pathophysiology chapter 15

Elsevier items and derived items © 2010, 2005 by Saunders, an imprint of Elsevier Inc.

CHAPTER 15ALTERATIONS IN BLOOD FLOW


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


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


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



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)


STRUCTURE OF BLOOD VESSELS



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


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


LYMPHATIC NETWORK


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


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



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



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



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



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



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



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


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)


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



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



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



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)


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


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



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



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)



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


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



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



Blood Vessels: Structural Alterations• Types of structural alterations

• Valvular incompetence• Arteriosclerosis/atherosclerosis• Aneurysms• Arteriovenous fistulas



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


ALTERATIONS IN ARTERIAL FLOW

Alterations in arterial flow result from:• Obstruction

• Arteriosclerosis/atherosclerosis• Inflammation• Vasospasm• Thrombi/emboli• Acute occlusion

• Mechanical alterations• Arteriovenous fistulas• Aneurysms


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



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



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



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


CLASSIFICATION OF ANEURYSMS


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


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


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



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



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



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


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


ALTERATIONS IN LYMPHATIC FLOW (CONT.)

Lymphedema• Typically affects extremities• Interventions are medical (external compression

therapy and exercise) and surgical (resection of subcutaneous fatty tissue)

pathophysiology chapter 15

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