Cardiovascular System

• Cardiovascular system


  Composed of the heart and the continuous system of blood vessels: the arteries, arterioles, capillaries, venules, and veins

  
• Pulmonary circulation


  Conveys arterial blood from the heart to the lungs and returns venous blood back to the heart
• Systemic circulation


  Conveys arterial blood from the heart to all other tissues and returns venous blood back to the heart
• Heart


  • Four-chamber muscular pump with two atria and two ventricles
  • Contains cardiac muscle for contraction
  • Fibrous skeleton for attachment of valves and separation of atrial and ventricular musculature
  • Conducting system for initiation and propagation of rhythmic contractions
  • Coronary vasculature (coronary arteries and cardiac veins)
• Heart wall layers


  • Epicardium (outer layer)
  • Myocardium (middle layer)
  • Endocardium (inner layer)
• Epicardium


  Visceral layer of serous pericardium, consisting of mesothelium with underlying connective and adipose tissue, contains coronary vasculature
• Myocardium


  Middle layer, consists of cardiac muscle
• Endocardium


  Inner layer, consists of endothelium, subendothelial connective tissue, and subendothelial layer containing cells of the conducting system
• Conducting system of the heart

  Sinoatrial (SA) node, atrioventricular (AV) node, AV bundle (Bundle of His), and Purkinje fibers

  
• Heart rate regulation


  Sympathetic nerves increase rate, parasympathetic nerves decrease rate, circulating hormones (epinephrine, norepinephrine), Ca2+, thyroid hormones
• Tunica intima


  Innermost layer of blood vessel wall, includes endothelium, subendothelial connective tissue, internal elastic membrane
• Tunica media


  Middle layer of blood vessel wall, comprised of circumferentially arranged vascular smooth muscle cells with elastic lamellae
• Tunica adventitia


  Outermost layer of blood vessel wall, composed of primarily collagen with few elastic fibers, contains vasa vasorum and nervi vasorum
• Endothelial cells


  Actively interact with underlying vascular smooth muscle cells and connective tissue, maintain selective permeability, prevent blood clotting, modulate vascular resistance, regulate immune responses
• Artery types


  • Large arteries (elastic arteries)
  • Medium arteries (muscular arteries)
  • Small arteries (including arterioles)
• Elastic arteries


  Tunica media has multiple layers of vascular smooth muscle cells separated by elastic lamellae, no fibroblasts
• Muscular arteries


  Tunica media has more smooth muscle and less elastic lamellae than elastic arteries, prominent internal elastic membrane in tunica intima
• Small arteries and arterioles

  Differentiated by number of smooth muscle cell layers in tunica media, arterioles have 1-2 layers and regulate vascular resistance
• Arteriovenous shunts


  Allow blood to bypass capillaries by providing direct routes between arteries and veins, regulated by precapillary sphincters
• Vein types


  • Venules
  • Small veins
  • Medium veins
  • Large veins
• Veins


  Do not have an internal elastic membrane, have a relatively thin tunica media and more pronounced tunica adventitia
• Postcapillary venules


  Collect blood from capillary network, characterized by presence of pericytes
• High endothelial venules


  In lymphoid tissues, lined by cuboidal endothelium facilitating lymphocyte migration
• Capillary types


  • Continuous capillaries
  • Fenestrated capillaries
  • Discontinuous or sinusoidal capillaries
• Continuous capillaries


  Characterized by uninterrupted vascular endothelium
• Fenestrated capillaries


  Characterized by numerous openings in capillary wall and continuous basal lamina
• Discontinuous or sinusoidal capillaries

  Larger diameter with large openings, intercellular gaps, and discontinuous basal lamina
• Pericytes


  Associated with capillaries, represent undifferentiated mesenchymal stem cells
• Lymphatic vessels


  Convey interstitial fluids from tissues to bloodstream, smallest are lymphatic capillaries
• All lymphatic vessels possess valves that prevent backflow of lymph
• The epicardium contains fatty connective tissue and vessels of the coronary circulation in the region of the atrium
• The myocardium is thicker in the ventricle than the atrium, as the ventricles need to generate more force to pump blood throughout the body
• The three layers of the heart wall are the epicardium, myocardium, and endocardium

  
• The three layers of the blood vessel wall are the tunica intima, tunica media, and tunica adventitia
• Differentiating arteries and veins

  Veins do not have an internal elastic membrane, unlike arteries
• Purkinje fibers


  Hypertrophied cardiac muscle fibers specialized for conducting impulses, contain high mitochondria and glycogen, pale staining cytoplasm
• Under H&E staining, Purkinje fibers appear pale staining compared to normal cardiac muscle cells
• Under PAS staining, Purkinje fibers stain strongly positive due to their high glycogen content, while normal cardiac muscle cells stain less intensely
• Under PTAH staining, Purkinje fibers stain less intensely than normal cardiac muscle cells
• Impulse conducting system of the heart

  1. Sinoatrial node

  2. Atrioventricular node

  3. Atrioventricular bundle (Bundle of His)

  4. Purkinje fibers