The gall bladder is an organ where bile is stored and concentrated, before it is released into the small intestines.
The main function of the gall bladder is to store bile.
The bile stored in the bladder is concentrated , by removal of some water and electrolytes. This happens through active transport of sodium ions across the epithelia of the gall bladder. This creates an osmotic pressure that causes water and other electrolytes such as chloride to be reabsorbed.
When foods containing fats enter the digestive tract, it stimulates the secretion of cholecystokinin. This causes the rhythmic contraction of gall bladder , therby realising its contents.