The gall bladder is a reservoir for bile. During fasting, resistance to
flow through the sphincter is high, and bile excreted by the liver is
diverted to the gall bladder. After feeding, the resistance to flow
through the sphincter of Oddi is reduced, the gall bladder contracts,
and the bile enters the duodenum. These motor responses
of the biliary tract are in part effected by the hormone CCK.
The second main function of the gall bladder is concentration
of bile by active absorption of water, sodium chloride and bicarbonate by the mucous membrane of the gall bladder.

The hepatic bile that enters the gall bladder becomes concentrated
5–10 times, with a corresponding increase in the proportion of
bile salts, bile pigments, cholesterol and calcium.
The third function of the gall bladder is the secretion of mucus
– approximately 20 ml is produced per day. With total obstruction
of the cystic duct in a healthy gall bladder, a mucocele develops
on account of this function of the mucosa of the gall bladder