<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> <Table> <Tr> <Td> Substance </Td> <Td>% of filtrate reabsorbed </Td> <Td> Comments </Td> </Tr> <Tr> <Td> salt and water </Td> <Td> approximately two - thirds </Td> <Td> Much of the mass movement of water and solutes occurs through the cells, passively across the luminal membrane via transcellular transport, followed by active resorption across the basolateral membrane via the Na / K / ATPase pump . The solutes are absorbed isotonically, in that the osmotic potential of the fluid leaving the proximal tubule is the same as that of the initial glomerular filtrate . </Td> </Tr> <Tr> <Td> organic solutes (primarily glucose and amino acids) </Td> <Td> 100% </Td> <Td> Glucose, amino acids, inorganic phosphate, and some other solutes are resorbed via secondary active transport through co-transporters driven by the sodium gradient out of the nephron . </Td> </Tr> <Tr> <Td> potassium </Td> <Td> approximately 65% </Td> <Td> Most of the filtered potassium is resorbed by two paracellular mechanisms - solvent drag and simple diffusion . </Td> </Tr> <Tr> <Td> urea </Td> <Td> approximately 50% </Td> <Td> Paracellular fluid reabsorption sweeps some urea with it via solvent drag . As water leaves the lumen, the concentration of urea increases, which facilitates diffusion in the late proximal tubule . </Td> </Tr> <Tr> <Td> phosphate </Td> <Td> approximately 80% </Td> <Td> Parathyroid hormone reduces reabsorption of phosphate in the proximal tubules, but, because it also enhances the uptake of phosphate from the intestine and bones into the blood, the responses to PTH cancel each other out, and the serum concentration of phosphate remains approximately the same . </Td> </Tr> <Tr> <Td> citrate </Td> <Td> 70%--90% </Td> <Td> Acidosis increases absorption . Alkalosis decreases absorption . </Td> </Tr> </Table>

Explain how epithelial cells of the proximal convoluted tubule are adapted for reabsorption