Chapter 21: The Cardiovascular System: Blood Vessels

1. Chapter 21: The Cardiovascular System: Blood Vessels

2. Vessel Structure - General • All vessels same basic structure • 3 wall layers (or tunics) • Tunica adventitia (externa) - elastic and laminar fibers • Tunica media • thickest layer • elastic fibers and smooth muscle fibers • Tunica interna (intima) • endothelium – non-stick layer • basement membrane • internal elastic lamina • Lumen - opening

3. Structure/function relationship changes as move through cardiovascular tree • Tunic thickness and composition variable throughout cardiovascular tree

4. Vessel Structure – Elastic Arteries • Elastic (conducting) arteries • Near heart • Thick walls • More elastic fiber, less smooth muscle • Lose elasticity with aging

5. Vessel Structure - Elastic Arteries • Aorta and elastic arteries • Can vasoconstrict or vasodilate • Large arteries expand, absorb pressure wave then release it with elastic recoil - Windkessel effect • Help to push blood along during diastole • With aging have less expansion and recoil

6. Vessel Structure – Muscular Arteries • Muscular (distributing) arteries • Deliver blood to organs • More smooth muscle • Less elastic fibers

7. Vessel Structure - Arterioles • Arterioles • Distribution of blood in organs • Composition varies depending on position - more muscle, less elasticity nearer heart • Regulate flow from arteries to capillaries • Flow = ΔP/R • vary resistance by changing vessels size • Site of blood pressure regulation

8. Vessel Structure - Capillaries • Microcirculation connects arteries and veins • Found in nearly every tissue in body • Higher the metabolic rate, more capillaries in tissue • Muscle many caps (>600/mm2) • Cartilage none

9. Vessel Structure - Capillaries • Allow exchange of nutrients and wastes between blood and tissue • Capillary structure - simple • Basal lamina - connective tissue • Endothelial cells • Structure/function

10. Flow Regulation • Regulation by vessels with smooth muscle • Metarterioles • connect arterioles to venules through capillary bed • allows flow through capillary bed w/out flow through caps

11. Flow Regulation • True capillaries • Pre-capillary sphincter • ring of smooth muscle • open/close to control flow • regulated by chemicals • Intermittent vasomotion – caps open for flow 5-10X min

12. Types of Capillaries • 3 types of capillaries • Continuous capillaries • continuous endothelial cells except for cleft between cells • tight junctions between endothelial cells prevent most things from leaving caps • most capillaries in body

13. Types of Capillaries • Fenestrated capillaries • fenestrations (slits) allow for filtration of small substances • glomerular capillaries in kidney

14. Types of Capillaries • Sinusoid capillaries • wider gaps between endothelial cells allowing RBC’s to exit the caps • found in liver

15. Vessel Structure - Veins • Venules • Collect blood from caps carry it to veins • Structure changes with position • Become more vessel-like (walls) as move from capillaries

16. Vessel Structure – Veins • Veins • Interna thicker than arteries • Media thinner, less muscle • Externa thick • Valves • Pressure low • High compliance - change volume easily with small change in pressure • Varicose veins

17. Vessel Structure - Histology • Very different morphology under light microscopy • Tunica media thickness differentiates artery from vein

18. Artery Vein Vein Artery

19. Vessel Structure/Function • At rest • 60% of blood located in veins and venules • Serve as reservoirs for blood, “storing” it until needed • Particularly veins of abdominal organs, skin • ANS regulates volume distribution • Vasoconstrict • Vasodilate • Open areas of circulation to be supplied with blood • veins at rest • caps during exercise • Can “shift” volumes to other areas as needed

20. Vessel Structure/Function  0.75 L/min Rest CO = 5 L/min

21. Vessel Structure/Function CO = 25 L/min Heavy Exercise  20 L/min  0.75 L/min Rest CO = 5 L/min

22. Physiology of Circulation • Flow = ΔP/R • or CO = MAP/R • MAP - mean arterial pressure • Higher pressure to lower pressure with resistance (R) factor • Blood pressure • Pressure of blood on vessel wall • Measurement of pressure of a volume in a space • Systole/diastole - 120/80 • BP falls progressively from aorta to O mm Hg at RA

23. Regulation of Blood Flow • Resistance - opposition to blood flow from blood and vessel wall friction • Factors that affect resistance (R) • Viscosity - V R • thickness of blood • dehydration, polycythemia • R proportional to vessel length • garden hose vs. straw • obesity • Vessel diameter • changes in diameter affect flow • vessel wall drag – blood cells dragging against the wall • laminar flow – layers of flow • R inverse proportional to radius4 • decrease in r by 1/2R 16X • only important in vessels that can change their size actively

24. Regulation of Pressure, Resistance • Systemic vascular resistance (Total Peripheral Resistance - TPR) • All vascular resistance offered by systemic vessels • Which vessels change size? • Resistance highest in arterioles • Largest pressure drop occurs in arterioles • Relationship of radius to resistance in arterioles important due to smooth muscle in walls

25. Systemic Blood Pressure • Arterial Blood Pressure • Pulsatile in arteries due to pumping of heart • Systolic/diastolic • Pulse pressure = systolic (minus) diastolic • - Windkessel effect on pulse pressure? • - Decreases pulse pressure • - What is the effect of hardening of the arteries on pulse pressure? • - Increases pulse pressure

26. Systemic Blood Pressure • Capillary Blood Pressure • Relatively low blood pressure • Low pressure good for caps because: • caps are fragile - hi pressure tears them up • caps are very permeable - hi pressure forces a lot of fluid out

