What causes the gallbladder to collapse?

It's empty: The gallbladder largely functions as a bag that concentrates bile made in the liver. Between meals, bile flows from the liver into the bile duct, and it is "sucked" into the gallbladder where it is concentrated between 8-20 x. When you eat, various stimuli including cck release stimulate gallbladder contaction, and the concentrated bile is then emptied into the small intestine to mix with food.