Blood Vessels and Circulation PDF

Blood Vessels and Circulation General Anatomy of Blood Vessels Blood Pressure Capillary Exchange Venous Return and Circulatory Shock 20-1 Anatomy of Blood Vessels Copyright © The McGraw-Hill Companies, Inc. Permission requ...

Blood Vessels and Circulation General Anatomy of Blood Vessels Blood Pressure Capillary Exchange Venous Return and Circulatory Shock 20-1 Anatomy of Blood Vessels Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Capillaries Artery: Tunica interna Tunica media Tunica externa Nerve Vein Figure 20.1a (a) 1 mm © The McGraw-Hill Companies, Inc./Dennis Strete, photographer arteries carry blood away from heart veins carry blood back to heart capillaries are site of nutrient exchange 20-2  Vessel Wall tunica interna (tunica intima) – lines the blood vessel, is exposed to blood – endothelium – simple squamous epithelium overlying a basement membrane and a sparse layer of loose connective tissue selectively permeable barrier secretes chemicals that stimulate dilation or constriction of the vessel normally repels blood cells and platelets (prevents clots) 20-3  Vessel Wall tunica media – middle layer – smooth muscle, collagen, and elastic tissue – strengthens vessel and prevents blood pressure from rupturing them – vasomotion – changes in diameter of the blood vessel brought about by smooth muscle 20-4  Vessel Wall tunica externa (tunica adventitia) – outermost layer – loose connective tissue that often merges with that of neighboring blood vessels, nerves, or other organs – anchors the vessel and provides passage for small nerves, lymphatic vessels – vasa vasorum: small vessels that supply blood to the outer layers of large vessels 20-5 Figure 20.2 20-6  Arteries arteries - sometimes called resistance vessels because they have strong, resilient tissue structure that resists blood pressure – carry blood away from heart – largest arteries are more elastic – stretch from force of heartbeat and then snap back – medium-sized arteries are more muscular – greater control of vasomotion – small arteries are called arterioles – pulse is felt in arteries, because force of heartbeat (bp) is strong there 20-7  Capillaries capillaries - site where nutrients, wastes, and hormones are exchanged between the blood and tissue fluid (exchange vessels) – the ‘business end’ of the cardiovascular system – absent or scarce in tendons, ligaments, epithelia, cornea and lens of the eye three capillary types distinguished by structural differences that account for their greater or lesser permeability 20-8 Three Types of Capillaries 1. continuous capillaries - occur in most tissues – endothelial cells have tight junctions forming a continuous tube with intercellular clefts allow passage of solutes such as glucose – pericytes wrap around the capillaries and contain the same contractile protein as muscle contract and regulate blood flow 20-9 Continuous Capillary Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pericyte Basal lamina Intercellular cleft Pinocytotic vesicle Endothelial cell Erythrocyte Tight junction 20-10 Three Types of Capillaries 2. fenestrated capillaries - In organs that require rapid absorption or filtration (kidneys, sm. intestine) endothelial cells riddled with holes called filtration pores (fenestrations) spanned by very thin glycoprotein layer allows passage of only small molecules 20-11  Fenestrated Capillary Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Endothelial cells Nonfenestrated Erythrocyte area Filtration pores (fenestrations ) Basal lamina Intercellular (a) cleft (b) 400 µm b: Courtesy of S. McNutt 20-12 Three Types of Capillaries 3. sinusoids (discontinuous capillaries) - liver, bone marrow, spleen – irregular (not always tubular) blood-filled spaces with large gaps between cells – allow proteins (albumin), clotting factors, and new blood cells to enter the circulation 20-13  Sinusoid in Liver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Macrophage Endothelial cells Erythrocytes in sinusoid Liver cell (hepatocyte) Microvilli Sinusoid 20-14  Capillary Beds capillaries organized into networks – usually supplied by a single metarteriole thoroughfare channel - metarteriole that goes through capillary bed to venule (small vein) precapillary sphincters control which beds are well perfused – when sphincters open: capillaries perfused with blood – when sphincters closed: blood bypasses the capillaries flows through thoroughfare channel to venule 3/4 of the body’s capillaries are shut down at a given time 20-15 Capillary Bed Sphincters Open Precapillary Thoroughfare sphincters channel Metarteriole Capillaries Arteriole Venule (a) Sphincters open when sphincters are open, the capillaries are well perfused 20-16 Capillary Bed Sphincters Closed Arteriole Venule (b) Sphincters closed when the sphincters are closed, little to no blood flow occurs 20-17 (skeletal muscles at rest) Veins (Capacitance Vessels) Carrying more blood Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Distribution of Blood than arteries at any moment in time thinner walls, flaccid, Pulmonary circuit 18% less muscle and elastic Veins 54% Heart tissue Systemic circuit 12% 70% collapse when empty, Arteries 11% expand easily Capillaries 5% steady blood flow relatively low blood pressure 20-18  Veins postcapillary venules – smallest veins – even more porous than capillaries so