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The capsular space is the fluid-receiving space within the glomerular capsule, between its outer wall and the visceral surface formed by podocytes. Glomerular filtration delivers water and filterable solutes into this space, which is continuous with the lumen of the proximal tubule. It belongs to the tubular-fluid pathway, rather than the lumen carrying blood through glomerular capillaries.^[ref]

A useful tracing exercise follows two routes through a renal corpuscle. Blood enters and leaves through vessels; filtered fluid crosses the glomerular filtration barrier, enters the capsular space and exits into the tubule. A section showing both routes does not show direct mixing of whole blood with the capsular contents.

The receiving hydrostatic pressure also opposes further filtration. If every other term in a simplified trans-barrier pressure model is fixed, raising receiving pressure by $5\mathrm{mmHg}$ lowers net driving pressure by the same amount. This hypothetical subtraction explains why arterial pressure alone is insufficient. In 1971, direct glomerular and proximal-tubular pressure measurements in rats were combined to estimate the driving force for ultrafiltration.^[ref]

The glomerular capsule, or Bowman's capsule, is the cup-like epithelial structure surrounding the glomerulus in a renal corpuscle. Its outer wall and its visceral layer, formed by podocytes, enclose a capsular space. Fluid crossing the glomerular filtration barrier enters that space and then passes into the proximal tubule.^[ref]

The space, its contents and its wall are distinct. In a section, identify the capillary tuft first, then the narrow receiving region around it, then the outer capsule. A renal corpuscle includes the tuft and capsule together; the capsule alone is not a second glomerulus. This observation prevents an anatomical label from being mistaken for an independent filtration unit.

The receiving compartment also matters mechanically. In a simplified filtration model, increasing hydrostatic pressure in the capsular space opposes fluid movement from the glomerular capillaries. Measuring only arterial pressure would therefore miss one side of the trans-barrier driving difference. Direct glomerular and tubular pressure measurements were an important part of the 1971 rat experiments establishing the physical basis of glomerular filtration.^[ref]

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]