27. Systemic Blood Pressure • Venous return • Volume of blood flowing back to heart from systemic veins • Depends on pressure difference (ΔP) from beginning of venules (16 mmHg) to heart (0 mmHg) • Any change in RA pressure changes venous return

28. Help for venous return • Skeletal muscle pump • muscles squeeze veins • force blood back to heart • valves prevent back flow • Respiratory pump • inhaling pulls air into lungs • helps to pull blood back into thorax

29. Velocity of Blood Flow • Velocity of blood flow - inversely related to total cross sectional area (CSA) of vessels • Aorta • Total CSA 3-5 cm2 • Velocity 40 cm/sec • Capillaries • Total CSA 4500-6000 cm2 • Velocity 0.1 cm/sec • Vena Cava • Total CSA 14 cm2 in vena cava • Velocity 5-20 cm/sec

30. Vessel Structure - Function • Capillary Function • Site of exchange between blood and tissues • Delivery of nutrients and removal of wastes • Slow flow allows time for exchange • Mechanisms of nutrient exchange • Diffusion - O2, CO2, glucose, AA's, hormones diffuse down [ ] gradients • If lipid soluble, can travel through cell • If water soluble, between cells

31. Capillary Fluid Exchange • Fluid movement • Fluid filtered and reabsorbed across capillary wall • Starling’s law of the capillaries • Forces driving the movement of fluid • Hydrostatic pressure capillary (HPc) • Hydrostatic pressure interstitial fluid (HPif) • Osmotic pressure capillary(OPc) • Osmotic pressure interstitial fluid (OPif) • Net filtration pressure (NFP) is a sum of all

32. Capillary Fluid Exchange • On average 85% of fluid filtered at arteriole end is reabsorbed at venular end

33. Maintaining Blood Pressure - Short Term Mechanisms - CNS • Neural Control - Cardiac centers in medulla • Vasomotor center • medullary area dedicated to control of blood vessels • sends sympathetic output to blood vessels • Vasoconstricts or vasodilates as needed • tone - normal amount of vasoconstriction or vasodilation • can vary tone which varies delivery of blood • receives input from different sources • baroreceptors • chemoreceptors

34. Maintaining Blood Pressure – Short term mechanisms – CNS reflexes • Baroreceptor initiated reflex • Located at carotid sinus and aortic arch • Monitor changes in blood pressure • Regulate activity of Sympathetic Nervous System (vascular tone)

35. Maintaining Blood Pressure – Short term mechanisms – CNS reflexes • Chemoreceptor initiated reflexes • Carotid bodies, aortic bodies • Monitors changes in chemicals (O2, CO2, [H+]) •  CO2,  H+, O2 (stresses) result in  sympathetic activity and  BP

36. Maintaining Blood Pressure – Short term mechanisms – CNS reflexes • Influence of Higher Brain Centers (areas above medulla) - Cortex and Hypothalamus • Not involved in minute to minute regulation • Influence vasomotor center depending on conditions • public speaking • temperature regulation

37. Maintaining Blood Pressure - Short Term Mechanisms - Hormones • Renin - Angiotensin - Aldosterone • Renin • enzyme from kidney • results in formation of Angiotensin II (AII) • AII • vasoconstrictor • stimulates ADH, thirst • stimulates aldosterone - Na+ reabsorption • Why/how would these things affect blood pressure?

38. Maintaining Blood Pressure - Short Term Mechanisms - Hormones • Adrenal medulla - Epi and Norepi •  CO (HR,  SV) • Constrict abdominal, cutaneous arterioles/venules • Dilate cardiac, skeletal muscle beds • Why/how would this affect blood pressure?

39. Maintaining Blood Pressure -Short Term Mechanisms - Hormones • Antidiuretic Hormone (ADH) • Osmoreceptors in hypothalamus • Retains fluid (inhibited by alcohol) • Vasoconstriction at high levels • Why/how would this affect blood pressure?

40. Maintaining Blood Pressure -Short Term Mechanisms - Hormones • ANP (atrial natriuretic peptide) • Released from atrial cells in response to BP • Vasodilator, Na+ and water loss, opposes Aldosterone • Why/how would this affect blood pressure?

41. Maintaining Blood Pressure - Long Term Regulation • Renal mechanism • Volume in a space • Regulate space in the short term – we just talked about it! • nervous control • hormones • Regulate volume in the long term • The kidneys! •  BP,  urine flow to  BP •  BP,  urine flow to  BP

42. Control of Blood Flow • Autoregulation (local control) - local automatic adjustment of blood flow to match tissue needs • Physical changes • Warming -  vasodilation • Cooling -  vasoconstriction • Chemical changes - metabolic products • Vasodilators • Vasoconstrictors • Myogenic control • smooth muscle controls resistance •  stretch  contraction,  stretch  relaxation

43. Blood Flow in Special Areas • Skeletal Muscle • Wide variability in amount of flow • Sympathetic regulation from brain in response to level of activity • α receptors - vasoconstrict • β receptors - vasodilate • Metabolic regulation in tissue • low O2 vasodilate to increase flow • hi O2 vasoconstrict to decrease flow • Brain • Very little variability in flow • Stores few nutrients so flow must be maintained! • Metabolic regulation

44. Blood Flow in Special Areas • Skin • Supplies nutrients, aids in temperature regulation, provides a blood reservoir • Metabolic and sympathetic regulation • Lungs • Low pressure (25/10), low resistance • Flow regulated by O2 availability in the lungs • hi O2 vasodilate to increase flow – opposite to muscle • low O2 vasoconstrict to decrease flow – opposite to muscle • Heart • Variable flow depending on activity • Metabolic and sympathetic regulation

45. Regulation of Blood Pressure CO = MAP/R MAP = CO x R