The Circulatory System: Blood Vessels and Circulation PDF

Chapter 20 The Circulatory System: Blood Vessels and Circulation ANATOMY & PHYSIOLOGY The Unity of Form and Function TENTH EDITION KENNETH S. SALADIN © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC...

Chapter 20 The Circulatory System: Blood Vessels and Circulation ANATOMY & PHYSIOLOGY The Unity of Form and Function TENTH EDITION KENNETH S. SALADIN © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. General Anatomy of the Blood Vessels Expected Learning Outcomes: Describe the structure of a blood vessel. Describe the types of arteries, capillaries, and veins. Trace the general route usually taken by the blood from the heart and back again. Describe some variations on this route. 2 General Anatomy of the Blood Vessels Three principal categories of blood vessels: Arteries Carry blood away from heart Veins Carry blood back to heart Capillaries Connect smallest arteries to smallest veins to create a circuit 3 The Vessel Wall Walls of arteries and veins have three layers called tunics Tunica interna (tunica intima) Lines the blood vessel and is exposed to blood Consists of the endothelium—simple squamous epithelium Selectively permeable barrier Secretes chemicals that stimulate dilation or constriction Normally repels blood cells and platelets to prevent clotting When tissue around vessel is inflamed, endothelial cells produce cell-adhesion molecules Cell adhesion causes leukocytes to congregate in tissues where their defensive actions are needed 4 The Vessel Wall Tunics of the vessel wall (continued): Tunica media Middle layer Consists of smooth muscle, collagen, and elastic tissue Strengthens vessels and prevents blood pressure from rupturing them Contraction of muscle controls blood vessel diameter Tunica externa (tunica adventitia) Outermost layer Consists of 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 outer half of wall in the larger vessels 5 Histology of the Blood Vessels Figure Access the text alternative for these images 20.2 6 Arteries Arteries are divided into three classes by size: Conducting (elastic or large) arteries Biggest arteries Examples: aorta, common carotid, subclavian, pulmonary trunk, and common iliac arteries Internal elastic lamina at the border between tunica interna and media External elastic lamina at the border between media and externa Expand during systole, recoil during diastole Expansion takes pressure of smaller downstream vessels Recoil maintains pressure during relaxation, keeps blood flowing 7 Arteries Arteries are divided into three classes by size (continued): Distributing (muscular or medium) arteries Distribute blood to specific organs Examples: brachial, femoral, renal, and splenic arteries Smooth muscle layers constitute three-fourths of wall thickness Internal and external elastic laminae are thick Resistance (small) arteries Thicker tunica media in proportion to their lumen than large arteries and very little tunica externa Arterioles—smallest of the resistance arteries 200 mm diameter; only 1 to 3 layers of smooth muscle Control amount of blood to various organs 8 Arteries Metarterioles (thoroughfare channels) Short vessels that link arterioles directly to venules in some places (for example, in mesenteries) Provide shortcuts allowing blood to bypass capillary beds 9  Aneurysm Aneurysm—weak point in artery or heart wall Forms a thin-walled, bulging sac that pulsates with each heartbeat and may rupture at any time Dissecting aneurysm: blood accumulates between tunics of artery and separates them, usually because of degeneration of the tunica media Most common sites: abdominal aorta, renal arteries, and arterial circle at base of brain Can cause pain by putting pressure on other structures Can rupture causing hemorrhage Result from congenital weakness of blood vessels, trauma, or bacterial infections Most common cause is atherosclerosis and hypertension 10 Arterial sense organs In walls of some major vessels, sensory structures monitor blood pressure and chemistry Transmit information to brainstem to regulate heart rate, blood vessel diameter, and respiration Carotid sinuses Baroreceptors in walls of internal carotid artery Monitor blood pressure Transmit signals through glossopharyngeal nerve Allow for baroreflex 11 Arterial sense organs Arterial sense organs (continued): Carotid bodies Oval bodies near branch of common carotids They are chemoreceptors—monitor blood chemistry Transmit signals through glossopharyngeal nerve to brainstem respiratory centers Adjust respiratory rate to stabilize pH, CO2, and O2 Aortic bodies One to three chemoreceptors in walls of aortic arch Same structure