Cholecystokinin (a hormone) is secreted by duodenum in response to presence of food. It stimulates flow of pancreatic enzymes and contraction of gall bladder. Enzymes are biological catalysts which taken part in various chemical reactions of the body. Bile pigments are secreted by liver. Lipids include fats, waxes and oils, steroids and sterols (e.g., cholesterol). These are heterogenous groups of fat and fat like substances.