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Blood Vessels. Blood vessel structure. Five types of blood vessels: Arteries Arterioles Capillaries Venules Veins Larger blood vessels served by own blood vessels located within their walls Vasa vasorum. Vessel structure. Arterial walls have 3 tunics tunica int ern a Endothelium

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blood vessel structure
Blood vessel structure
  • Five types of blood vessels:
    • Arteries
    • Arterioles
    • Capillaries
    • Venules
    • Veins
  • Larger blood vessels served by own blood vessels located within their walls
    • Vasa vasorum
vessel structure
Vessel structure
  • Arterial walls have 3 tunics
    • tunica interna
      • Endothelium
      • Basement membrane
      • Internal elastic lamina
    • tunica media
      • Thickest layer
      • Elastic fibres
      • Smooth muscle
      • External elastic lamina (only in muscular arteries)
    • tunica externa
      • Elastic and collagen fibres
arteries
Arteries
  • Elastic (conducting) arteries
    • Largest diameter arteries
    • Tunica media contains high proportion of elastic fibres
      • Store elastic energy
        • Helps keep blood moving during diastole
    • “conduct” blood from heart to smaller muscular arteries
arteries1
Arteries
  • Muscular (distributing) arteries
    • Medium sized arteries
      • Tunica media contains
        • high proportion of smooth muscle
          • Very active in vasoconstriction and vasodilation
    • distribute blood to various parts of body
arterioles
Arterioles
  • Arterioles
    • small, almost microscopic arteries
      • deliver blood to capillaries
    • key regulators of systemic vascular resistance
  • Metarterioles
    • Emerge from arterioles
      • Supply capillary beds
      • Distal end has no smooth muscle
        • thoroughfare channel
capillaries
Capillaries
  • Microscopic vessels (microcirculation)
    • Distribution varies with metabolic activity of tissue
    • Prime function is exchange of nutrients and wastes via interstitial fluid
      • Walls consist of only endothelium and basement membrane
capillaries1
Capillaries
  • True capillaries
    • Emerge from arterioles or metarterioles
    • flow regulated by precapillary sphincter
    • Flow intermittent (vasomotion)
      • Caused by alternating contraction/relaxation of metarterioles and pre-capillary sphincters
    • RBC move in single file
capillary exchange
Capillary exchange
  • Three different types of capillaries
    • Continuous capillaries
      • uninterrupted lining
    • Fenestrated capillaries
      • many fenestrations/pores
    • Sinusoidal capillaries
      • large fenestrations and intercellular clefts
      • incomplete basement membrane
veins
Veins
  • Veins:
    • Composed of essentially same 3 tunics as arteries
      • Tunica interna thinner
      • Tunica media thinner
        • Less smooth muscle and elastic fibres
      • Tunica externa
        • Thickest layer - collagen and elastic fibres
        • Lack elastic lamina of arteries
    • Many contain valves to prevent backflow of blood.
capillary exchange1
Capillary exchange
  • Substances enter and leave capillaries by three methods:
    • diffusion (most important)
    • transcytosis (vesicular transport)
    • bulk flow (filtration and absorption)
      • Important for regulation of relative volumes of blood and interstitial fluid
        • Driven by balance between hydrostatic and osmotic pressures (Net filtration pressure)
          • Volume of fluid and and solute reabsorbed normally almost same as volume filtered (Starling’s Law of the Capillaries)
clinical note edema
Moderate Edema: Fluid accumulation in leg resulting in loss of contours of malleoli. Extensor tendons offoot also no longer visualized.

