CVS Histo

• HISTOLOGY OF THE CARDIOVASCULAR SYSTEM

  
• Cardiovascular system


  Transport system that carries blood and lymph to and/or from tissues of the body

  
• Constituents of the cardiovascular system


  • Heart
  • Blood vessels
  • Lymph vessels

  
• Pulmonary circulation


  Conveys blood from the heart to the lungs and vice versa

  
• Systemic circulation


  Conveys blood from the heart to other tissues of the body and vice versa

  
• The heart is a muscular pump that maintains unidirectional flow of blood and consists of four chambers. It has valves that prevent the backflow of blood.

  
• Heart


  • Consists of four chambers (right and left atria and ventricles)
  • Valves guard the exit of the chambers and prevent backflow of blood
  • Heart wall constituents: cardiac muscle, fibrous skeleton, impulse conducting system
• Layers of the heart wall


  • Endocardium
  • Myocardium
  • Epicardium

  
• Endocardium


  Innermost layer, consists of endothelium and subendothelial connective tissue, contains specialised cardiac muscle fibres
• Myocardium


  Middle layer, consists of cardiac muscle

  
• Epicardium


  Outermost layer, consists of mesothelium lining underlying connective tissue, contains blood vessels and nerves, and adipose tissue
• Purkinje fibres are wider in diameter compared to ordinary cardiac muscle fibres, have larger glycogen space, and myofibrils arranged at the periphery. They are the terminal part of the conducting system and function to receive, conduct and transmit contractile impulse to the myocardium.
• General histological pattern of blood vessels

  Tunica intima (innermost layer), tunica media (middle layer), tunica adventitia (outermost layer)

  
• Tunica intima


  • Innermost layer
  • Contains endothelium with basal lamina,
  • Contains a subendothelial layer with loose connective tissue and occasional smooth muscle cells,
  • Contains internal elastic membrane/laminae in arteries & arteriole (variable in veins)
• Tunica media


  • Middle layer
  • Contains circumferentially arranged layers of smooth muscle,
  • Contains variable amounts of elastin, reticular fibres and proteoglycans
  • Contains fenestrated external elastic membrane arranged in a circular layer
  • Thicker in arteries
• Tunica adventitia


  • Outermost layer
  • Primarily composed of longitudinally arranged collagenous tissue and elastic fibres,
  • May contain external elastic laminae,
  • Also contains vasa vasorum and nerves
  • Thicker in veins
• Contrast between arteries and veins

  ->Tunica intima always has IEL in arteries, variable in veins

  ->Arteries have thicker tunica media than tunica adventitia

  ->Veins have thicker tunica adventitia than tunica media
• Large (elastic) artery


  • Tunica intima has inconspicuous/indistinct internal elastic lamina,
  • Tunica media has multiple layers of smooth muscle and fenestrated elastic lamellae
  • Tunica adventitia is relatively thin compared to TM
  • Function: conveys blood from the heart to the systemic and pulmonary circulation
• Large vein


  • Tunica intima has prominent valves & indistinct boundary between TI & TM
  • Tunica media is relatively thin, contains circumferentially arranged layers of smooth muscle and collagen fibres, and fibroblasts
  • Tunica adventitia is the thickest layer, contains longitudinally arranged smooth muscle cells with collagen and elastic fibers
  • Function: Returns blood to the heart
• Medium (muscular) artery

  Medium artery has

  ->Prominent internal elastic lamina & sparse sub-endothelial layer in tunica intima,

  ->Thicker tunica media with multiple layers of smooth muscle and few elastic fibres

  ->Tunica adventitia with prominent EEL, vasa vasorum & nerves

  ->Function: distributes blood to all organs and maintains steady blood pressure and flow
• Small arteries

  • Tunica intima has endothelium and smaller subendothelial layer, tunica media may have up to 8 layers of smooth muscle, tunica adventitia is thin
• Arterioles

  • Tunica intima has endothelium, tunica media typically has 1-3 layers of smooth muscle, tunica adventitia is relatively large compared to other layers
• Muscular venule and post-capillary venule


  • Muscular venules have 1-2 layers of smooth muscle in tunica media, post-capillary venules are associated with pericytes
• Capillaries


  Consist of a layer of endothelial cells and their corresponding basal lamina, tunica media consists of pericytes, facilitate exchange of gases, metabolites and waste products
• Classification of capillaries

  • Continuous
  • Fenestrated
  • Discontinuous/sinusoidal
• Lymphatic vessels

  Convey fluids from tissues into the bloodstream, distribute lymphocytes, antibodies and other immune components, more permeable than blood capillaries, lined by endothelium and contain valves
• General pattern of the heart and blood vessels

  Inner -> Outer: Lumen, Epithelial lining, Muscular wall, Surrounding connective tissue
• Heart wall constituents - Cardiac muscle

  • Branched & cylindrical

  • Single, central nucleus per cell

  • Intercalated discs – join adjacent cells • Faint striations • Glycogen in juxtanuclear space
• Heart wall constituents - Impulse Conduction system

  Intrinsic regulation of the heart using SA node, AV node, AV bundle & Purkinje fibers – larger, more glycogen

  • Modified cardiac cells – 4x faster

  • Function: coordinates contractions of atria & ventricles
• Heart wall constituents - Fibrous skeleton

  • Consists of:

  ->4 fibrous rings surrounding valve orifices (fibrous rings = dense, irregular CT)

  ->2 fibrous trigones connecting the rings

  ->Membranous part of IV & IA septae

  • Composition = dense, irregular CT

  • Function: Attachment site for valves & myocardium & Electrical insulation
• Medium vein

  ->Tunica intima with endothelium, thin subendothelial layer & thin IEL->Tunica media is smaller than TA & thinner than of TM of media artery

  ->Thicker tunica adventitia than tunica media

  ->Function: carries blood to larger veins
• Classification of capillaries- continuous

  Continuous:

  • Continuous endothelium and basement membrane

  • Endothelial lining with occluding junctions.

  • May be surrounded by pericytes

  • Location: muscle, lungs and CNS
• Classification of capillaries - fenestrated

  Fenestrated:

  • Fenestrated endothelium and continuous basement membrane

  • Facilitates fluid and metabolite absorption

  • Location: endocrine glands, kidney, gallbladder, intestinal tract
• Classification of capillaries - discontinuous/sinusoidal

  Discontinuous/sinusoidal:

  • Discontinuous endothelium and basement membrane

  • Wider lumen and irregularly shaped

  • Location: liver, spleen and bone marrow