Question

In the kidneys the pressure gradient or effective filtration pressure (EFP) is determined by

• A. Blood colloidal osmotic pressure
• B. Glomerular hydrostatic pressure
• C. Capsular hydrostatic pressure
• D. All of the above

Answer 1

The correct option is D All of the above

Effective filtration pressure (EFP) refers to the net driving forces for water and solute transport across the glomerular membrane. EFP is a function of three variables - 1) the hydrostatic pressure driving fluid out of the glomerular capillary, 2) the hydrostatic pressure driving fluid into the Bowman's capsule and 3) the colloid osmotic gradient bringing fluid into the glomerular capillary. Using these variables, the EFP is expressed as:
EFP = $P_{cap}-(P_{Bow}+CO{P}_{cap})$
Thus hydrostatic and osmotic pressures play a important role in determining the pressure gradient or effective filtration pressure. When hydrostatic pressure exceeds osmotic pressure, filteration occurs and when osmotic pressure exceeds hydrostatic pressure, reabsorption occurs. Thus, $P_{cap}$ describes outward forces promoting filtration and $CO{P}_{cap}+P_{Bow}$ describes inward forces opposing filtration. The colloid osmotic pressure in Bowman's capsule $CO{P}_{Bow}$ is not expressed in this equation because glomerular filtrate normally contains little protein.