Blood Vessels

1. Blood Vessels • Blood is carried in a closed system of vessels that begins and ends at the heart • In adult, blood vessels stretch 60,000 miles! • The three major types of vessels are arteries, capillaries, and veins • Arteries carry blood away from the heart (O2 rich, except pulmonary) • They branch, diverge, fork • Veins carry blood toward the heart (O2 poor, except pulmonary) • They join, merge, converge • Capillaries contact tissue cells and directly serve cellular needs

2. 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

3. Generalized Structure of Blood Vessels Figure 19.1b

4. Tunics • Tunica interna (tunica intima) • Endothelial layer that lines the lumen of all vessels • Continuation w/ the endocardial lining of the heart • Flat cells, slick surface • In vessels larger than 1 mm, a subendothelial connective tissue basement membrane is present • Tunica media • Circularly arranged smooth muscle and elastic fiber layer, regulated by sympathetic nervous system • Controls vasoconstriction/vasodilation of vessels

5. Tunics • Tunica externa (tunica adventitia) • Collagen fibers that protect and reinforce vessels • Contains nerve fibers, lymphatic vessels, & elastin fibers • Larger vessels contain vasa vasorum that nourish more external tissues of the blood vessel wall

6. Blood Vessel Anatomy Table 19.1

7. Elastic (Conducting) Arteries • Thick-walled arteries near the heart; the aorta and its major branches • Large lumen allow low-resistance conduction of blood • Contain elastin in all three tunics • Inactive in vasoconstriction • Serve as pressure reservoirs expanding and recoiling as blood is ejected from the heart

8. Muscular (Distributing) Arteries and Arterioles • Muscular arteries – distal to elastic arteries; deliver blood to body organs • Account for most of the named structures in lab • Have thickest tunica media (proportionally) of all the vessels • Active in vasoconstriction • Arterioles – smallest arteries; lead to capillary beds • Control flow into capillary beds via vasodilation and constriction

9. Capillaries • Capillaries are the smallest blood vessels (microscopic) • Walls consisting of a thin tunica interna, one cell thick • Allow only a single RBC to pass at a time • Pericytes on the outer surface stabilize their walls • Tissues have a rich capillary supply • Tendons, ligaments are poorly vascularized • Cartilage, cornea, lens, and epithelia lack capillaries

10. Vascular Components Figure 19.2a, b

11. Types of Capillaries • There are three structural types of capillaries: • Continuous • Fenestrated • Sinusoids

12. Continuous Capillaries • Continuous capillaries are abundant in the skin and muscles • Endothelial cells provide an uninterrupted lining • Adjacent cells are connected with tight junctions • Intercellular clefts allow the passage of fluids & small solutes • Brain capillaries do not have clefts (blood-brain barrier)

13. Fenestrated Capillaries • Found wherever active capillary absorption or filtrate formation occurs (e.g., small intestines, endocrine glands, and kidneys) • Characterized by: • An endothelium riddled with pores (fenestrations) • Greater permeability than other capillaries

14. Sinusoids • Highly modified, leaky, fenestrated capillaries with large lumens • Found in the liver, bone marrow, lymphoid tissue, and in some endocrine organs • Allow large molecules (proteins and blood cells) to pass between the blood and surrounding tissues • Blood flows sluggishly (e.g. in spleen) allowing for removal of unwanted debris and immunogens

15. Capillary Beds • A microcirculation moving from arterioles to venules • Consist of two types of vessels: • Vascular shunts – metarteriole–thoroughfare channel connecting an arteriole directly with a postcapillary venule • True capillaries – the actual exchange vessels • Terminal arteriole feeds metarteriole which is continuous w/ the thoroughfare channel which joins the postcapillary venule

16. Blood Flow Through Capillary Beds • True capillaries: 10-100/bed • Branch from, then return to, the capillary bed • Precapillary sphincter • Cuff of smooth muscle that surrounds each true capillary at the metarteriole and acts as a valve to regulate flow • Blood flow is regulated by vasomotor nerves and local chemical conditions • True bed is open after meals, closed between meals • True bed is rerouted to skeletal muscles during physical exercise

17. Capillary Beds Figure 19.4a

18. Capillary Beds Figure 19.4b

19. Venous System: Venules • Venules are formed when capillary beds unite • Postcapillary venules – smallest venules, composed of endothelium and a few pericytes • Allow fluids and WBCs to pass from the bloodstream to tissues

