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Blood Vessels

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)

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Blood Vessels

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  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

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