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CHAPTER 15ALTERATIONS IN BLOOD FLOW
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ORGANIZATION OF THE CIRCULATORY AND LYMPHATIC SYSTEMS
• Circulatory circuit• Absorption and delivery of nutrients• Oxygen uptake and delivery• Removal of waste products
• Lymphatic circulation• Specialized system of channels and tissues• Reabsorbs fluid that leaks from vascular network
into the interstitium and returns it to the general circulation
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ORGANIZATION OF THE CIRCULATORY AND LYMPHATIC SYSTEMS (CONT.)
• Vessel structure• Arterioles: primary tissue is smooth muscle• Venules: connective tissue predominates• Capillaries: single layer of endothelial cells• Changes occur with aging
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CHANGES IN THE CIRCULATORY SYSTEM DUE TO AGING
• Thickening of the basement membrane in microvasculature• Narrowed vessel lumen• Impaired exchange of oxygen, nutrients and metabolic
wastes• Decreased elastin and increased collagen
• Decreased elasticity of arteries• Increased fibrosis in media and thickening of intima• Increased systolic blood pressure and arterial
insufficiency
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ORGANIZATION OF THE CIRCULATORY AND LYMPHATIC SYSTEMS
Anatomy of Arteries and Veins• Three microscopically distinct layers
(tunicae)• Intima: endothelial cells in direct contact with the
blood as it flows through the vessel• Media: smooth muscle tissue (thickest section in
arteries)• Adventitia: collagenous connective tissue (thickest
section in veins)
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STRUCTURE OF BLOOD VESSELS
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ORGANIZATION OF THE CIRCULATORY AND LYMPHATIC SYSTEMS
Anatomy of Capillaries• Single thickness of endothelial cells attached to a
basement membrane• Spaces between endothelial cells determine the
capillary permeability• Blood-brain barrier• Kidney capillary beds
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ORGANIZATION OF THE CIRCULATORY AND LYMPHATIC SYSTEMS (CONT.)Lymphatic Structure• Thin walled and resemble veins• Range in size from capillaries to vessels of
increased diameter• Intermittent valves that extend into lumen• Capillary walls contain contractile fibers
that propel lymph along the vessel
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LYMPHATIC NETWORK
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PRINCIPLES OF FLOW
Blood Flow, Pressure, and Resistance• Blood flow—movement along a pressure gradient
within the vascular bed • Pressure—blood moves from areas of higher
pressure (arteries) to an area of lower pressure (veins); the greater the pressure difference, the greater the blood flow
• Resistance—Opposing forces that deter blood flow; as resistance increases, blood flow decreases
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PRINCIPLES OF FLOW (CONT.)
Blood Flow, Pressure, and Resistance• Poiseuille’s law represents resistance• Determinants of resistance
• Vessel length• Vessel radius• Blood viscosity
• Ohm’s law represents the variables of driving pressure and resistance and their effect on blood flow
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PRINCIPLES OF FLOW (CONT.)
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PRINCIPLES OF FLOW (CONT.)
Velocity and Laminar and Turbulent Flow• Velocity: a measure of the distance traveled in a
given interval of time (centimeters/second)• Parabolic profile of laminar flow: velocity of the
layers of blood flow within the vessel vary, with blood in the center moving faster than blood in outer layers
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PRINCIPLES OF FLOW (CONT.)
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PRINCIPLES OF FLOW (CONT.)
Velocity and Laminar and Turbulent Flow• Turbulent flow is an interruption in the forward
current of blood flow by crosswise flow• Turbulent flow may manifest as:
• Bruit• Thrill• Thrombus formation
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PRINCIPLES OF FLOW (CONT.)
Wall Tension and Compliance• Law of Laplace represents the relationship
between distending pressure and wall tension• An increase in radius or distending pressure
results in increased wall tension
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PRINCIPLES OF FLOW (CONT.)
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PRINCIPLES OF FLOW (CONT.)
Dynamics in the Microcirculation: Capillaries and Lymphatics• Exchange of gases and nutrients• Dependent on pressure gradient between the
capillary and the interstitium through balance of:• Capillary hydrostatic pressure• Interstitial fluid colloid osmotic pressure• Plasma colloid osmotic pressure• Interstitial fluid pressure
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PRINCIPLES OF FLOW (CONT.)
Dynamics in the Microcirculation: Capillaries and Lymphatics• Starling’s hypothesis: net filtration is equal to the
combined forces fostering filtration minus the combined forces opposing filtration
• Capillary fluid pressure and plasma colloid osmotic pressure are most clinically relevant
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PRINCIPLES OF FLOW (CONT.)
