<P> The proximal tubule as a part of the nephron can be divided into two sections, pars convoluta and pars recta . Differences in cell outlines exist between these segments, and therefore presumably in function too . </P> <P> Regarding ultrastructure, it can be divided into three segments, oS1, S2, and S3: </P> <Table> <Tr> <Th> Segment </Th> <Th> Gross divisions </Th> <Th> Ultrastructure divisions </Th> <Th> Description </Th> </Tr> <Tr> <Td> Proximal tubule </Td> <Td> convoluted </Td> <Td> S1 </Td> <Td> Higher cell complexity </Td> </Tr> <Tr> <Td> S2 </Td> <Td> </Td> </Tr> <Tr> <Td> straight </Td> </Tr> <Tr> <Td> S3 </Td> <Td> Lower cell complexity </Td> </Tr> </Table> <Tr> <Th> Segment </Th> <Th> Gross divisions </Th> <Th> Ultrastructure divisions </Th> <Th> Description </Th> </Tr>

When sodium ions are reabsorbed through the wall