Glomerular filtration

Glomerular filtration is transfer of water and filterable solutes from glomerular blood plasma across the glomerular filtration barrier into the capsular space. Most blood cells and much of the plasma protein remain in the circulation. The resulting fluid begins the tubular pathway; its composition can subsequently change through reabsorption and secretion, so filtration is not identical to final renal excretion.

A simplified pressure model writes the filtered volumetric flow rate aswhere $K_f$ combines hydraulic permeability and filtering area, $P$ denotes hydrostatic pressure, $\pi$ denotes oncotic pressure, and subscripts $g,c$ refer to glomerular capillary and capsular fluid. Here the pressure terms represent effective averages, with freely transmitted water pressure and a simplified protein-retention assumption. Actual pressures and concentrations vary along a capillary.^[ref]

For a hypothetical calculation, take $P_g=55$, $P_c=15$, ${\pi }_g=30$ and ${\pi }_c=0$, all in $\mathrm{mmHg}$. The net driving pressure is $10\mathrm{mmHg}$. If $K_f=2\mathrm{mL}\mathrm{mi}{\mathrm{n}}^{-1}\mathrm{mmH}{\mathrm{g}}^{-1}$, then $Q_f=20mL/min$. These chosen values illustrate the model, rather than specifying normal human measurements. Increasing receiving-space pressure reduces filtration even if capillary pressure stays fixed.

In 1971, investigators used rats with unusually accessible surface glomeruli and servo-null micropipette measurements to test these driving forces directly. Combining pressure measurements with single-nephron filtration measurements made it possible to estimate the barrier's hydraulic behavior, instead of inferring it from final urine alone.^[ref]