also exchange fluid with surrounding tissues medium veins have venous valves – skeletal muscle pump propels venous blood back towards heart venous sinuses – veins with especially thin walls, large lumens, and no smooth muscle – often more of a chamber, not a tube (posterior of heart, superior to brain) 20-19 Skeletal Muscle Pump Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. To heart Valve open Venous blood Valve closed (a) Contracted skeletal muscles (b) Relaxed skeletal muscles 20-20  Venous Return venous return – the flow of blood back to the heart – blood pressure is the most important force in venous return, but also… gravity drains blood from head and neck skeletal muscle pump in the limbs thoracic (respiratory) pump – inhalation - thoracic cavity expands and thoracic pressure decreases, abdominal pressure increases, forcing blood upward cardiac suction of expanding atrial space 20-21 Venous Return and Exercise exercise increases venous return: – heart beats faster, harder increasing CO & BP – vessels of skeletal muscles, lungs, and heart dilate – increased respiratory rate, increased action of thoracic pump – increased skeletal muscle pump venous pooling occurs with inactivity – venous pressure not enough to force blood upward (esp. from legs) – with prolonged standing, CO (cardiac output) may be low enough to cause dizziness 20-22  Anastomoses veins are more redundant than arteries; some veins are paired and run alongside a single artery paired veins are connected by multiple cross-channels called anastomoses anastomoses can be artery-vein or artery-artery as well 20-23 ? 20-24  Blood Pressure blood pressure (bp) – the force that blood exerts against a vessel wall measured at brachial artery of arm using sphygmomanometer two pressures are recorded: – systolic pressure: peak arterial BP taken during ventricular contraction (ventricular systole) – diastolic pressure: minimum arterial BP taken during ventricular relaxation (diastole) between heart beats normal value, young adult: 120/75 mm Hg 20-25  Abnormalities of Blood Pressure hypertension – high bp – resting BP > 140/90 – consequences can weaken small arteries and cause aneurysms or scarring hypotension – chronic low resting BP – caused by blood loss, dehydration, anemia 20-26  Hypertension hypertension – most common cardiovascular disease, affecting about 30% of Americans over 50 “the silent killer” – major cause of heart failure, stroke, kidney failure damages heart – myocardium enlarges until overstretched and inefficient renal arterioles thicken in response to stress – drop in renal BP leads to salt retention and worsens overall hypertension Treatments – Reducing stress, reducing salt intake, bp medications (beta-blockers, among others) 20-27  Blood Pressure Importance of arterial elasticity – expansion and recoil maintains steady flow of blood throughout cardiac cycle, smoothes out pressure fluctuations and decreases stress on small arteries BP rises with age – arteries less elastic and absorb less systolic force BP determined by cardiac output, blood volume and peripheral resistance (more in following chapters) 20-28  Arterial Sense Organs sensory structures in the walls of certain vessels that monitor blood pressure and chemistry – send info to medulla that serves to regulate heart rate, vasomotion, and respiration – carotid sinuses – baroreceptors (pressure sensors) in walls of internal carotid artery monitor blood pressure – carotid bodies - chemoreceptors (monitor blood chemistry) oval bodies near branch of common carotids adjust resp. rate to stabilize pH, CO2, and O2 – aortic bodies – chemoreceptors in aortic arch same function as carotid bodies 20-29  BP Changes With Distance Copyright © The McGraw-Hill Companies, Inc. 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Systemic blood pressure (mm Hg) 120 100 Systolic pressure 80 60 Diastolic pressure 40 20 0 A C L a rt e S te V A V av S in ap les m r rt La en ies en ae m s or a ar e s r g ri e c ve les er al ie al ill rg ta u ae l s io l ar e ve in Increasing distance from left ventricle s 20-30  Two Purposes of Vasomotion 1. general method of raising or lowering BP throughout the whole body – important in supporting brain during hemorrhage or dehydration 2. rerouting blood from one region to another for perfusion of individual organs (more common) – either centrally or locally controlled during exercise, blood flow reduced to kidneys and digestive tract and increased to skeletal muscles metabolite accumulation in a tissue affects local circulation without affecting circulation elsewhere in the body 20-31  Blood Flow in Response to Needs Aorta Superior mesenteric artery Constricted Dilated (Reduced (Increased flow flow to to intestines) Intestines) Common iliac arteries Constricted Dilated Reduced flow to legs Increased flow to legs (a) (b) arterioles shift blood flow with changing priorities 20-32  Blood Flow Comparison Copyright © The McGraw-Hill Companies, Inc. 