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Blood Vessels

Blood Vessels

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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 • The three major types of vessels are arteries, capillaries, and veins • Arteries carry blood away from the heart, veins carry blood toward the heart • Capillaries contact tissue cells and directly serve cellular needs

  2. Classes of Blood Vessels • Arteries • Carry blood away from heart • Arterioles • Are smallest branches of arteries • Capillaries • Are smallest blood vessels • Location of exchange between blood and interstitial fluid • Venules • Collect blood from capillaries • Veins • Return blood to heart

  3. Vascular Components Figure 19.2a, b

  4. Generalized Structure of Blood Vessels • Arteries and veins are composed of three tunics • tunica interna, • tunica media • tunica externa • Capillaries are composed of endothelium with sparse basal lamina

  5. Figure 21-1 Comparisons of a Typical Artery and a Typical Vein Tunica externa Tunica media Tunica intima Smooth muscle Lumenof vein Internal elasticmembrane External elastic membrane Lumenof artery Endothelium Elastic fiber Artery and vein LM  60 ARTERY

  6. Figure 21-1 Comparisons of a Typical Artery and a Typical Vein Tunica externa Tunica media Tunica intima Lumenof vein Smoothmuscle Lumenof artery Endothelium Artery and vein LM  60 VEIN

  7. Figure 21-2 Histological Structure of Blood Vessels Elastic Artery Internal elasticlayer Tunicaintima Endothelium Tunica media Tunica externa Muscular Artery Tunica externa Tunica media Endothelium Tunica intima Arteriole Smooth muscle cells(Media) Endothelium Basement membrane

  8. Figure 21-2 Histological Structure of Blood Vessels Large Vein Tunica externa Tunica media Endothelium Tunica intima Medium-Sized Vein Tunica externa Tunica media Endothelium Tunica intima Venule Tunica externa Endothelium

  9. A Comparison of a Typical Artery and a Typical Vein

  10. Structure of vessel walls • The walls of blood vessels are too thick to allow diffusion between blood stream and surrounding tissues or the tissues of the blood vessels. • The walls of large vessels contain small blood vessels that supply both tunica media and externa – vasa vasorum

  11. 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 • Withstand and smooth out large blood pressure fluctuations • Serve as pressure reservoirs

  12. Muscular (Distributing) Arteries and Arterioles • Muscular arteries – distal to elastic arteries; deliver blood to body organs • Have thick tunica media with more smooth muscle • Active in vasoconstriction • Arterioles – smallest arteries; lead to capillary beds • Control flow into capillary beds via vasodilation and constriction

  13. Capillaries • Capillaries are the smallest blood vessels • 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 • There are three structural types of capillaries: continuous, fenestrated, and sinusoids

  14. Figure 21-2 Histological Structure of Blood Vessels Capillaries Fenestrated Capillary Continuous Capillary Pores Endothelial cells Endothelial cells Basement membrane Basement membrane

  15. Figure 21-4 Capillary Structure Basementmembrane Endothelial cell Nucleus Endosomes Endosomes Fenestrations,or pores Boundarybetweenendothelialcells Boundarybetweenendothelialcells Gap betweenadjacent cells Basementmembrane Basementmembrane Fenestrated capillary Sinusoid Continuous capillary

  16. 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 • Continuous capillaries of the brain: • Have tight junctions completely around the endothelium • Constitute the blood-brain barrier

  17. Figure 21-4a Capillary Structure Basementmembrane Endothelial cell Nucleus Endosomes Boundarybetweenendothelialcells Basementmembrane Continuous capillary

  18. 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

  19. Figure 21-4b Capillary Structure Basementmembrane Endothelial cell Nucleus Endosomes Fenestrations,or pores Boundarybetweenendothelialcells Basementmembrane Fenestrated capillary

  20. 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

  21. Figure 21-4c Capillary Structure Gap betweenadjacent cells Sinusoid

  22. Capillary Beds • Vascular shunts – Metarteriole--is a vessel that emerges from an arteriole, passes through the capillary network and empties into a venule. • Proximal portions are surrounded by scattered smooth muscle cells whose contraction and relaxation help regulate the amount and force of the blood. • Distal portion has no smooth muscle fibers and is called a thoroughfare channel. • True capillaries – 10 to 100 per capillary bed, capillaries branch off the metarteriole and return to the thoroughfare channel at the distal end of the bed • At their site of origin, there is a ring of smooth muscle fibers called a precapillary sphincter that controls the flow of blood entering a true capillary

  23. Capillary Beds Figure 19.4a

  24. Capillary Beds Figure 19.4b

  25. Venous System: Venules • Venules are formed when capillary beds unite • Postcapillary venules – smallest venules, composed of endothelium and a few pericytes • Large venules have one or two layers of smooth muscle (tunica media)

  26. Venous System: Veins • Veins are: • 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 • Veins have much lower blood pressure and thinner walls than arteries • Venous sinuses – specialized, flattened veins with extremely thin walls (e.g., coronary sinus of the heart and dural sinuses of the brain)

