Osmotically active pool

Bile formation occurs by processes that are not hilly defined. It takes place in canaliculi, minute passages lined by specialized modihcations of the hepatocyte membrane, that ultimately unite to form bile ductules. Hepatic bile contains 5% to 15% total solids, the major component of which is bile acids. The increase in biliary water and electrolyte excretion caused by this osmotic effect represents the bile acid-dependent fraction of bile flow. Even with severe depletion of the circulating bile acid pool, as is seen with bile duct diversion, some bile flow continues. The active transport of sodium and of glutathione and bicarbonate is mediated by Na-K-ATPase, which is responsible for the bile acid-independent flow of bile (up to 40% of total flow). Hormones such as secretin increase bile flow by stimulating secretion of sodium, bicarbonate, and chloride. Hormone-dependent flow accounts for 20% to 25% of the total. 

Aqueous humor is formed in the ciliary body and its epithelium (Figs. 92-1 and 92-2) through both filtration and secretion. Because ultrafiltration depends on pressure gradients, blood pressure and lOP changes influence aqueous humor formation. Osmotic gradients produced by active secretion of sodium and bicarbonate, and possibly other solutes such as ascorbate from the ciliary body epithelial cells into the aqueous humor, result in movement of water from the pool... 

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