Vessel structure

• Cardiovascular system


  The scope of the system of endothelial tubes is to reach organs
• Arteries


  • Carry blood away from the heart
• Veins


  • Carry blood toward the heart
• Capillaries


  • Contact tissue cells and directly serve cellular needs
• Lymphatics


  • One-way system, carries excess interstitial fluids toward the heart
• Components of lymphatic system

  • Lymph nodes
• Artery and vein structure


  Three distinct layers or tunics
• Tunica intima or interna

  • Presents an endothelium
  • Lies on a basement layer surrounded by a lamina propria (loose connective tissue contractile cells: myointimal cells with phagocytic capacities that can synthesize collagen and elastin)
  • Surrounded by an internal elastic lamina (sheet-like layer of fenestrated elastic material)
• Tunica media


  • Made of smooth muscle fibres, elastic fibres that can be organized in sheet and collagen fibres that support framework, and limit the distensibility of the vessel (are more prominent in veins)
  • The deepest layer is the external elastic lamina
• Tunica adventitia or externa


  • Mostly made of connective tissues and in larger vessels it contains vasa vasorum and nerves
• Arteries classified by size

  • Elastic arteries
  • Muscular arteries
  • Arterioles
• Elastic arteries


  • Large size, conduct blood to organs, close to heart, act as pressure reservoirs during systole and recoil during diastole, able to push blood during diastole, tunica intima contains elastic propria and myointimal cells, tunica media has elastic tissue organized in lamellae with fenestration, less smooth muscle cells
• Muscular arteries


  • Medium size, deliver blood according to functional need, more active in vasoconstriction, more smooth muscle cells, tunica intima subendothelial layer is thinner and wavy, tunica media smooth muscle cells organized in spiral fashion to maintain blood pressure, controlled by autonomic nervous system
• Arterioles


  • Small size, control amount of blood to organs through vasoconstriction and vasodilation, smooth muscle fibres circularly arranged in 1-2 layers, have tone (partial contraction) and are major determinants of systemic blood pressure, tunica interna is thin without subendothelial, tunica media is purely muscular, tunica externa is thinner than media and fibro-elastic
• Smaller arterioles


  Give rise to capillaries and venules, compose the microcirculation
• Metarterioles


  Narrow vessels arising from arterioles that give rise to capillaries, important regulators of blood flow, constituted by one layer of smooth muscle cells
• Arterial anastomosis


  Arteries joined to one another so one can supply the territory of the other
• Microvascular bed


  • Site of microcirculation, tasked with providing nutrients and removing metabolic byproducts, maximizes distribution of nutrients and oxygen by touching nearly every cell, minimizes space occupied to allow room for other cell types, achieved by complex but highly organized branching pattern
• Capillary bed


  • Originates from terminal arteriole or metarteriole, composed of larger capillaries with continuous blood flow and smaller capillaries with intermittent blood flow regulated by precapillary sphincters
• Postcapillary venules


  Primary sites of leukocyte extravasation during inflammation
• Arteriovenous shunt


  Bypasses the microvascular bed, connecting arterioles directly to venules, important for thermoregulation
• Capillaries


  The smallest blood vessels, made of a single layer of thin endothelial cells, that allows only a single RBC to pass at a time
• Capillaries


  • There is no tunica media and adventitia
  • Pericytes cells on the outer surface stabilize their walls (undifferentiated connective cells with contractile properties)
  • Capillaries are not innervated as there are not muscle cells
  • The blood flow is intermittent and dependent on functional demands
  • Only 1/4 of the capillaries are open at a given time
• Continuous capillaries


  The least permeable, lack of fenestrations, endothelial cells and basal lamina provide an uninterrupted lining, transport takes place mostly through caveolae and pinocytotic vesicles, adjacent cells are connected with tight junctions that allow the passage of small molecules, found in the brain, lungs, muscles
• Fenestrated capillaries


  Typical of releasing/absorbing organs where active capillary absorption or filtration occurs, found in the intestine, kidney, endocrine glands
• Discontinuous or sinusoids capillaries

  Very permeable, have a large lumen with large fenestrae (30-40 μm diam) and large intercellular clefts, don't have diaphragms and have a discontinuous basal lamina, the endothelium can be discontinuous as well, found where there is an intimate relationship between blood and parenchyma, as in the liver, bone marrow, lymphoid tissue and in some endocrine organs