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]
Anatomy and Physiology 2e by J. Gordon Betts and others. Section25.4: renal corpuscle and capsular space
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 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 Dynamics of Glomerular Ultrafiltration in the Rat by Barry M. Brenner and others. Abstract: servo-null micropipette measurements and pressure difference; quantitative correction noted on publisher page
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