Blood Vessels

1. Blood Vessels

2. 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

3. 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

4. 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

5. 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

6. Distribution during exercise

7. 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

8. 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

9. 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

10. 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

11. Small veins formed from merging of several capillaries Venules merge to form veins Venules

12. 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.

13. 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)

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. Control of blood pressure and flow • Negative feedback mechanisms control: • Heart rate • Stroke volume • Systemic vascular resistance • Blood volume

21. Neural regulation of blood pressure and flow • Cardiovascular centre (medulla oblongata) • controls • heart rate • contractility (SV) • blood vessel diameter (resistance)

22. 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

23. Higher brain regions • Higher brain regions synapse with CV centre to regulate cardiovascular responses

24. Proprioceptors • Proprioceptors synapse with CV centre • Allows for rapid adjustments in HR and BP

25. 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

26. 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

27. 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

28. Autoregulation of blood flow • Autoregulation • Ability of a tissue to automatically adjust its blood flow to match its metabolic needs

29. 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

30. Measuring blood pressure • Systolic and diastolic BP identified using auscultatory method by Korotkoff sounds

31. 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)