20. Venous System: Veins • Veins: • Formed when venules converge • Composed of three tunics, with a thin tunica media and a thick tunica externa consisting of collagen fibers and elastic networks • Accommodate a large blood volume • Capacitance vessels (blood reservoirs) that contain 65% of the blood supply

21. Venous System: Veins • Veins have much lower blood pressure and thinner walls than arteries • To return blood to the heart, veins have special adaptations • Large-diameter lumens, which offer little resistance to flow • Valves (resembling semilunar heart valves), which prevent backflow of blood • Venous sinuses – specialized, flattened veins with extremely thin walls (e.g., coronary sinus of the heart and dural sinuses of the brain)

22. Vascular Anastomoses • Merging blood vessels, more common in veins than arteries • Arterial anastomoses provide alternate pathways (collateral channels) for blood to reach a given body region • If one branch is blocked, the collateral channel can supply the area with adequate blood supply • E.g. occur around joints, abdominal organs, brain, & heart • E.g. poorly anastomized organs are retina, kidney, spleen • Thoroughfare channels are examples of arteriovenous anastomoses

23. Physiology of Circulation: Blood Flow • Actual volume of blood flowing in a given period: • Is measured in ml per min. • Is equivalent to cardiac output (CO), considering the entire vascular system • Is relatively constant when at rest • Varies widely through individual organs

24. Blood Pressure (BP) • Force / unit area exerted on the wall of a blood vessel by its contained blood • Expressed in millimeters of mercury (mm Hg) • Measured in reference to systemic arterial BP in large arteries near the heart • The differences in BP within the vascular system provide the driving force that keeps blood moving from higher to lower pressure areas

25. Resistance • Resistance – opposition to flow (friction) • Measure of the amount of friction blood encounters • Generally encountered in the systemic circulation away from the heart • Referred to as peripheral resistance (PR) • The three sources of resistance are: • Blood viscosity • Total blood vessel length • Blood vessel diameter

26. Resistance Factors: Viscosity and Vessel Length • Resistance factors that remain relatively constant are: • Blood viscosity: • Internal resistance to flow that exist in all fluids • Increased viscosity increases resistance (molecules ability to slide past one another) • Blood vessel length: • The longer the vessel, the greater the resistance • E.g. one extra pound of fat = 1 mile of small vessels required to service it = more resistance

27. Resistance Factors: Blood Vessel Diameter • Changes in vessel diameter are frequent and significantly alter peripheral resistance • The smaller the tube the greater the resistance • Resistance varies inversely with the fourth power of vessel radius • For example, if the radius is doubled, the resistance is 1/16 as much Fluid flows slowly Fluid flows freely

28. Resistance Factors: Blood Vessel Diameter • Small-diameter arterioles are the major determinants of peripheral resistance • Fatty plaques from atherosclerosis: • Cause turbulent blood flow • Dramatically increase resistance due to turbulence

29. Blood Flow, Blood 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 F = P/R

30. Systemic Blood Pressure • Any fluid driven by a pump thru a circuit of closed channels operates under pressure • The fluid closest to the pump is under the greatest pressure • The heart generates blood flow. • Pressure results when flow is opposed by resistance…eg. Systemic Blood Pressure

31. Systemic Blood Pressure • Systemic pressure: • Is highest in the aorta • Declines throughout the length of the pathway • Is 0 mm Hg in the right atrium • The steepest change in blood pressure occurs in the arterioles which offer the greatest resistance to blood flow

32. Systemic Blood Pressure Figure 19.5

33. Arterial Blood Pressure • Arterial BP reflects two factors of the arteries close to the heart • Their elasticity (how much they can be stretched) • The volume of blood forced into them at any given time • Blood pressure in elastic arteries near the heart is pulsatile (BP rises and falls)

34. Arterial Blood Pressure • Systolic pressure – pressure exerted on arterial walls during left ventricular contraction pushing blood into the aorta (120 mm Hg) • Diastolic pressure – aortic SL valve closes preventing backflow and the walls of the aorta recoil resulting in pressure drop • lowest level of arterial pressure during a ventricular cycle

35. Arterial Blood Pressure • Pulse pressure – the difference between systolic and diastolic pressure • Felt as a pulse during systole as arteries are expanded • Mean arterial pressure (MAP) – pressure that propels the blood to the tissues • Because diastole is longer than systole… • MAP = diastolic pressure + 1/3 pulse pressure • E.g. systolic BP = 120 mm Hg • E.g. diastolic BP = 80 mm Hg • MAP = 93 • MAP & pulse pressure decline w/ distance from the heart • At the arterioles blood flow is steady

