In vertebrates the gallbladder (cholecyst, gall bladder, biliary vesicle) is a small organ that aids mainly in fat digestion and concentrates bile produced by the liver. In humans, the loss of the gallbladder is, in most cases, easily tolerated. The surgical removal of the gallbladder is called a cholecystectomy. The gallbladder is a hollow system that sits just beneath the liver. In adults, the gallbladder measures approximately 8 centimeters (3.1 in) in length and 4 centimeters (1.6 in) in diameter when fully distended. It is divided into three sections: fundus, body, and neck. The neck tapers and connects to the biliary tree via the cystic duct, which then joins the common hepatic duct to become the common bile duct. At the neck of the gallbladder is a mucosal fold called Hartmann's pouch, where gallstones commonly get stuck. The angle of the gallbladder is located between the costal margin and the lateral margin of the rectus abdominis muscle. When food containing fat enters the digestive tract, it stimulates the secretion of cholecystokinin (CCK). In response to CCK, the adult human gallbladder, which stores about 50 milliliters (1.7 U.S. fl oz; 1.8 imp fl oz) of bile, releases its contents into the duodenum. The bile, originally produced in the liver, emulsifies fats in partly digested food. During storage in the gallbladder, bile becomes more concentrated which increases its potency and intensifies its effect on fats.