Mini MCAT passage: Effects of pressure on GFR

The glomerular filtration rate (GFR) is used to determine how well the kidneys are functioning. The glomerulus in the kidney is a high-pressure system that is optimal for fluid filtration. In addition to differences in hydrostatic pressure (P) between the glomerular capillary and Bowman’s capsule, disparities in oncotic pressure (π) can also direct fluid movement. Oncotic pressure is determined by the concentration of protein present such that protein pulls fluid into the space it inhabits. The formula in Figure 1 defines the relationship between hydrostatic and oncotic pressure and how they determine the glomerular filtration rate.

Figure 1. The effects of hydrostatic and oncotic pressure on GFR.

GFR = k [(${\text{P}}_G$‍ + ${\text{π}}_B$‍) - (${\text{P}}_B$‍ + ${\text{π}}_G$‍)

where,

k = Filtration constant

${\text{P}}_G$‍ = Hydrostatic pressure in the glomerular capillary

${\text{π}}_B$‍ = Oncotic pressure in Bowman’s capsule

${\text{P}}_B$‍ = Hydrostatic pressure in Bowman’s capsule

${\text{π}}_G$‍ = Oncotic pressure in the glomerular capillary