Schematic representation of the glomerular capillary hydraulic and oncotic pressure in normal kidneys (A and B) and pathologic kidneys with decrease of the total ultrafiltration surface (C and D). The difference between the hydraulic pressure difference [PGC, glomerular capillary hydraulic pressure-PT hydraulic pressure in Bowman's space) and the intracapillary oncotic pressure (∏GC) represents the effective filtration pressure gradient. In normal condition (A), the PGC-PT slightly decreases along the glomerular capillary axe and the ∏GC increases leading to equilibrium between the opposing forces to filtration. If renal perfusion pressure and PGC increase (B), the point of equilibrium is reached earlier along the axe due to increase of filtration fraction. GFR does not change and only increase of renal plasma flow and decrease of filtration fraction causes the GFR to increase (B). GFR is likely to increase with rise of renal perfusion pressure if the filtration surface is impaired, the point of equilibrium not being reached (C and D). Note the role of plasma oncotic pressure. Infusion of crystalloid decreases plasma oncotic pressure due to hemodilution favoring the net filtration pressure while infusion of colloids increases plasma oncotic pressure therefore reducing GFR. GFR, glomerular filtration rate.