H2o

Reabsorption

Epithelium

Lumen

Fig. 9.24 Main mechanisms for secretion and reabsorption of fluid and electrolytes. In secretion the active mechanism (AT) is an electrogenic secretion of CI". This is achieved by Cl~ entry into the cells through a variety of co- or counter-transport mechanisms (fuelled ultimately by the Na+ gradient) and Cl~ exit to the lumen through Cl~ channels that can be regulated by a variety of mechanisms. When the CI" channels are open, a net CI" current flows, generating a transepithelial potential. Na+ ions flow down this electrical gradient passively (P) largely through the paracellular route, and water follows using the osmotic gradient (O) generated by the ions. In reabsorption, the active process is Na+ reabsorption. This is achieved by Na+ entering the luminal border through a variety of co- and counter-transport processes down its electrochemical gradient, and being expelled to the interstitial fluid by the Na pump. This generates a current and a lumen-positive transepithelial gradient. CI" and water follow passively down the electrical and osmotic gradients.

cyclase, and activates CI- channels through a cAMP-dependent protein kinase (Fig. 9.25).

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