THE PROXIMAL TUBULE REABSORBS essentially all the filtered organic solutes, most of the filtered phosphate, 80% of the filtered bicarbonate, and 60% of the filtered sodium chloride. Approximately 70% of the filtered water is also reabsorbed by this segment. Despite these very high rates of proximal tubule solute transport, the osmolality of the luminal fluid decreases by only 5 mosmol/kgH2O fro...