Relationship between glomerular filtration rate and the tubular flow rate at the end of the proximal tubule. Late proximal flow rate is used as an experimentally controllable surrogate for the true TGF signal, the NaCl concentration at the macula densa. Values given are valid for the rat. Arrows with open heads indicate increasing levels of nitric oxide and angiotensin II. The solid line represents the relationship for the control state; the dotted lines indicate resetting of the TGF curve to the right following prolonged administration of benzolamide or ECV expansion or to the left following ECV depletion. The open circles indicate the operating points for the different conditions. ECV, extracellular volume.