Blood vessels
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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


  • 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


  • 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

    • 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


  • 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


  • 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


Small veins formed from merging of several capillaries

Venules merge to form 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.


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


  • 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

    • 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 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


    • 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)