Circulatory system 3

• Heart

  • A four-chambered muscular organ, approximately the size of a man's clenched fist, triangular in shape with base uppermost and apex inferiorly, located in the middle mediastinum between the 2 lungs. Base is found just below the 2nd rib, apex forms the lower border and is found at the level of the 5th intercostal space, and rests on the diaphragm [aortic notch]

  
• Pericardium

  Loose-fitting sac that covers the heart, consists of an outer fibrous portion and an inner serous portion (parietal and visceral layers)

  
• Pericardial space

  Potential space between the parietal and visceral layers of the pericardium, contains a few drops of pericardial fluid which serves as a lubricant
• functions of Pericardium

  • Protects the heart, allowing it to beat unimpeded as long as fluid is maintained in the potential space, and the layers do not stick together

  
• Myocardium

  Consists of involuntary, striped/striated muscle, thickest in the ventricles, thinnest in the atria, covered directly by visceral pericardium

  
• Endocardium

  The innermost layer, lines the interior of the myocardium, composed of a single layer of squamous cells upon a base of connective tissue, also called the endothelium of the heart

  
• Atria

  The upper two chambers of the heart
• Ventricles

  The lower two chambers of the heart, larger and have thicker walls than the atria
• Left ventricle

  • Thickest because the aorta leaves from it and it has to pump oxygenated blood around the body (systemic circulation)

  
• Pulmonary artery

  Leaves the right ventricle, taking deoxygenated blood to the lungs

  
• Pulmonary veins

  Enter the left atrium, returning oxygenated blood from the lungs
• Superior and inferior vena cava

  Enter the right atrium, returning blood from the systemic circulation

  
• Inter-atrial septum


  Separates the two atria
• Inter-ventricular septum


  Separates the two ventricles
• Cuspid valves

  Mechanical devices that permit blood flow in one direction only, separating the atria from the ventricles

  
• Tricuspid valve

  Separates the right ventricle from the right atria, has three flaps

  
• Bicuspid/mitral valve

  Separates the left ventricle from the left atrium, has two flaps

  
• Semi-lunar valves

  Half-moon shaped flaps that grow outward from the internal lining of the aorta and the pulmonary artery, allowing blood to flow in one direction only - away from the heart
• Chordae tendinae

  Fine tendinous cords that extend between the ventricular surfaces of the cuspid valves and projections of the heart muscles in the ventricles called papillary muscles, anchoring the flaps of the cuspid valves

  
• Left coronary artery

  Branches into the anterior descending artery (serves left and right ventricles) and the circumflex artery (serves left atrium and left ventricle)

  
• Right coronary artery

  Branches into the posterior descending artery (supplies left and right ventricles) and the marginal artery (supplies right atrium and right ventricle)

  
• Coronary arteries

  • Ventricles have more blood supply because they do more work, forcing blood into the aorta and pulmonary arteries respectively

  
• Anastomoses

  Alternative connections or bypasses in the heart's blood supply, providing a means for arterial blood to travel if the main route becomes blocked

  
• Coronary sinus

  Large vascular sinus on the posterior aspect of the heart, where most deoxygenated blood from the myocardium drains

  
• Veins draining into the coronary sinus

  • Great cardiac vein
  • Middle cardiac vein
  • Small cardiac vein
  • Anterior cardiac veins

  
• Sino-atrial (SA) node

  Small mass of modified cardiac muscle fibres in the wall of the right atrium, close to the opening of the superior vena cava, initiates the heartbeat

  
• Atrioventricular (AV) node of Tawara

  Small mass of specialized cardiac muscle in the inter-atrial septum, just above the level of the mitral and tricuspid valves, relays the impulse from the SA node to the atrioventricular bundle

  
• Atrioventricular (AV) bundle of His and Purkinje fibres

  Bundle of specialized cardiac muscle originating in the AV node, extending along the interventricular septum and continuing as Purkinje fibres into the papillary muscles and lateral walls of the ventricles

  
• Conduction mechanism

  1. Stimulation of SA node by autonomic nerves

  2. Impulse spreads through atrial muscles

  3. Atria contract

  4. Impulse spreads to AV node

  5. Impulse relayed by bundle of His and Purkinje fibres to the right and left ventricles

  6. Ventricles contract
• Cardiac plexus

  Contains sympathetic and parasympathetic nerve fibres that supply the heart, most ending at the SA node

  
• Ischaemic heart disease: lessening of blood supply to heart muscle

  
• Valvular insufficiency: Where valves lose the capacity to close tightly resulting in backflow of blood from ventricles to atria
• Fallot's Tetralogy: Combination of four anomalies: 1. Stenosis of the pulmonary artery, 2. Defect in interventricular septum 3. Over riding aorta 4. Enlarged right ventricle

  
• Myocardial infarction: Death of heart muscle cells due to deprivation of blood supply
• Systemic circulation


  The flow of blood through the system of blood vessels carrying oxygenated blood from the left ventricle of the heart to all organs and tissues, facilitating exchange of substances, then returning deoxygenated blood to the heart via the superior and inferior vena cava

  
• Systemic circulation

  1. Oxygenated blood enters left atrium via pulmonary veins

  2. Mitral valve opens, allowing blood into left ventricle

  3. Left ventricle contracts, aortic valve opens, blood forced out to organs and tissues

  4. Blood moves from arteries to arterioles, capillaries, venules, veins, facilitating exchange

  5. Deoxygenated blood drains into superior and inferior venae cavae, returning to right atrium

  
• Pulmonary circulation

  Circulation of blood from the right ventricle of the heart to the lungs and back to the heart via the left atrium, facilitating gas exchange

  
• Pulmonary circulation


  1. Pulmonary artery leaves right ventricle, bifurcates into right and left pulmonary arteries

  2. Pulmonary arteries divide further into branches serving the lungs

  3. Gas exchange occurs between alveoli and surrounding capillaries

  4. Deoxygenated blood collects in pulmonary veins and enters left atrium

  
• Portal circulation

  Includes all veins draining deoxygenated blood from the abdominal portion of the digestive tract, begins with capillaries and merges to form the portal vein (splenic and superior mesenteric veins at L2) which delivers nutrients to the liver

  
• Foetal circulation

  Circulation in a baby before birth, where the baby gets oxygen and food from the mother's blood instead of its own lungs and digestive tract, all blood circulates through the placenta