and function as carotid bodies, but innervation is by vagus nerve 12 Baroreceptors and Chemoreceptors in the Arteries Superior to the Heart Figure Access the text alternative for these images 20.4 13 Capillaries Capillaries are exchange vessels—where gasses, nutrients, wastes, and hormones pass between the blood and tissue fluid Part of the microvasculature (microcirculation), which also includes arterioles and venules Nearly every cell in body is close to a capillary Exceptions: capillaries absent or scarce in tendons, ligaments, epithelia, cornea, and lens of the eye Composed of endothelium and basal lamina Three capillary types distinguished by permeability: Continuous capillaries Fenestrated capillaries Sinusoids 14 Types of Blood Capillaries c: S. McNutt Access the text alternative for these images 15 Capillaries Types of capillaries: Continuous capillaries Found in most tissues Endothelial cells held together by tight junctions; form a continuous tube with cells separated by gaps— intercellular clefts Small solutes (such as glucose) can pass, but most plasma protein and other large molecules (blood cells, platelets) cannot Basal lamina—thin protein-carbohydrate layer that surrounds endothelium Pericytes wrap around the capillaries and contain the same contractile protein as muscle Contract and regulate blood flow; can differentiate into endothelial cells, muscle cells to help with vessel growth and repair 16 Capillaries Types of capillaries (continued): Fenestrated capillaries Found in organs that require rapid absorption or filtration (kidneys, small intestine) Endothelial cells contain filtration pores (fenestrations) 20 to 100 nm in diameter Spanned by very thin glycoprotein membrane much thinner than cell’s plasma membrane Allow passage of only small molecules Proteins and larger particles stay in bloodstream 17 Capillaries Types of capillaries (continued): Sinusoids Found in liver, bone marrow, spleen Irregular blood-filled spaces Endothelial cells separated by wide gaps with no basal lamina, and the cells often have large fenestrations Allow proteins (albumin), clotting factors, and new blood cells to enter the circulation 18 Capillaries Capillary beds—networks of 10 to 100 capillaries Usually supplied by a single arteriole or metarteriole Drain into venule or distal end of metarteriole At any given time, 75% of body’s capillaries are shut down Most control involves constriction of upstream arterioles Precapillary sphincters control flow in capillary beds supplied with metarterioles When sphincters are relaxed, capillaries are well perfused with blood When sphincters contract, they constrict the entry to the capillary and blood bypasses the capillary 19 Perfusion of a Capillary Bed Figure Access the text alternative for these images 20.7 20 Veins Veins are the capacitance vessels of the cardiovascular system Thin-walled and flaccid Collapse when empty, expand easily Greater capacity for blood containment than arteries At rest, about 64% of blood is in veins, 13% in systemic arteries Have steady blood flow (unlike pulses in arteries) Subjected to relatively low blood pressure Averages 10 mm Hg with little fluctuation 21 Veins Types of veins, listed from smallest to largest: Postcapillary venules Smallest veins (10 to 20 um diameter) Consist of tunica interna with only a few fibroblasts around it; no muscle Even more porous than capillaries, so also exchange fluid with surrounding tissues Leukocytes leave bloodstream through venule walls 22  Veins Types of veins (continued): Muscular venules Receive blood from postcapillary venules Up to 1 mm in diameter One or 2 layers of smooth muscle in tunica media; thin tunica externa Medium veins Up to 10 mm in diameter Thin tunica media, thick tunica externa, and tunica interna forms venous valves Varicose veins may result from failure of these valves Skeletal muscle pump propels venous blood back to heart 23 Veins Types of veins (continued): Large veins Diameter >10 mm Smooth muscle in all three tunics Relatively thin tunica media with moderate amount of smooth muscle Tunica externa is thickest layer, contains longitudinal bundles of smooth muscle Examples of large veins: the venae cavae, pulmonary veins, internal jugular veins, and renal veins Venous sinuses Occur in select locations in the body Modified veins with specially thin walls, large lumens, and no smooth muscle Not capable of vasoconstriction Examples include dural sinuses of brain, coronary sinus of heart 24  Varicose Veins Blood pools in the lower legs of people who stand 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 with irregular dilations Hereditary weakness, obesity, and pregnancy promote