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PRINCIPLES OF FLOW (CONT.)
Dynamics in the Microcirculation: Capillaries and Lymphatics• Changes in capillary permeability that allows
plasma proteins to leak out can result in edema• Impairment of lymphatic flow allowing fluid to
collect in the interstitium is referred to as lymphedema
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CONTROL OF FLOW
• Blood flow controlled by central mechanisms mediated by:• Autonomic nervous system• Venous and thoracic pumps• Intrinsic autoregulatory mechanisms
• Lymphatic flow controlled by:• Increasing interstitial fluid colloid osmotic pressure• Stimulation of contractile fibers (lymphatic pumps)
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CONTROL OF FLOW (CONT.)
Control of Blood Flow• Extrinsic mechanisms
• Sympathetic nervous system (SNS)—medulla–vasomotor center: responds to direct stimulation and to excitatory/inhibitory afferent stimuli
• Smooth muscle in vessels is innervated by adrenergic fibers from the SNS
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CONTROL OF FLOW (CONT.)
Extrinsic Mechanisms• Release of norepinephrine results in
vasoconstriction via α1 receptors• Release of epinephrine results in vasodilation via
β2-adrenergic receptors• Venous system flow controlled by pressure
gradient from veins and venous and thoracic pumps
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CONTROL OF FLOW (CONT.)
Intrinsic Mechanisms• Autoregulation is the ability of blood vessels
within organs to maintain a relatively constant blood flow regardless of changes in arterial pressure• High BP leads to stretch that results in constriction of
arterioles and precapillary sphincters• Low BP results in vessel dilation
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CONTROL OF FLOW (CONT.)
Control of Lymphatic Flow• Movement of lymph is enhanced by:
• Lymphatic pumps• Compression of lymphatic channels (ambulation)• Intrathoracic pressure changes (deep breathing)• Lymphatic contractions (increased BP)
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GENERAL MECHANISMS THAT CAUSE ALTERED FLOW
• Reduction in flow impairs ability to transport gases and nutrients to body tissues resulting in:• Hypoxia• Ischemia• Venous engorgement• Venous obstruction
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GENERAL MECHANISMS THAT CAUSE ALTERED FLOW (CONT.)
Blood Vessels: Obstructions• May involve arterial or venous system• Obstruction results in reduced flow beyond the
obstruction (downstream) and increased pressure before the obstruction (upstream)
• In the arterial system obstruction manifests as distal ischemia, in the venous system obstruction manifests as edema
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GENERAL MECHANISMS THAT CAUSE ALTERED FLOW (CONT.)
Thrombosis• Thrombus is a stationary blood clot formed within
a vessel or a chamber of the heart• Arterial thrombosis results in ischemia• Venous thrombosis results in edema• Prophylactic interventions include oral/parenteral
anticoagulant therapy, or drugs that block platelet activation/aggregation
• Once a thrombus has formed, anticoagulant therapy is initiated to prevent clot enlargement
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GENERAL MECHANISMS THAT CAUSE ALTERED FLOW (CONT.)
Embolus• Material that forms a clot within the bloodstream• Embolus leaving L ventricle = ischemic stroke• Embolus leaving R ventricle = pulmonary
embolus• Treatment may involve anticoagulant therapy,
embolectomy (for thromboemboli) or a filter in the inferior vena cava (Greenfield filter)
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GENERAL MECHANISMS THAT CAUSE ALTERED FLOW (CONT.)
Vasospasm• Sudden constriction of arterial smooth muscles
resulting in obstruction of flow• May be mediated by hormonal changes, food
additives, or environmental factors• Manifestations include:
• Prinzmetal angina• Hemorrhagic stroke• Migraine headaches
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GENERAL MECHANISMS THAT CAUSE ALTERED FLOW (CONT.)Inflammation• Can increase risk of thrombotic process• Vasculitis—inflammation of the intima of an
artery• Phlebitis—inflammation of the lining of a
vein• Arteritis—inflammatory process of
autoimmune origin in arteries
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GENERAL MECHANISMS THAT CAUSE ALTERED FLOW (CONT.)
Mechanical Compression• External forces to the vascular system may result
in partial or complete obstruction• May be caused by:
• Trauma• Tight casts, dressings, or stockings• Compartment syndrome
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GENERAL MECHANISMS THAT CAUSE ALTERED FLOW (CONT.)
