Understanding Blood Vessels and Circulation

1. Chapter 19. Vessels and Circulation

2. Quiz Thurs • On Chap 18 Heart • Exam next Thurs on 18, 19 (more about this next time)

3. Overview • Classes of blood vessels and their structures • Cardiovascular physiology • Circulatory pressures • Capillary exchange • Cardiovascular regulation • Vascular diseases • Vessels to know (repeat from lab)

4. Blood Vessels • Closed system of tubes that starts and ends at the heart Arteries: large vessels that carry blood away from heart Arterioles: smallest branches of arteries Capillaries: smallest blood vessels with a small diameter and thin walls; location of exchange between blood and interstitial fluid (exchange vessels) Venules: smaller branches of veins collect blood from capillaries Veins: Larger vessels that return blood to heart

5. Structure of Vessel Walls Figure 21-1

6. Generalized Structure of Blood Vessels • Arteries and veins are composed of three tunics – tunica interna, tunica media, and tunica externa • Lumen – central blood-containing space surrounded by tunics • Capillaries are composed of endothelium with sparse basal lamina

7. Artery and Vein Walls – 3 layers • Tunica Interna (Intima): innermost layer • endothelial lining of all vessels • In vessels larger than 1 mm, a subendothelial connective tissue basement membrane is present • In arteriesonly theinternal elastic membrane is a layer of elastic fibers in outer margin • Tunica Media: middle layer • concentric sheets of smooth muscle in loose connective tissue • regulated by sympathetic nervous system • Controls vasoconstriction/vasodilation of vessels • External elastic membrane (arteries only) separates tunica media from tunica externa • Thickest layer in a small artery

8. Artery and Vein Walls – 3 Layers • Tunica Externa (adventitia): outer layer • Connective tissue sheath with collagen fibers • Anchors vessel to adjacent tissues • Thick in veins • Larger vessels contain vasa vasorum • In arteries: • collagen fibers • elastic fibers • In veins: • elastic fibers • smooth muscle cells

9. Vasa Vasorum • Small arteries and veins found in the walls of large arteries and veins • These are the blood supply for the large vessels • Supply cells of tunica media and tunica externa with oxygen and nutrients • Why don’t you need these in capillaries?

10. Arteries vs. Veins at a glance • Arteries and veins run side-by-side • Arteries have thicker walls and higher blood pressures • Collapsed artery has small, round lumen • Vein has a large, flat lumen • Vein lining contracts, artery lining does not • Artery lining folds • Arteries more elastic • Veins have valves • Arteries thick t. media, veins thick t.externa

11. Vessel Composition

12. Arteries • Elasticity allows arteries to absorb pressure waves that come with each heartbeat • Contractility: arteries change diameter, controlled by sympathetic division of ANS • Vasoconstriction: contraction of arterial smooth muscle by the ANS, shrinking lumen • Vasodilation: The relaxation of arterial smooth muscle, enlarging lumen

13. Vasoconstriction and Vasodilation • Active processes that affect: • afterload on heart (how?) • peripheral blood pressure (how?) • capillary blood flow Note: elasticity of the arteries also allows them to expand and contract passively in response to changes in blood pressure

14. Structure of Blood Vessels Figure 21-2

15. Vascular Components

16. Artery Characteristics • From heart to capillaries, arteries change characteristics (along a continuum): • Elastic arteries (conducting arteries) • Large vessels (e.g. and aorta) dmax = 2.5cm, lumen allow low-resistance conduction of blood • Contain elastin in all three tunics • Withstand and even out large blood pressure fluctuations • Serve as pressure reservoirs • Tunica media has many elastic fibers and few muscle cells • Muscular arteries (distribution arteries) • Medium-sized davg =0.4cm • Account for most arteries • Thick tunica media has many muscle cells • Active in vasoconstiction • Arterioles (resistance vessels) • Smallest arteries (d ≤ 30micons) • Have little or no tunica externa, thin or incomplete media • Contol blood flow into capillaries by change diameter in response to ANS, local conditions

17. Artery Diameter and Resistance • Small muscular arteries and arterioles changes diameter with sympathetic or endocrine stimulation (vasomotor response) • Decreasing diameter increases resistance, the force opposing blood flow • Arterioles also calledresistance vessels

18. Aneurysm • Bulge in an arterial wall • Caused by weak spot in elastic fibers • Pressure may rupture vessel

19. Capillaries • Are smallest vessels with thin walls (davg = 8 microns) • We have about 10 billion or 25,000 miles • Have only a tunica interna, one cell thick • Pericytes on the outer surface stabilize their walls • Microscopic capillary networks permeate all active tissues • Blood flow through caps is slow • Exchange occurs here: materials diffuse between blood and interstitial fluid • All living cells no more than 125um from a cap

