= 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> as
$$
Q_f=K_f\bigl[(P_g-P_c)-(\pi_g-\pi_c)\bigr],
$$
where $K_f$ 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.<The Dynamics of Glomerular Ultrafiltration in the Rat by Barry M. Brenner and others>[Introduction and Abstract: hydrostatic/colloid-osmotic driving forces and ultrafiltration coefficient]{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 $K_f=2\,\mathrm{mL\,min^{-1}\,mmHg^{-1}}$, then $Q_f=20\,\mathrm{mL/min}$. 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.<The Dynamics of Glomerular Ultrafiltration in the Rat by Barry M. Brenner and others>[Abstract and Methods]{ref}
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