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Capillary Fluid Exchange: Mechanisms and Regulation

This lecture explores the different mechanisms involved in capillary-tissue fluid exchange, including diffusion, osmosis, filtration, and vesicular transport. It also discusses the regulation of capillary blood flow and factors that affect the net amount of fluid leaving the capillaries.

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Capillary Fluid Exchange: Mechanisms and Regulation

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  1. Marieb’s Human Anatomy and Physiology Ninth Edition Marieb w Hoehn Chapter 19 Blood Vessels Lecture 4 Part 1b: Blood Flow and Fluid Exchange

  2. Differences in Blood Flow: At Rest/Exercise Figures from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

  3. Regulation of Capillary Blood Flow Figure from: Hole’s Human A&P, 12th edition, 2010 • Precapillary sphincters • may close a capillary • respond to needs of the cells • low oxygen and nutrients cause sphincter to relax

  4. Exchange in the Capillaries • major mechanism involved in exchange of solutes is diffusion • substances move in and out along the length of the capillaries according to their respective concentration gradients • Fluid movement in systemic capillaries is determined by two major factors • 1. hydrostatic pressure;varies along portions of capillary • 2. osmotic pressure; remains about the same along the length of the capillary Excess tissue fluid is drained via lymphatics Figure from: Hole’s Human A&P, 12th edition, 2010

  5. Forces Acting Across Capillary Walls Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 What would increase the net amount of fluid leaving the capillaries? What would decrease the net amount of fluid leaving the capillaries?

  6. Venules and Veins • Venule • thinner wall than arteriole; larger lumen • less smooth muscle and elastic tissue than arteriole • Vein • thinner wall than artery; larger lumen • three layers to wall but middle layer is poorly developed • some have flaplike valves • carries blood under relatively low pressure • serves as blood reservoir • are able to constrict (sympathetic innervation)

  7. Venous Valves Figure from: Hole’s Human A&P, 12th edition, 2010 Valves aid one-way blood flow since pressure is low in veins If the walls of veins near valves become weakened, valves may fail, blood will pool, vessels will become distended, e.g., varicose veins

  8. Blood Volumes in Vessels Figure from: Hole’s Human A&P, 12th edition, 2010 Important for control of blood pressure At any one time, most of the body’s blood is in the venous system; thus they are a major factor influencing venous return to the heart.

  9. Venous Blood Flow • not just a direct result of heart action • depends on skeletal muscle contraction (skeletal muscle pump) • depends on breathing (respiratory pump) • depends on venoconstriction Figure from: Hole’s Human A&P, 12th edition, 2010

  10. Review • Capillary-tissue exchange is dependent upon • Diffusion (Exchange of solutes) • Osmosis (colloid osmotic pressure) • Filtration • Vesicular transport Exchange of fluid

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