20. 3 Types of Capillaries • Continuous • Abundant in skin and muscles • Have complete endothelial lining, connected by tight junctions • Small clefts permit diffusion of water, small solutes, and lipid soluble material (but NOT blood cells or plasma proteins) • thymus and brain have specialized continuous capillaries (barriers) • Fenestrated • Have pores in endothelial lining (not gaps between cells) • Permit more rapid exchange of water and larger solutes between plasma and interstitial fluid • Found in: choroid plexus, kidneys, intestinal tract, and? • Sinusoids • Modified fenestrated capillaries • Very leaky, with large gaps between adjacent endothelial cells that allow large molecules (plasma proteins) and cells through • Found only in: Liver, spleen, bone marrow, other lymphoid tissues, used for phagocyte monitoring, plasma protein entry from liver

21. Capillary Structure Figure 21-4

22. Capillary Networks • One arteriole gives rise to several capillary beds • Each Capillary bed connects 1 arteriole to 1 venule • Each capillary entrance guarded by precapillary sphincter

23. Capillary Bed – key parts • Capillaries • 10 to 100 capillaries per capillary bed, they branch off the metarteriole and return to the thoroughfare channel at the distal end of the bed • Thoroughfare Channels • Are direct capillary connections between arterioles and venules • Controlled by smooth muscle segments called metarterioles found at channel entrance • Collaterals • Multiple arteries contribute to one capillary bed and allow circulation if one artery is blocked • Arterial anastomosis =fusion of two collateral arteries • Arteriovenous Anastomoses • direct connections between arterioles and venules allow blood to bypass the capillary bed

24. Capillary Networks Figure 21-5

25. Precapillary Sphincters • Guards entrance to each capillary • Open and close, causing capillary blood to flow in pulses • Vasomotion: • Contraction and relaxation cycle of capillary sphincters • Causes blood flow in capillary beds to constantly change routes • Contract/relax on the order of 10 times/min • Causes capillary flow to pulse • Controlled by autoregulation (see later)

26. Closed precapillary sphincters

27. Capillary Volume • At rest, blood is flowing in about 25% of your capillaries • When you begin to exercise, vessels must redistribute blood within the capillary network (you can’t just open up more capillaries) • If all your capillaries open at once: shock

28. Veins • Collect blood from capillaries in all tissues and organs and return it to heart • Larger in diameter than arteries, but have thinner walls and much lower blood pressures • Tunica externa is usually thickest layer • Capacitance vessels (blood reservoirs) that contain 65% of the blood supply • Classified on the basis of size

29. Vein Characteristics • Venules • Collect blood from capillaries • Average diameter of 20um, resemble capillaries in structure • Allow fluids and WBCs to pass from the bloodstream to tissues • Medium Sized Veins • thin tunica media and few smooth muscle cells • Thickest part is tunica externa with longitudinal bundles of elastic fibers and collagen • Size ranges from 2-9mm • LargeVeins (e.g. sup/inf vena cava) • have all 3 tunica layers • thick tunica externa with thin tunica media

30. Vein Valves • Valves are folds of tunica interna • Resemble semilunar heart valves • Prevent blood from flowing backward • Compression from muscular contractions (even rotation in isometric contractions) pushes blood toward heart • Not so important when laying down • Compartmentalize blood flow: blood return from below heart is like a boat traversing several locks to get up a hill

31. Valves in the Venous System Figure 21-6

32. Blood Distribution Figure 21-7

33. Blood Distribution • Heart, arteries, and capillaries: • 30–35% of blood volume • Venous system: • 60–65% • Fully 1/3 of venous blood is in the large venous networks of the liver, bone marrow, and skin (some of this is part of the venous reserve)

34. Capacitance • The ability to stretch or the relationship between blood volume and blood pressure • Veins (capacitance vessels) stretch more than arteries (8x as much as arteries) • Lower resistance = higher capacitance = expands easily at low pressures • Means veins can accommodate large changes in blood volume

35. Veins Response to Blood Loss • Vasomotor centers (medulla) stimulate sympathetic nerves • Venoconstriction = smooth muscles in systemic medium sized veins constrict • Affects blood pressure in venous system but major effect is to cause veins in liver, skin and lungs to redistribute venous reserve back to arterial system (about 20% of total blood)

