The secretion of bile

Summary

Separate active transport mechanisms may account for the entry of bile salts and bilirubin into the biliary canaliculi from the hepatic parenchymal cells. Extracellular fluid follows in response to osmotic gradients and possibly also by an independent secretory mechanism. This “primary bile” probably is modified, as it passes down the bile ducts, by bidirectional movements of electrolytes across the epithelium. At least one active transport process involving bicarbonate is involved in a secretory mechanism in the distal ducts. The control of bile secretion involves three major factors: the presence of bile salts, gastrointestinal hormones, and vagal innervation. It is likely that the hepatic circulation may also influence bile secretion; presumably, the bile salt concentration of portal venous blood determines the secretion of bile salts by the parenchymal cells, but the details of the mechanism are missing. Gastrin, secretin, cholecystokinin-pancreozymin, and glucagon all increase bile flow, particularly by increasing the flow of sodium chloride and bicarbonate solutions. Therefore, the control of bile secretion can be divided into the secretion of bile salts and the secretion and reabsorption of water and electrolyte. The latter seems to be particularly subject to neurohumoral influences. Factors determining the secretion of bilirubin are less well understood but may include the effects of glucagon.