Blood Vessels: Structural Alterations• Types of structural alterations
• Valvular incompetence• Arteriosclerosis/atherosclerosis• Aneurysms• Arteriovenous fistulas
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GENERAL MECHANISMS THAT CAUSE ALTERED FLOW (CONT.)
Lymphatic Vessels• Changes in capillary or interstitial oncotic
pressure increase filtration into tissues resulting in edema
• Lymphedema results from impairment in the circulation of lymph
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ALTERATIONS IN ARTERIAL FLOW
Alterations in arterial flow result from:• Obstruction
• Arteriosclerosis/atherosclerosis• Inflammation• Vasospasm• Thrombi/emboli• Acute occlusion
• Mechanical alterations• Arteriovenous fistulas• Aneurysms
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ALTERATIONS IN ARTERIAL FLOW (CONT.)
Arteriosclerosis/Atherosclerosis• Includes three pathologic processes:
• Mönckeberg sclerosis• Arteriolar sclerosis• Atherosclerosis
• Underlying pathologic condition leading to hypertension, cardiac and renal disease, peripheral arterial disease, and stroke
• Identification and interventions directed toward modifiable risk factor reduction
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ALTERATIONS IN ARTERIAL FLOW (CONT.)
Thromboangiitis Obliterans (Buerger Disease)• Rare inflammatory condition affecting small and
medium-size arteries and veins of upper and lower extremities resulting in varying degrees of obstruction
• Treatment entails smoking cessation (if applicable) and use of prostaglandins
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ALTERATIONS IN ARTERIAL FLOW (CONT.)
Raynaud Syndrome• Extreme vasoconstriction producing cessation of
flow to fingers and toes• Intrinsic structural factors, extrinsic
neuroregulation, and locally produced mediators are involved
• Treatment directed toward enhancing circulation through biofeedback, relaxation, calcium-channel blockers, sympatholytic drugs, and prostaglandins
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ALTERATIONS IN ARTERIAL FLOW (CONT.)
Aneurysms• Localized arterial dilations• Classified as true or false aneurysms• Frequently found in cerebral circulation and
thoracic and abdominal aorta• Dissecting aortic aneurysms are a medical
emergency and treated medically and/or surgically
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CLASSIFICATION OF ANEURYSMS
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ALTERATIONS IN ARTERIAL FLOWAcute Arterial Occlusion• Absence of arterial circulation—emergency• May result from thrombi/emboli or
mechanical compression• Classic signs and symptoms (6 Ps)
• Pallor• Paresthesia• Paralysis• Pain• Polar • Pulseless
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ALTERATIONS IN VENOUS FLOW
• Incompetent valves (obesity, pregnancy, right heart failure, prolonged standing) producing varicose veins, chronic venous insufficiency, and obstruction by deep vein thrombosis
• Accompanied by edema, venous stasis ulcers, and pain
• Deep vein thrombosis can be life threatening
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ALTERATIONS IN VENOUS FLOW (CONT.)
Valvular Incompetence• Overstretching of the valves owing to excessive
venous pressures resulting in backflow of blood• Results in:
• Venous insufficiency• Varicose veins
• Treatment is directed toward smoking cessation and regular exercise
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ALTERATIONS IN VENOUS FLOW (CONT.)
Varicose Veins• Impaired venous return results in increased
capillary pressure causing edema, and superficial, darkened, raised, and tortuous veins
• Greater saphenous vein most commonly affected• Treatment directed toward increasing venous
flow and reducing venous pressure; sclerotherapy and surgical interventions may be used for severe cases
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ALTERATIONS IN VENOUS FLOW (CONT.)
Chronic Venous Insufficiency• Results when valvular incompetence involves the
deep veins• Venous stasis ulcers are typically present• Venous ulcers treated with compression therapy
and infection control
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ALTERATIONS IN VENOUS FLOW (CONT.)
Deep Vein Thrombosis• Most frequently due to thrombus in a deep vein of
the lower extremity• May be asymptomatic• Treated aggressively with anticoagulation therapy• Previous DVT is a risk factor for further
hypercoagulation
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ALTERATIONS IN LYMPHATIC FLOW
Lymphedema• Occurs when normal flow is obstructed or altered
in some fashion• Primary lymphedema due to congenital anomaly
or dysfunction of the lymphatic system• Secondary lymphedema associated with a
disease process or iatrogenic
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ALTERATIONS IN LYMPHATIC FLOW (CONT.)
Lymphedema• Typically affects extremities• Interventions are medical (external compression
therapy and exercise) and surgical (resection of subcutaneous fatty tissue)