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Lecture 9

Lecture 9.docx

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University of Otago
Daryl Schwenke

Capillaries and Veins Objectives:  Understand the functional anatomy of the microcirculation: more than one design  Understand the balance of fluid exchange through the capillary wall; role of the lymphatic system  Understand the blood reservoir function of the veins. Capillary Circulation:  Several routes between arterioles and venules. Primary function of capillaries is the exchange of nutrients and metabolic products:  Thin walled = no smooth muscle  Rich branching network  Large total cross sectional area  Very low blood flow velocity Capillary: Fluid Exchange:  Water is driven across capillary wall by a balance between: 1. Hydrostatic (perfusion) pressure – moves water out of the capillary 2. Colloid osmotic pressure – generated by plasma proteins dissolved in the plasma: draws water into the capillary NB: Lymphatics drain excess fluid from ISF back into systemic circulation. Balance of fluid exchange:  Small changes in capillary pressure can alter the amount of water shifting in or out of the capillaries.  A major determinant of capillary pressure is the amount of resistance n the small arterioles up stream  Some capillary beds are designed : a) for net filtration b) highly efficient lymph absorption (lungs) Veins:  Veins are low resistance, high capacitance vessels = high compliance  The slope of a P-V curve of a blood vessel is a measure of a vessel wall’s compliance.  Veins have: steep P-V relationship initially (can therefore hold a large volume for a small change in pressure), until there is a sudden plat
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