Figure 3.

Feedback relationship between GFR and proximal nephron Na reabsorption. While GFR directly determines Na reabsorption and thereby late proximal (LP) flow rate (solid arrow and inset 1), LP flow rate inversely affects GFR (broken arrow and inset 2). The intersection of the two relationships when plotted in the same graph (inset 3) defines the operating point of this feedback system. LP flow rate is an experimentally controllable surrogate of NaCl concentration at the macula densa sensor site. Bold arrows indicate two principal perturbations: a change in filtration forces and a change in Na reabsorption. Insets 4 and 5 indicate that GFR will tend to return to normal with changes in the former but deviate from normal with changes in the latter.