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At rest Moderate exercise Total cardiac output 5 L/min Total cardiac output 17.5 L/min Other Other Coronary 350 mL/min Coronary 400 mL/min 200 mL/min 750 mL/min Cutaneous 1,900 mL/min Cutaneous 300 mL/min Muscular 1,000 mL/min Cerebral 750 mL/min Cerebral Renal 700 mL/min 600 mL/min Digestive Muscular 1,350 mL/min 12,500 mL/min Renal Digestive 1,100 mL/min 600 mL/min during exercise: – increased perfusion of lungs, myocardium, skeletal muscles – decreased perfusion of kidneys and digestive tract 20-33  Capillary Exchange capillary exchange – two way movement of fluid across capillary walls – water, oxygen, glucose, amino acids, lipids, minerals, antibodies, hormones, wastes, carbon dioxide, ammonia chemicals get through capillary wall by three routes: 1. through endothelial cell cytoplasm 2. intercellular clefts between endothelial cells 3. filtration pores (fenestrations) of the fenestrated capillaries 20-34  Capillary Exchange - Diffusion Always diffusion is the most important form of capillary down exchange conc. – glucose and oxygen diffuse out of the blood gradient – carbon dioxide & other waste diffuse into blood capillary diffusion can only occur if: – the solute can permeate the plasma membranes of the endothelial cell, or – can find passages large enough to pass through water soluble substances must pass through filtration pores and intercellular clefts large particles like proteins are held back 20-35 Capillary Exchange Filtration pores Transcytosis Diffusion through endothelial cells Intercellular clefts 20-36 Filtration and Reabsorption fluid (water) filters out of the arterial end of the capillary and osmotically reenters at the venous end – delivers materials to the cells & removes wastes opposing forces – blood hydrostatic pressure drives fluid out of capillary (physical force) high on arterial end of capillary, low on venous end – colloid osmotic pressure (COP) draws fluid into capillary due to plasma proteins (albumin)- more in blood capillaries reabsorb about 85% of the fluid they filter 20-37  Capillary Filtration and Reabsorption capillary filtration at arterial end capillary reabsorption at venous end Figure 20.17 20-38  Capillary Filtration and Reabsorption capillary filtration at arterial end capillary reabsorption at venous end Figure 20.17 20-39  Capillary Filtration and Reabsorption (numbers not important, just the Copyright © The McGraw-Hill Companies, Inc. 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Figure 20.17 20-40  Edema edema – the accumulation of excess fluid in a tissue – when fluid filters into a tissue faster than it is absorbed three primary causes: 1. increased capillary filtration kidney failure, histamine release, old age, poor venous return 2. reduced capillary absorption hypoproteinemia, liver disease, dietary protein deficiency 3. obstructed lymphatic drainage surgical removal of lymph nodes 20-41 Consequences of Edema tissue necrosis – oxygen delivery and waste removal impaired severe edema circulatory shock – excess fluid in tissue spaces causes low blood volume and low blood pressure pulmonary edema: – suffocation threat cerebral edema: – headaches, nausea, seizures, and coma 20-42 ? 20-43  Varicose Veins blood pools in lower legs when standing for long periods, stretching the veins – cusps of the valves pull apart in enlarged superficial veins, further weakening vessels – blood backflows and further distends the vessels, their walls grow weak and develop into varicose veins hereditary weakness, obesity, and pregnancy also contribute hemorrhoids are varicose veins of the anal canal 20-44  Circulatory Shock circulatory shock – cardiac output is insufficient to meet the body’s needs – Could be from cardiogenic shock - inadequate pumping of heart (MI) – Or could be low venous return (LVR) – too little blood is returning to the heart 1. hypovolemic shock - most common -loss of blood volume: trauma, burns, dehydration 2. obstructed venous return shock -tumor or aneurysm compresses a vein 20-45 Vascular Shock and Others 3. venous pooling (vascular) shock long periods of standing, sitting or widespread vasodilation neurogenic shock - loss of vasomotor tone, vasodilation – emotional shock, brainstem injury 4. septic shock bacterial toxins trigger vasodilation and increased capillary permeability 5. anaphylactic shock severe immune reaction to antigen, histamine release, generalized vasodilation, increased capillary permeability 20-46  TIAs and CVAs transient ischemic attacks (TIAs) – brief episodes of cerebral ischemia (oxygen deficit) – caused by spasms of diseased cerebral arteries – dizziness, loss of vision, weakness, paralysis, headache or aphasia – lasts from a moment to a few hours – often early warning of impending stroke stroke - cerebral vascular accident (CVA) – sudden death of brain tissue caused by ischemia atherosclerosis, thrombosis, ruptured aneurysm – effects range from unnoticeable to fatal blindness, paralysis, loss of sensation, loss of speech common 20-47  Aneurysm aneurysm - weak point in an artery or heart wall – forms thin-walled, bulging sac that pulses with each heartbeat may rupture at any time, causing hemorrhage – dissecting aneurysm - blood accumulates between the tunics of the artery can cause pain – result from congenital weakness of the blood vessels or result of trauma or bacterial infections such as syphilis most common cause is atherosclerosis and hypertension 20-48  Phlebitis Phlebitis: inflammation of a vein, most often in the leg – Caused by a blood clot in the vein – Blood clots are often caused by insufficient venous return From surgery, bed rest, prolonged sitting (such as on long flight) – DVT (deep vein thrombosis) – if clot breaks loose from leg, can enter and block lung 20-49