxygenation.
    • Left Side: Receives oxygenated blood from the lungs.
      • Pumps this blood to body tissues via the systemic circuit.

Anatomy of the Heart

  • Chambers of the Heart

    • Receiving Chambers:
    • Right Atrium: Receives blood from systematic circuit (via superior vena cava, inferior vena cava).
    • Left Atrium: Receives blood from pulmonary circuit (via pulmonary veins).
    • Pumping Chambers:
    • Right Ventricle: Pumps blood into the pulmonary circuit.
    • Left Ventricle: Pumps blood into systemic circulation.

  • Physical Characteristics

    • Size: Approximately the size of a fist.
    • Location:
    • In mediastinum between the second rib and the fifth intercostal space.
    • Two-thirds of the heart lies to the left of the midsternal line.
    • Anterior to the vertebral column and posterior to the sternum.

  • Base and Apex

    • Base: Posterior surface leaning toward right shoulder.
    • Apex: Points toward the left hip; palpated between fifth and sixth ribs, just below the left nipple.

Coverings of the Heart: Pericardium

  • Structure: Double-walled sac

    • Superficial Fibrous Pericardium: Protects and anchors the heart; prevents overfilling.
    • Deep Serous Pericardium:
    • Parietal layer lines fibrous pericardium.
    • Visceral layer (epicardium) covers the heart; space between layers contains pericardial fluid reducing friction.

  • Homeostatic Imbalance: Pericarditis

    • Inflammation of the pericardium leads to friction rub sounds; excessive fluid causes cardiac tamponade which compresses the heart.

Layers of the Heart Wall

  • Three Layers:
    • Epicardium: Visceral layer of serous pericardium.
    • Myocardium:
    • Contains contractile cardiac muscle cells.
    • Cardiac skeleton supports the structure and regulates action potential spread.
    • Endocardium: Linings of heart chambers.

Chambers and Septa

  • Four Chambers:
    • Atria (2 superior):
    • Smaller with thinner walls; fill the ventricles.
    • Ventricles (2 inferior):
    • Thicker walls, functioning as pumps.
    • Interatrial Septum separates atria; Interventricular Septum separates ventricles.

Heart Valves

  • Function of Valves: Ensure unidirectional blood flow, open and close in response to pressure changes.

  • Atrioventricular (AV) Valves: Prevent backflow into atria when ventricles contract.

    • Tricuspid Valve (right AV valve)
    • Mitral Valve (left AV valve, bicuspid valve)
    • Anchored by chordae tendineae.

  • Semilunar Valves: Prevent backflow into ventricles when they relax.

    • Aortic Semilunar Valve
    • Pulmonary Semilunar Valve.

Pathway of Blood Through the Heart

  • Pulmonary Circuit:

    1. Right atrium → tricuspid valve → right ventricle → pulmonary semilunar valve → pulmonary trunk → pulmonary arteries → lungs.
    2. Blood becomes oxygenated in the lungs, then returns to the left atrium via pulmonary veins.

  • Systemic Circuit:

    1. Left atrium → mitral valve → left ventricle → aortic semilunar valve → aorta → systemic circulation.

Coronary Circulation

  • Provides the heart muscle itself with blood, primarily during relaxation.
  • Anatomy & Functions:
    • Left ventricle receives a significant portion of blood supply.
    • Contains anastomoses allowing alternative blood routes, however, cannot compensate for occluded coronary arteries.