Bulk flow - Factors that influence bulk flow

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• Bulk flow helps regulate the relative volumes of blood and interstitial fluid. • Flow from blood to interstitium is called filtration. ■ Flow from interstitium to blood is called reabsorption. ■ Four factors determine the net direction of capillary exchange. ■ These factors include: • Blood hydrostatic pressure (BHP), which pushes fluid out of the capillary and promotes filtration. • Blood colloid osmotic pressure (BCOP), which acts as if it "pulls" fluid into the capillary and promotes reabsorption. • Interstitial fluid osmotic pressure (IFOP), which is the slight pull of fluids towards the interstitium promoting filtration. • Interstitial fluid hydrostatic pressure (IFHP), which is near zero under normal conditions. Net filtration pressure and lymph formation • The net filtration pressure (NFP) is the force promoting filtration minus the force promoting reabsorption. ■ At the arterial end of an ideal capillary, the filtration pressures are stronger. The result: net filtration. ■ At the venous end of an ideal capillary, the reabsorption pressures are stronger. The result: net reabsorption. ■ About 90% of the fluid that is filtered at the arterial end is reabsorbed at the venous end. ■ The remaining fluid drains into lymphatic capillaries to form lymph. Edema ■ An abnormal buildup of interstitial fluid results in swelling known as edema. • Major causes of edema include: • Increased capillary hydrostatic pressure • Increased capillary permeability • Decreased blood colloidal osmotic pressure • Blockage of lymphatic drainage Blood velocity ■ Blood velocity is inversely related to total cross-sectional area; the greater the area, the slower the velocity. • Capillaries have a higher total cross-sectional area; therefore blood flows more slowly. ■ Slower blood flow allows greater time for exchange of materials. ■ Arteries and veins have lower total cross-sectional areas; blood flows more quickly. • Blood velocity slows as blood travels away from heart, increases as it returns to heart.

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