Fig. 2. The MR cell. With this configuration of transport elements, both Na⁺ and Cl^– can be absorbed from ambient NaCl, and Cl^– can be taken up from Na⁺-free solution. As pictured, it lacks a basolateral mechanism for eliminating HCO₃^– and hence would be unable to absorb Na⁺ from Cl^–-free media. Whether this is, in fact, the case remains to be determined. The apical Cl^– channel is not shown, since it would be closed in FW.