problems Hemorrhoids are varicose veins of the anal canal 25  Simplest and most common route for blood: Heart → arteries → arterioles → capillaries → venules → veins Circulatory In this route, blood passes through Routes only one network of capillaries from the time it leaves the heart until it returns Alternate pathways may involve two capillary beds or no capillary beds 26  Alternate circulatory pathways: Portal system Blood flows through two consecutive capillary networks before returning to Circulatory heart Examples: Routes In kidneys Between hypothalamus and anterior pituitary Between intestines and liver 27  Alternate circulatory pathways (continued): Anastomosis Convergence between two vessels other than capillaries Arteriovenous anastomosis Circulatory (shunt)—artery flows directly into Routes vein, bypassing capillaries Venous anastomosis—one vein empties into another; most common type of anastomosis Arterial anastomosis—two arteries merge; provides collateral (alternative) routes of blood supply 28 Variations in Circulatory Pathways Figure 20.11 Access the text alternative for these images 29 Blood Pressure, Resistance, and Flow Expected Learning Outcomes: Explain the relationship between blood pressure, resistance, and flow. Describe how blood pressure is measured and expressed. Show how pulse pressure and mean arterial pressure are calculated. Describe three factors that determine resistance to blood flow. Explain how vessel diameter influences blood pressure and flow. Describe some local, neural, and hormonal influences on blood flow. 30 Blood Pressure, Resistance, and Flow Blood supply to a tissue expressed in terms of flow and perfusion Flow—amount of blood flowing through an organ, tissue, or blood vessel in a given time (mL/min) Perfusion—flow per given volume or mass of tissue in a given time (mL/100g/min) At rest, total flow is quite constant and is equal to the cardiac output (5.25 L/min) Hemodynamics—the physical principles of blood flow based on pressure and resistance The greater the pressure difference between two points, the greater the flow; the greater the resistance, the less the flow 31 Blood Pressure Blood pressure (BP)—force blood exerts against a vessel wall Measured at brachial artery using sphygmomanometer A close approximation of pressure at exit of left ventricle 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 BP value for young adult: 120/75 mm Hg 32 Blood Pressure Measures of stress on blood vessels: Pulse pressure Difference between systolic and diastolic pressure Important measure of driving force on circulation and of stress exerted on small arteries by pressure surges generated by the heart Mean arterial pressure (M A P) Diastolic pressure + (one-third of pulse pressure) Average blood pressure that most influences risk level for edema, fainting (syncope), atherosclerosis, kidney failure, and aneurysm 33 Blood Pressure Since pressure varies across the cardiac cycle, blood flow in arteries is pulsatile Speed surges from 40 cm/s to 120 cm/s Blood spurts intermittently from an open artery In capillaries and veins, blood flows at steady speed Bleeding from veins tends to be slow and steady BP tends to rise with age Arteriosclerosis—stiffening of arteries due to deterioration of elastic tissues of artery walls Atherosclerosis—build up of lipid deposits that become plaques 34 Changes in Blood Pressure Relative to Distance from the Heart Figure 20.13 Access the text alternative for these images 35 Blood Pressure Hypertension Chronic resting blood pressure higher than 130/80 Can weaken arteries, cause aneurysms, promote atherosclerosis Hypotension Chronic low resting BP Caused by blood loss, dehydration, anemia No specific numerical criterion for hypotension 36 Blood Pressure BP determined by three variables: Cardiac output Blood volume Resistance to flow Blood volume is regulated mainly by kidneys Except for beating of the heart, kidneys have the largest influence on blood pressure of any organ 37 Peripheral Resistance Peripheral resistance—opposition to flow that blood encounters in vessels away from the heart Resistance hinges on three variables: Blood viscosity Vessel length Vessel radius Variables affecting resistance: Blood viscosity Stems mainly from plasma proteins (albumin) and R B Cs Low R B Cs (anemia) or albumin (hypoproteinemia) reduces viscosity and speeds up blood flow High viscosity and flow declines with polycythemia and dehydration Vessel length The farther liquid travels through tube, the more cumulative friction it encounters Pressure and flow decline with distance 38 Peripheral Resistance Variables affecting resistance (continued): Vessel radius