<P> Straight segments descend into the outer medulla . They terminate at a remarkably uniform level and it is their line of termination that establishes the boundary between the inner and outer stripes of the outer zone of the renal medulla . </P> <P> As a logical extension of the nomenclature described above, this segment is sometimes designated as S3 . </P> <P> The proximal tubule regulates the pH of the filtrate by exchanging hydrogen ions in the interstitium for bicarbonate ions in the filtrate; it is also responsible for secreting organic acids, such as creatinine and other bases, into the filtrate . </P> <P> Fluid in the filtrate entering the proximal convoluted tubule is reabsorbed into the peritubular capillaries . This is driven by sodium transport from the lumen into the blood by the Na / K ATPase in the basolateral membrane of the epithelial cells . Sodium reabsorption is primarily driven by this P - type ATPase . This is the most important transport mechanism in the PCT . </P>

Where does primary active transport of sodium occur in proximal convoluted tubule cells