Bulk Flow: Filtration and Reabsorption
Bulk Flow : is a passive process where large numbers of ions, molecules, or particles in a fluid move together in the same direction. These substances move at rates greater than by diffusion alone. Bulk flow occurs from an area of high pressure to an area of low pressure and continues as long as the pressure difference exists. Filtration : Pressure-driven movement of fluid and solutes from blood capillaries into interstitial fluid Reabsorption : Pressure-driven movement of fluid and solutes from interstitial fluid into blood capillaries Starling’s Law of the Capillaries: states that overall, the volume of fluid and solutes reabsorbed normally is almost as large as the volume filtered.
2 pressures promote filtration :
1) Blood Hydrostatic Pressure (BHP)
2) Interstitial Fluid Osmotic Pressure (IFOP)
2 pressures promote reabsorption :
1) Blood Colloid Osmotic Pressure (BCOP)
2) Interstitial Fluid Hydrostatic Pressure (IFHP)
The balance of these pressures is called Net Filtration Pressure (NFP) and determines whether volumes of blood and interstitial fluid remain steady or change. Blood Hydrostatic Pressure (BHP) - Within vessels, the hydrostatic pressure is due to the pressure that water in blood plasma exerts against blood vessel walls. BHP “ pushes ” fluid out of the capillaries into the interstitial fluid. BHP is about 35mmHg at the arterial end of a capillary and about 16mmHg at the capillary’s venous end.
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