Greatest control over blood flow Blood exhibits laminar flow—(flow in layers) and dilating a vessel allows more blood to flow without friction of vessel wall, speeds up flow Vasomotion—collective term for vasoconstriction (narrowing of vessel) and vasodilation (widening of vessel) 39 Peripheral Resistance From aorta to capillaries, blood velocity (speed) decreases for three reasons: Blood has traveled a greater distance Friction has reduced speed Smaller radii of arterioles and capillaries More resistance Number of vessels and their total cross-sectional area becomes greater and greater From capillaries to vena cava, velocity increases again Since veins are larger, they create less resistance than capillaries Large amount of blood forced into smaller channels Blood in veins never regains velocity it had in large arteries Veins are further from the pumping heart Veins are more compliant (they stretch more) than arteries 40 Peripheral Resistance Arterioles are most significant point of control over peripheral resistance and flow On proximal side of capillary beds and best positioned to regulate flow into the capillaries Outnumber any other type of artery, providing the most numerous control points More muscular in proportion to their diameter Highly capable of changing radius Arterioles produce half of the total peripheral resistance 41 Regulation of Blood Pressure and Flow Vasomotion is a quick and powerful way of altering blood pressure and flow Three ways of controlling vasomotor activity: Local control Neural control Hormonal control 42 Regulation of Blood Pressure and Flow Local control: Autoregulation—ability of tissues to regulate their own blood supply If tissue is inadequately perfused, wastes accumulate, stimulating vasodilation which increases perfusion (CO2 , H+delivers Bloodstream , K + , lactate, adenos oxygen andine)removes metabolites When wastes are removed, vessels reconstrict Homeostatic dynamic equilibrium that adjusts perfusion to the tissue’s metabolic needs 43 Regulation of Blood Pressure and Flow Neural control: Remote control of vessels by the central and autonomic nervous systems Vasomotor center of medulla exerts sympathetic control over blood vessels throughout the body Stimulates most vessels to constrict, but dilates vessels in cardiac muscle to meet demands of exercise Vasomotor center is the integrating center for three autonomic reflexes Baroreflexes Chemoreflexes Medullary ischemic reflex 44 Regulation of Blood Pressure and Flow Neural control (continued): Baroreflex—negative feedback response to changes in blood pressure Rising BP is detected by baroreceptors in carotid sinuses Glossopharyngeal nerve sends signals to brainstem Results in inhibition of sympathetic cardiac and vasomotor neurons, excitation of vagal fibers to slow heart rate and thus reduce BP Decrease in BP has the opposite effect Governs short-term regulation of BP, such as adjustments for rapid changes in posture Not helpful in correcting chronic hypertension—after 2 days or less, the receptors adjust their set point 45 Regulation of Blood Pressure and Flow Neural control (continued): Chemoreflex—response to changes in blood O2 and CO2 chemistry, especially pH and concentrations of Chemoreceptors include aortic bodies (in aortic arch and subclavian arteries) and carotid bodies (in external carotid arteries) Primary role is to adjust respiration to changes in blood chemistry; secondary role is vasomotion: Hypoxemia, hypercapnia, and acidosis stimulate chemoreceptors Act through vasomotor center to cause widespread vasoconstriction Increases BP and lung perfusion, which increases gas exchange Also stimulates breathing, so ventilation of lungs better matches their increased perfusion 46 Regulation of Blood Pressure and Flow Hormonal control: Some hormones also influence blood pressure, either by vasoactive effects or adjusting water balance Angiotensin II Potent vasoconstrictor that raises blood pressure Synthesis of angiotensin II requires angiotensin-converting enzyme (A C E) Hypertension often treated with A C E inhibitors which block production of angiotensin II (lowering BP) 47 Regulation of Blood Pressure and Flow Hormonal control (continued): Aldosterone PromotesNa + retention by the kidneys Water follows osmotically, + Nasoretention increases water retention, and BP is supported Natriuretic peptides Secreted by the heart IncreaseNa + excretion by the kidneys, reducing blood volume and pressure Have a generalized vasodilator effect, which also lowers BP 48 Regulation of Blood Pressure and Flow Hormonal control (continued): Antidiuretic hormone (A D H) Promotes water retention and raises BP At pathologically