Fig. 7.

Fluid and salt reabsorption in the renal proximal tubule. (A) The geometric and transport properties of the proximal tubule. The new filtrate c_f= c_o (moles/cm³) is generated by the glomerulus at z = 0. As the filtrate flows along the lumen of the tubule, salt is transported out of the tubule across apical membranes at a rate j_A (moles/cm²/s) and water follows at a rate u_A (cm/s). The osmolarity of the transported solution, j_A/u_A (moles/cm³), is initially c_o/0.86 (Eq. 22 and related text), implying c_f is decreasing with increasing distance from the glomerulus. The transport equations are derived in the text, where approximate solutions are presented. (B) A graphical representation of the solution for c_f as a function of distance from the glomerulus. Based on parameter values in Table 2, ε = 10⁻³ and the total water permeabilities of apical and basolateral membranes are the same, hence α = 1. Initially c_f = c_o = 300 mM, but it exponentially declines to its steady-state value of (1−(1+α)ε)c_o = 399.4 mM with a length constant λ = 0.3 cm. At steady state, the osmolarities of the transported solution and the filtrate are the same: c_f = j_A/u_A.