In the proximal tubule of the kidney, Na+ and HCO3- reabsorption vary propo
rtionally with changes in axial flow rate. This feature is a critical compo
nent of glomerulotubular balance, but the basic mechanism by which the tubu
le epithelial cells sense axial flow remains unexplained. We propose that t
he microvilli, which constitute the brush border, are physically suitable t
o act as a mechanosensor of fluid flow. To examine this hypothesis quantita
tively, we have developed an elastohydrodynamic model to predict the forces
and torques along each microvillus and its resulting elastic bending defor
mation. This model indicates that: 1) the spacing of the microvilli is so d
ense that there is virtually no axial velocity within the brush border and
that drag forces on the microvilli are at least 200 times greater than the
shear force on the cell's apical membrane at the base of the microvilli; 2)
of the total drag on a 2.5-mu m microvillus, 74% appears within 0.2 mu m f
rom the tip; and 3) assuming that the structural strength of the microvillu
s derives from its axial actin filaments, then a luminal fluid flow of 30 n
l/min produces a deflection of the microvillus tip which varies from about
1 to 5% of its 90-nm diameter, depending on the microvilli length. The micr
ovilli thus appear as a set of stiff bristles, in a configuration in which
changes in drag will produce maximal torque.