Every collecting duct (CD) of the rat inner medulla is uniformly surrounded by about four abutting ascending vasa recta (AVR) running parallel to it. One or two ascending thin limbs (ATLs) lie between and parallel to each abutting AVR pair, opposite the CD. These structures form boundaries of axially running interstitial compartments. Viewed in transverse sections, these compartments appear as four interstitial nodal spaces (INSs) positioned symmetrically around each CD. The axially running compartments are segmented by interstitial cells spaced at regular intervals. The pairing of ATLs and CDs bounded by an abundant supply of AVR carrying reabsorbed water, NaCl and urea make a strong argument that mixing of NaCl and urea within the INSs, and countercurrent flows play a critical role in generating the inner medullary osmotic gradient. This study fully supports that hypothesis. We quantified interactions of all structures comprising INSs along the corticopapillary axis for 2 rodent species, the Munich-Wistar rat and the kangaroo rat. Results show remarkable similarities in configurations of INSs, suggesting that the structural arrangement of INSs is a highly conserved architecture that plays a fundamental role in renal function. The number density of INSs along the corticopapillary axis directly correlates with a loop population that declines exponentially with distance below the outer medullary-inner medullary boundary. Axial configurations are consistent with discrete association between near-bend loop segments and INSs, and with upper loop segments lying distant from INSs.