Bile (gall) is a bitter-tasting greenish-yellow alkaline fluid produced by the liver, stored in the gall bladder, and secreted into the duodenum of vertebrates. It assists the digestion and absorption of fats by the action of bile salts, which chemically reduce fatty substances and decrease the surface tension of fat droplets so that they are broken down and emulsified. Bile may also stimulate gut muscle contraction (peristalsis). Bile also contains the bile pigments, bilirubin and biliverdin, which are produced by the breakdown of the blood pigment haemoglobin.