<P> The principal cells are the sodium - transporting cells in the collecting duct system . </P> <P> Although only a fragment of total reabsorption happens here, it is the main part of intervention . This is e.g. done by endogenous production of aldosterone, increasing reabsorption . Since the normal excretion rate of sodium is ~ 100mmoles / day, then a regulation of the absorption of still more than 1000 mmoles / day entering the collecting duct system has a substantial influence of the total sodium excreted . </P> <Table> Characteristics of Na reabsorption <Tr> <Th> Characteristic </Th> <Th_colspan="3"> proximal tubule </Th> <Th_colspan="3"> loop of Henle </Th> <Th> Distal convoluted tubule </Th> <Th_colspan="4"> Collecting duct system </Th> </Tr> <Tr> <Td> S1 </Td> <Td> S2 </Td> <Td> S3 </Td> <Td> descending limb </Td> <Td> thin ascending limb </Td> <Td> thick ascending limb </Td> <Td> connecting tubule </Td> <Td> initial collecting tubule </Td> <Td> cortical collecting ducts </Td> <Td> medullary collecting ducts </Td> </Tr> <Tr> <Td> reabsorption (%) </Td> <Td_colspan="3"> 67% </Td> <Td> </Td> <Td> </Td> <Td> 25% </Td> <Td_colspan="4"> 5% </Td> <Td> 3% </Td> </Tr> <Tr> <Td> reabsorption (m moles / day) </Td> <Td_colspan="3"> ~ 17,000 </Td> <Td> </Td> <Td> </Td> <Td> ~ 6,400 </Td> <Td_colspan="4"> ~ 1,300 </Td> <Td> ~ 700 </Td> </Tr> <Tr> <Td> Concentration (m M) </Td> <Td> 142 </Td> <Td> </Td> <Td> 142 </Td> <Td> </Td> <Td> </Td> <Td> 100 </Td> <Td> 70 </Td> <Td> </Td> <Td> </Td> <Td> 40 </Td> <Td> </Td> </Tr> <Tr> <Td> electrical driving force (m V) </Td> <Td> - 3 </Td> <Td> </Td> <Td> + 3 </Td> <Td> </Td> <Td> </Td> <Td> + 15 </Td> <Td> - 5 to + 5 </Td> <Td> </Td> <Td> </Td> <Td> - 40 </Td> <Td> </Td> </Tr> <Tr> <Td> chemical driving force (m V) </Td> <Td> 0 </Td> <Td> </Td> <Td> 0 </Td> <Td> </Td> <Td> </Td> <Td> - 9 </Td> <Td> - 19 </Td> <Td> </Td> <Td> </Td> <Td> - 34 </Td> <Td> </Td> </Tr> <Tr> <Td> electrochemical driving force (m V) </Td> <Td> - 3 </Td> <Td> </Td> <Td> + 3 </Td> <Td> </Td> <Td> </Td> <Td> + 6 </Td> <Td> - 24 to - 14 </Td> <Td> </Td> <Td> </Td> <Td> - 74 </Td> <Td> </Td> </Tr> <Tr> <Td> apical transport proteins </Td> <Td> SGLT, Na - H antiporter </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> (passively) </Td> <Td> Na - K - 2Cl symporter (Na - H antiporter and passively) </Td> <Td> sodium - chloride symporter </Td> <Td> </Td> <Td> </Td> <Td> ENaC </Td> <Td> </Td> </Tr> <Tr> <Td> basolateral transport proteins </Td> <Td_colspan="11"> Na / K - ATPase </Td> </Tr> <Tr> <Td> Other reabsorption features </Td> <Td_colspan="3"> isosmotic </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td_colspan="4"> by principal cells, stimulated by aldosterone </Td> </Tr> <Tr> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> </Table> <Tr> <Th> Characteristic </Th> <Th_colspan="3"> proximal tubule </Th> <Th_colspan="3"> loop of Henle </Th> <Th> Distal convoluted tubule </Th> <Th_colspan="4"> Collecting duct system </Th> </Tr>

Where does the highest rate of tubular reabsorption occur