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.