  27. 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 • Thoroughfare channels are examples of arteriovenous anastomoses

  28. Blood Flow • The purpose of cardiovascular regulation is to maintain adequate blood flow through the capillaries to the tissues • Actual volume of blood flowing through a vessel, an organ, or the entire circulation in a given period: • Is measured in ml/min. • Is equivalent to cardiac output (CO), considering the entire vascular system • Is relatively constant when at rest • Varies widely through individual organs

  29. Blood Flow Through Tissues • Blood flow, or tissue perfusion, is involved in: • Delivery of oxygen and nutrients to, and removal of wastes from, tissue cells • Gas exchange in the lungs • Absorption of nutrients from the digestive tract • Urine formation by the kidneys • Blood flow is precisely the right amount to provide proper tissue function

  30. Brain Heart Skeletal muscles Skin Kidney Abdomen Other Total blood flow at rest 5800 ml/min Total blood flow during strenuous exercise 17,500 ml/min Figure 19.13

  31. Circulatory Shock • Circulatory shock – any condition in which blood vessels are inadequately filled and blood cannot circulate normally • Results in inadequate blood flow to meet tissue needs • Three types include: • Hypovolemic shock – results from large-scale blood loss or dehydration • Vascular shock – normal blood volume but too much accumulate in the limbs (long period of standing/sitting) • Cardiogenic shock – the heart cannot sustain adequate circulation

  32. Blood flow • Capillary blood flow is determined by the interplay between: • Pressure • Resistance • The heart must generate sufficient pressure to overcome resistance

  33. Some terms and definitions • Define blood pressure and name the units used • Define resistance and name the sources of resistance

  34. Physiology of Circulation: Blood Pressure (BP) • Force per 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 • Absolute pressure is less important than pressure gradient

  35. Figure 21-10c Relationships among Vessel Diameter, Cross-Sectional Area, Blood Pressure, and Blood Velocity within the Systemic Circuit Averagebloodpressure(mm Hg) Average blood pressure

  36. Physiology of Circulation: Resistance • Opposition to flow • Measure of the amount of friction blood encounters • Generally encountered in the peripheral systemic circulation • Because the resistance of the venous system is very low (why?) usually the focus is on the resistance of the arterial system: • Peripheral resistance (PR) is the resistance of the arterial system

  37. Physiology of Circulation: Resistance • Three important sources of resistance • Blood viscosity • Total blood vessel length • Blood vessel diameter

  38. Flow  1 / 2 Friction and Vessel Length Resistance to flow  1 Internal surfacearea  1 Flow  1 Resistance to flow  2 Internal surface area  2 Friction and Vessel Diameter Greatest resistance,slowest flow near surfaces Leastresistance,greatest flowat center Vessel Length versus Vessel Diameter Diameter  2 cm Resistance to flow  1 Diameter  1 cm Resistance to flow  16 Factors Affecting Vascular Resistance Plaque deposit Turbulence Turbulence

  39. Resistance – constant factors • Blood viscosity • The “stickiness” of the blood due to formed elements and plasma proteins • Blood vessel length • The longer the vessel, the greater the resistance encountered

  40. Resistance – frequently changed factors • Changes in vessel diameter are frequent and significantly alter peripheral resistance • Small-diameter arterioles are the major determinants of peripheral resistance • Frequent changes alter peripheral resistance • Fatty plaques from atherosclerosis: • Cause turbulent blood flow • Dramatically increase resistance due to turbulence

  41. Figure 21-10a Relationships among Vessel Diameter, Cross-Sectional Area, Blood Pressure, and Blood Velocity within the Systemic Circuit Vesseldiameter (cm) Vessel diameter

  42. Figure 21-10b Relationships among Vessel Diameter, Cross-Sectional Area, Blood Pressure, and Blood Velocity within the Systemic Circuit Cross-sectionalarea(cm2) Total cross-sectional area of vessels

  43. Systemic Blood Pressure • The pumping action of the heart generates blood flow through the vessels along a pressure gradient, always moving from higher- to lower-pressure areas • Pressure results when flow is opposed by resistance • 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 between arteries to arterioles

  44. Figure 21-11 Pressures within the Systemic Circuit Systolic Pulsepressure Mean arterialpressure Diastolic mm Hg

  45. Arterial Blood Pressure • Arterial pressure is important because it maintains blood flow through capillaries • Blood pressure near the heart is pulsatile • Systolic pressure: pressure exerted during ventricular contraction • Diastolic pressure: lowest level of arterial pressure

  46. Arterial Blood Pressure • A pulse is rhythmic pressure oscillation that accompanies every heartbeat • Pulse pressure = difference between systolic and diastolic pressure • Mean arterial pressure (MAP): pressure that propels the blood to the tissues MAP = diastolic pressure + 1/3 pulse pressure • Pulse pressure and MAP both decline with increasing distance from the heart • Efficiency of the circulation can be assessed by taking pulse and blood pressure measurements