The gallbladder is a small sac and is the storage depot for bile. Bile is only concentrated in the gallbladder. The gallbladder is located at the back part surface of the liver. It is connected with the liver via the cystic duct and the hepatic bile duct. Together they form the common bile duct, which leads into the small intestine. Bile is continuously formed in bile canaliculi in the liver hepatocytes, were it is collected and secreted into the hepatic bile duct. Bile is a complex fluid containing water, electrolytes and a battery of organic molecules including bile acids (water-soluble derivatives of cholesterol), cholesterol, phospholipids and bilirubin. The rate of bile fluid secretion is dependent on the circadian rhythm of the animal (Vonk et al., 1979). 

Humans and most laboratory species have a gallbladder. However, the rat has no gallbladder. Bile from the rat liver flows directly through the (hepatic) bile duct into the small intestine (Hebel and Stromberg, 1988).

Composition and rate of bile excretion

Several studies describe that food composition, fasting/feeding and radiation are factors affecting biliary excretion and bile salt composition (in human and several animal species). The fasting state influences the lipid flow and composition of the bile. Not only food affects the composition and excretion rate of bile, also external factors do, e.g. irradiation and health condition.