36. Capillary Blood Pressure • Capillary BP ranges from 40 (beginning) to 20 (end) mm Hg • 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

37. Venous Blood Pressure • Venous BP is steady and changes little during the cardiac cycle • The pressure gradient in the venous system is only about 20 mm Hg (from venules to venae cavae) vs. 60 mm Hg from the aorta to the arterioles • A cut vein has even blood flow; a lacerated artery flows in spurts

38. Factors Aiding Venous Return • Venous BP alone is too low to promote adequate blood return and is aided by the: • Respiratory “pump” – pressure changes created during breathing suck blood toward the heart by squeezing local veins • Muscular “pump” – contraction of skeletal muscles surrounding deep veins “pump” blood toward the heart with valves prevent backflow during venous return • Smooth muscle contraction of the tunica media by the SNS

39. Factors Aiding Venous Return Figure 19.6

40. Maintaining Blood Pressure • Maintaining blood pressure requires: • Cooperation of the heart, blood vessels, and kidneys • Supervision of the brain

41. Maintaining Blood Pressure • The main factors influencing blood pressure are: • Cardiac output (CO) • Peripheral resistance (PR) • Blood volume • Blood pressure = CO x PR • Blood pressure varies directly with CO, PR, and blood volume • Changes in one (CO, PR, BV) are compensated by the others to maintain BP

42. Cardiac Output (CO) • CO = stroke volume (ml/beat) x heart rate (beats/min) • Units are L/min • Cardiac output is determined by venous return and neural and hormonal controls • Resting heart rate is controlled by the cardioinhibitory center via the vagus nerves • Stroke volume is controlled by venous return (end diastolic volume, or EDV) • Under stress, the cardioacceleratory center increases heart rate and stroke volume • The end systolic volume (ESV) decreases and MAP increases

43. Cardiac Output (CO) Figure 19.7

44. Controls of Blood Pressure • Short-term controls: • Are mediated by the nervous system and bloodborne chemicals • Counteract moment-to-moment fluctuations in blood pressure by altering peripheral resistance • Long-term controls regulate blood volume

45. Short-Term Mechanisms: Neural Controls • Neural controls of peripheral resistance: • Alter blood distribution in response to demands • Maintain MAP by altering blood vessel diameter • Neural controls operate via reflex arcs involving: • Baroreceptors & afferent fibers • Vasomotor centers of the medulla • Vasomotor fibers • Vascular smooth muscle

46. Short-Term Mechanisms: Vasomotor Center • Vasomotor center – a cluster of sympathetic neurons in the medulla that oversees changes in blood vessel diameter • Maintains blood vessel tone by innervating smooth muscles of blood vessels, especially arterioles • Cardiovascular center – vasomotor center plus the cardiac centers that integrate blood pressure control by altering cardiac output and blood vessel diameter • Vasomotor fibers (T1-L2) innervate smooth muscle of the arterioles • Vasomotor state is always in a state of moderate constriction

47. Short-Term Mechanisms: Vasomotor Activity • Sympathetic activity causes: • Vasoconstriction and a rise in BP if increased • BP to decline to basal levels if decreased • Vasomotor activity is modified by: • Baroreceptors (pressure-sensitive), chemoreceptors (O2, CO2, and H+ sensitive), higher brain centers, bloodborne chemicals, and hormones

48. Short-Term Mechanisms: Baroreceptor-Initiated Reflexes • When arterial blood pressure rises, it stretches baroreceptos located in the carotid sinuses, aortic arch, walls of most large arteries of the neck & thorax • Baroreceptors send impulses to the vasomotor center inhibiting it resulting in vasodilation of arterioles and veins and decreasing blood pressure • Venous dilation shifts blood to venous reservoirs causing a decline in venous return and cardiac output • Baroreceptors stimulate parasympathetic activity and inhibit the cardioacceletory center to decrease heart rate and contractile force

49. Short-Term Mechanisms: Baroreceptor-Initiated Reflexes • Declining blood pressure stimulates the cardioacceleratory center to: • Increase cardiac output and vasoconstriction causing BP to rise • Peripheral resistance & cardiac output are regulated together to minimize changes in BP

50. Short-Term Mechanisms: Baroreceptor-Initiated Reflexes • Baroreceptors respond to rapidly changing conditions like when you change posture • They are ineffective against sustained pressure changes, e.g. chronic hypertension, where they will adapt and raise their set-point