high concentrations, it also acts as a vasoconstrictor (hence its alternate name: arginine vasopressin) Epinephrine and norepinephrine Adrenal and sympathetic catecholamines bind to 𝛼-adrenergic receptors on smooth muscle of most vessels Stimulates vasoconstriction and raises BP 49 Two Purposes of Vasomotion Vasomotion serves two purposes: Generalized raising or lowering of BP Selectively modifying perfusion of an organ, rerouting blood from one region to another Generalized increase in blood pressure: Important in supporting cerebral perfusion during dehydration or hemorrhage Requires centralized control: medullary vasomotor center or hormones (angiotensin II, epinephrine) Generalized vasodilation lowers BP throughout body 50 Two Purposes of Vasomotion Rerouting of blood: Either centrally or locally controlled Example of central control: during exercise, sympathetic system reduces blood flow to kidneys and digestive tract and increases blood flow to skeletal muscles Example of local control: metabolite accumulation in a tissue affects local circulation without affecting circulation elsewhere in the body If a specific artery constricts, the pressure downstream drops, pressure upstream rises, and blood flows down path of least resistance Allows for rerouting of blood according to needs of body Examples: While dozing in armchair after meal, vasoconstriction in vessels to lower limbs reduces blood flow there, while vasodilation to digestive organs increases local blood flow During vigorous exercise, arteries in lungs, coronary circulation, and muscles dilate, while constriction occurs in kidneys and digestive tract 51 Redirection of Blood Flow in Response to Changing Metabolic Needs Figure Access the text alternative for these images 20.17 52 Differences in Systemic Blood Flow According to States of Physical Activity Figure Access the text alternative for these images 20.18 53 Filtration and Reabsorption Fluid filters out of arterial end of capillary and osmotically reenters venous end (is reabsorbed) Delivers materials to the cells, removes metabolic wastes Balance between osmosis and hydrostatic pressure Hydrostatic pressure drives fluid out of capillary High on arterial end of capillary, low on venous end Colloid osmotic pressure (C O P) draws fluid into capillary Results from plasma proteins (albumin)—more in blood Oncotic pressure = net C O P (blood C O P − tissue C O P) 54 Filtration and Reabsorption At the arterial end, the balance of hydrostatic and oncotic pressures results in a net filtration pressure (N F P) of 13 mm Hg out At the venous end, the balance results in a net reabsorption pressure of 7 mm Hg inward Blood pressure is lower, so oncotic pressure overrides hydrostatic pressure Overall result: capillary gives off fluid at arterial end and reabsorbs it at the venous end Capillaries reabsorb about 85% of the fluid they filter The rest (15% of filtered fluid) is absorbed by lymphoid system and ultimately returned to the blood 55 Edema Edema—accumulation of excess fluid in a tissue Fluid filters into issue faster than it is absorbed Three primary causes: Increased capillary filtration Kidney failure, histamine, old age, poor venous return Reduced capillary reabsorption Hypoproteinemia, liver disease, dietary protein deficiency Obstructed lymphatic drainage Surgical removal of lymph nodes 56 Edema Pathological consequences of edema: Tissue death Oxygen delivery and waste removal impaired Pulmonary edema Fluid in lungs; suffocation threat Cerebral edema Headaches, nausea, seizures, and coma Severe edema can cause circulatory shock Excess fluid in tissue spaces causes low blood volume and low blood pressure 57 Venous Return and Circulatory Shock Expected Learning Outcomes: Explain how blood in the veins is returned to the heart. Discuss the importance of physical activity in venous return. Discuss several causes of circulatory shock. Name and describe the stages of shock. 58 Mechanisms of Venous Return Venous return—flow of blood back to the heart, is achieved by five mechanisms: Pressure gradient Gravity Skeletal muscle pump Thoracic pump Cardiac suction 59 Mechanisms of Venous Return The pressure gradient Blood pressure is most important force in venous return Gravity Drains blood from head and neck The skeletal muscle pump Contracting limb muscles squeeze blood out of compressed part of vein Valves keep blood moving toward heart The thoracic (respiratory) pump Inhalation expands thoracic cavity, and blood is forced upward toward heart Cardiac suction During contraction of the ventricles, valves are pulled downward and atrial space expands 60 Venous Return and Physical Activity Exercise increases