Structure and function of the inner medullary collecting duct

Abstract

Until recently the IMCD was considered to be one segment that was homogeneous along its length with respect to its functional properties. However, because of morphologic differences observed along the IMCD and in order to facilitate the description of structural as well as functional parameters, we have recently proposed to subdivide the IMCD into three portions and use the following nomenclature: outer third (IMCD1), middle third (IMCD2), and inner third (IMCD3). These are arbitrary subdivisions. The changes along the IMCD are gradual and there are no abrupt transitions between the three segments. There is now evidence from physiologic studies that the IMCD consists of at least two functionally distinct segments which have been termed the initial IMCD and the terminal IMCD. As illustrated in Figure 1, the initial IMCD is the outer portion corresponding to the IMCD1, whereas the terminal IMCD is located in the papilla and corresponds to the IMCD2 and IMCD3. The present review will focus on the differences that exist along the IMCD with respect to both structure and function, and attempt to establish a subsegmentation that is consistent with structural as well as functional properties.