36. Cardiovascular Physiology Figure 21-8

37. Cardiovascular Physiology • Cardiac output = blood flow • Determined bypressure and resistance in the cardiovascular system • Force is proportional to pressure gradient • Pressure (P)= force the heart generates to overcome resistance • Absolute pressure is less important than pressure gradient • Pressure Gradient (P) = the difference between pressure at the heart and pressure at peripheral capillary beds resistance

38. Blood Flow, Pressure, and Resistance • Blood flow (F) is directly proportional to the difference in blood pressure (P) between two points in the circulation • If P increases, blood flow speeds up; if P decreases, blood flow declines • Blood flow is inversely proportional to resistance (R) • If R increases, blood flow decreases • R is more important than P in influencing local blood pressure • The differences in pressure within the vascular system provide the driving force that keeps blood moving, alwaysfrom higher to lower pressure area

39. Measuring Pressure • Blood pressure(BP): • arterial pressure (mm Hg) • Pressure required to move blood • Capillary hydrostatic pressure (CHP): • pressure within the capillary beds • pressure where diffusion and osmosis occur • Venous pressure: • pressure in the venous system • Circulatory Pressure: ∆P across the systemic circuit (about 100 mm Hg)

40. Resistance • Resistance – opposition to flow • Measure of the amount of friction blood encounters • Generally encountered in the systemic circulation • Referred to as peripheral resistance (PR) • Circulatory pressure must overcome total peripheral resistance of entire cardiovascular system which comes from 3 sources: • Vascular resistance (length and diameter) • Blood viscosity • Turbulence

41. Peripheral Resistance: vascular resistance • Vascular resistance (= major factor): R of blood vessels due to friction between blood and vessel walls depends on vessel length and vessel diameter • Adult vessel length is constant • Vessel diameter varies by vasodilation and vasoconstriction • R increases exponentially (4th power!) as vessel diameter decreases (double the radius, decrease resistance by 16x)

42. Peripheral Resistance: viscosity and Turbulence • Viscosity also increases resistance • Normal whole blood viscosity is about 4-5 times that of water, changes with hematocrit • Turbulence: swirling action that disturbs smooth flow of liquid • Occurs in heart chambers and great vessels • Atherosclerotic plaques cause abnormal turbulence

43. Overview of Circulatory Pressures: Arteries • Largest pressure gradient is between aorta and proximal end of capillary beds (100  35mmHg so gradient = 65) • This part of the system also has the highest resistance. • Both the pressure (CO) and the resistance (vasomotor tone) can be regulated, determining the rate of flow in the capillaries

44. Overview of Circulatory Pressures: Capillaries • Blood at the proximal (arterial) side of cap beds has pressure of 35mmHg • At distal end, where blood enters venules, pressure is 18mmHg • Low capillary pressure is desirable because high BP would rupture fragile, thin-walled capillaries • Low BP is sufficient to force filtrate out into interstitial space and distribute nutrients, gases, and hormones between blood and tissues

45. Overview of Circulatory Pressures:Veins • Blood entering venules is 18mmHg, enters right atrium at 0 - 2mmHg so gradient = 18mmHg (pretty small) • However, veins provide very low resistance and so they don’t require great pressures for blood to move • As blood gets closer to heart, veins get larger and larger, decreasing resistance. This doesn’t increase the pressure but it does increase the velocity of blood flow

46. Systemic Blood Pressure Figure 19.5

47. Vessel Diameter Cross-Sectional Area Average Systemic Blood Pressure Systemic Blood Velocity

48. What the graphs show • Arteries  caps (divergence) • Caps veins (convergence) • Blood pressure and velocity are proportional to the TOTAL cross sectional area of all vessels • As total cross-sectional area increases, avg. blood pressures and velocities decline • Velocity continues to decline until the veins, where cross sectional areas increase (reducing friction) • Velocity is slowest at capillaries and flow allows adequate time for exchange between blood and tissues

49. Pressures in the Systemic Circuit • Systolic pressure: • peak arterial pressure during ventricular systole • Diastolic pressure: • minimum arterial pressure during diastole • Pulse pressure: • difference between systolic pressure and diastolic pressure • Mean arterial pressure(MAP): • MAP = diastolic pressure + 1/3 pulse pressure

50. Pressure and Distance • MAP and pulse pressure decrease with distance from heart • Blood pressure decreases with friction • Pulse pressure decreases due to elastic rebound • Near the heart, BP pulses • By the arterioles, pulsing is gone (if you cut a vein it will bleed continuously; an artery spurts)