From: http://medicine.ucsd.edu/clinicalimg/extremities-edema.html

Clinical Note - Edema
  • Abnormal increase in interstitial fluid volume
    • Caused by imbalance between filtration and reabsorption
      • Result of either:
        • Excess filtration
          • Increased capillary blood pressure
          • Increased capillary permeability
        • Inadequate reabsorption
          • Decreased concentration of plasma proteins
hemodynamics
Hemodynamics
  • Blood flow
    • volume of blood that flows through a tissue per unit time
      • determined by blood pressure and resistance
        • Blood Flow = pressure gradient / resistance
blood pressure
Blood pressure
  • Blood pressure (BP)
    • pressure exerted on the walls of a blood vessel.
      • Reduces as move further away from heart
    • Difference between systolic and diastolic pressure is pulse pressure
    • MABP = DBP + 1/3PP
resistance
Resistance
  • Resistance
    • Opposition to blood flow due to friction between blood and vessel wall
      • Depends on:
        • Size of lumen
          • Resistance inversely proportional to 4th power of diameter of lumen (R  1/d4)
          • If diameter is halved, what happens to R?
        • Blood viscosity
          • Depends mostly on ratio of RBC to plasma
        • Total vessel length
venous return
Venous return
  • Venous return:
    • volume of blood returning to heart from systemic veins
    • maintained by:
      • pressure gradient established by heart
      • muscle pump
      • respiratory pump
      • valves
velocity of blood flow
Velocity of blood flow
  • Velocity of blood flow inversely related to cross-sectional area
    • Velocity
      • decreases as blood flows from aorta to capillaries
      • increases as blood flows from capillaries to heart
    • Velocity slowest in capillaries
      • Allows increased time for exchange
control of blood pressure and flow
Control of blood pressure and flow
  • Negative feedback mechanisms control:
    • Heart rate
    • Stroke volume
    • Systemic vascular resistance
    • Blood volume
neural regulation of blood pressure and flow
Neural regulation of blood pressure and flow
  • Cardiovascular centre (medulla oblongata)
    • controls
      • heart rate
      • contractility (SV)
      • blood vessel diameter (resistance)
cardiovascular centre
Cardiovascular centre
  • Cardiovascular centre receives input from:
      • Higher brain regions
      • Proprioceptors
      • Baroreceptors
      • Chemoreceptors
  • CV centre sends outputs via:
    • Sympathetic impulses
      • cardioaccelerator nerves
        • increase heart rate
        • increase contractility via regulation of Ca2+ channels
      • vasomotor nerves constrict blood vessel walls
    • Parasympathetic impulses
      • vagus nerves decrease heart rate
higher brain regions
Higher brain regions
  • Higher brain regions synapse with CV centre to regulate cardiovascular responses
proprioceptors
Proprioceptors
  • Proprioceptors synapse with CV centre
    • Allows for rapid adjustments in HR and BP
baroreceptor reflex
Baroreceptor reflex
  • Main baroreceptors located in carotid sinus and aorta
  • When BP falls baroreceptors stretched less
      • Send impulses to CV centre more slowly
        • Reduces parasympathetic stimulation of heart
        • Increases sympathetic stimulation of
          • heart
          • blood vessels
          • adrenal medulla
      • Increases blood pressure
chemoreceptor reflex
Chemoreceptor reflex
  • Chemoreceptors in carotid bodies and aortic bodies monitor blood levels of O2, CO2, and H+
      • initiate vasoconstriction when
        • Levels of O2 or pH decrease
        • Levels of CO2 increase
          • Increases BP and venous return
hormonal regulation of blood pressure
Hormonal regulation of blood pressure
  • Renin-Angiotensin system
    • Reduced blood volume or renal blood flow
      • Kidneys secrete renin
        • Leads to production of angiotensin II
          • Causes vasoconstriction (increases BP)
          • Stimulates secretion of aldosterone by adrenal cortex which increases sodium and water retention by kidney (increases blood volume)
  • Epinephrine and norepinephrine
    • Sympathetic stimulation of adrenal medulla by CV centre increases secretion of these hormones
      • Increase HR and contractility
      • Promote vasoconstriction (except in cardiac and skeletal muscle)
  • Antidiuretic hormone
    • Released by posterior pituitary gland in response to dehydration and/or reduced blood volume
      • Promotes vasoconstriction
      • Promotes water retention by kidneys
  • Atrial Natriuretic Peptide
    • Released by cells of right atrium in response to increased venous return
      • Lowers BP by promoting vasodilatation and renal excretion of sodium and water
autoregulation of blood flow
Autoregulation of blood flow
  • Autoregulation
    • Ability of a tissue to automatically adjust its blood flow to match its metabolic needs
autoregulation of blood flow1
Autoregulation of blood flow
  • Two types of stimuli cause autoregulatory changes in blood flow:
    • 1) Physical changes
      • Warming and cooling promote vasodilation and constriction
      • Myogenic response
        • stretch promotes increased smooth muscle tone to maintain relatively constant perfusion in face of fluctuations in pressure
    • 2) Chemical mediators
      • Vasodilators
        • Metabolically active cells release K+, H+, adenosine, La-
        • Endothelial cells release nitric oxide
        • Inflammation causes leukocytes to release kinins and histamine
      • Vasoconstrictors
        • Eicosanoids (local hormones)
        • - eg Thromboxane A2 - also activates platelets
        • Serotonin (from platelets)
        • Endothelins (from endothelial cells)
        • Superoxide radicals
      • Oxygen
        • Response to low O2 differs in systemic and pulmonary circulations
          • systemic blood vessels dilate
          • pulmonary blood vessels constrict
measuring blood pressure
Measuring blood pressure
  • Systolic and diastolic BP identified using auscultatory method by Korotkoff sounds
shock
Shock
  • Failure of CV system to deliver enough blood to meet cellular needs
    • 4 types:
      • Hypovolemic (low-volume)
        • Reduced venous return = reduced cardiac output
          • Compensatory mechanisms can maintain adequate blood flow and BP despite acute blood loss of up to 10% of total volume
      • Cardiogenic
        • Heart fails to pump adequately
      • Vascular (reduced systemic vascular resistance)
        • Anaphylactic shock – allergic reaction causes release of vasodilators
        • Neurogenic shock – malfunction of CV centre (ie head trauma)
        • Septic shock – bacterial toxins promote vasodilation
      • Obstructive
        • Blockage of blood vessels (ie pulmonary embolism)
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