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.