venous return for several reasons: Heart beats faster and harder, increasing CO and BP Vessels of skeletal muscles, lungs, and heart dilate and increase flow Increased respiratory rate, increased action of thoracic pump Increased skeletal muscle pump Venous pooling can occur with inactivity Accumulation of blood in limbs, venous pressure not enough to force blood upward With prolonged standing, CO may be low enough to cause dizziness or syncope Can usually be prevented by periodically tensing leg muscles 61 Circulatory Shock Circulatory shock—any state in which cardiac output is insufficient to meet body’s metabolic needs All forms of circulatory shock fall into two categories Cardiogenic shock—inadequate pumping of heart; usually result of myocardial infarction Low venous return (L V R) shock—cardiac output low because too little blood returns to heart 62 Circulatory Shock Three major forms of L V R shock: Hypovolemic shock Most common Loss of blood volume: trauma, burns, dehydration Obstructed venous return shock Tumor or aneurysm compresses a vein, impedes its blood flow Venous pooling shock Long periods of standing, sitting, or widespread vasodilation Neurogenic shock—results from sudden loss of vasomotor tone and vessels dilate; possible causes are brainstem trauma, emotional shock 63 Circulatory Shock Other types of shock: Share characteristics with venous pooling and hypovolemic shock Septic shock Bacterial toxins trigger vasodilation and increased capillary permeability Anaphylactic shock Severe immune reaction to antigen, histamine release, generalized vasodilation, increased capillary permeability 64 Circulatory Shock Responses to circulatory shock: Compensated shock Several homeostatic mechanisms bring about spontaneous recovery Example: If a person faints and falls to a horizontal position, gravity restores blood flow to the brain Decompensated shock When compensation fails Life-threatening positive feedback loops occur Condition gets worse causing damage to cardiac and brain tissue 65 Special Circulatory Routes Expected Learning Outcomes: Explain how the brain maintains stable perfusion. Discuss the causes and effects of strokes and transient ischemic attacks. Explain the mechanisms that increase muscular perfusion during exercise. Contrast the blood pressure of the pulmonary circuit with that of the systemic circuit, and explain why the difference is important in pulmonary function. 66  Brain Total blood flow to the brain fluctuates less than that of any other organ (700 mL/min) Seconds of deprivation causes loss of consciousness Four to 5 minutes causes irreversible brain damage Brain regulates its own blood flow Cerebral arteries dilate as systemic BP drops, constrict as BP rises Main chemical stimulus is pH Poor perfusion leads to CO2 accumulation in brain (hypercapnia) pH decreases and triggers vasodilation Low CO2 in brain (hypocapnia) leads to opposite result pH increases and stimulates vasoconstriction Occurs with hyperventilation May lead to ischemia, dizziness, syncope 67  Brain Transient ischemic attacks (T I As)—brief episodes of cerebral ischemia Caused by spasms of diseased cerebral arteries Dizziness, vision loss, weakness, paralysis, headache, aphasia Lasts from a moment to a few hours Often early warning of impending stroke Stroke (cerebral vascular accident, C V A)—sudden death of brain tissue caused by ischemia Can be caused by atherosclerosis, thrombosis, ruptured aneurysm Effects range from unnoticeable to fatal: blindness, paralysis, loss of sensation, loss of speech common 68 Skeletal Muscles Variable blood flow depending on state of exertion At rest: Arterioles constrict, most capillary beds shut down Total flow about 1 L/min During exercise: Arterioles dilate in response to muscle metabolites such as lactate, CO2 , and H Blood flow can increase 20-fold as blood diverted from digestive and urinary organs Muscular contraction impedes flow Isometric contraction causes fatigue faster than intermittent isotonic contractions 69 Lungs Lungs have a low pulmonary blood pressure Pressure in pulmonary arteries only 25/10 mm Hg Blood flow is slower, more time for gas exchange Oncotic pressure overrides blood (hydrostatic) pressure Pulmonary capillaries absorb fluid (almost no filtration) Prevents fluid accumulation in alveoli Unique response to hypoxia: Pulmonary arteries constrict in diseased area Redirects flow to better ventilated region Opposite as systemic arteries, which dilate in response to hypoxia to increase tissue perfusion 70  End of Main Content Because learning changes everything. ® www.mheducation.com © McGraw Hill LLC. 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