Bile or gall is a dark green fluid produced by the liver of human beings that aids the digestion of fats in the small intestine. In humans, bile is produced continuously by the hepatocytes of the liver and stored and concentrated in the gall bladder. After eating, the stored bile is discharged into the duodenum. Bile is composed of water, bile acids, cholesterol, sodium, potassium, and bilirubin (a waste product formed from the breakdown of red blood cells). Bile is slightly alkaline with a pH of about 7.5 to 8. Adult humans produce 400 to 800 ml of bile daily. Initially, hepatocytes secrete bile in the canaliculi, from which it flows into the bile ducts. The hepatic bile contains large quantities of bile acids, cholesterol, and other organic molecules. As the bile flow through the bile duct, it is modified by the addition of watery, bicarbonate-rich secretion from the ductal epithelial cells. The amount of bile secreted into the duodenum is controlled by the hormones cholecystokinin, secretin, gastrin, somatostatin, and also by the vagus nerve.
When the chyme reaches the duodenum, the upper portion of the duodenum and the muscle of the gall bladder wall begin to contract. This rhythmic contraction releases bile into the common bile duct, the passageway that connects the gall bladder to the small intestine. In addition to the bile, the pancreas releases lipases, which are digestive enzymes that are vital to fat digestion.

The pancreatic lipase (Steapsin) and intestinal lipase convert the fat into fatty acids and glycerol.

Fats are hydrophobic, i.e., they do not dissolve in water. Without proper digestive juices, small molecules of fats would combine together to form one large molecule of fat. Bile prevents this from happening. Bile molecules contain a hydrophobic end, as well as a hydrophilic, or water-absorbent, end. When bile enters the small intestine, bile molecules attach to the fat molecule. The fat molecule absorbs the hydrophobic end of the bile molecule, allowing the hydrophobic end to protrude from the fat molecule like a tail. This hydrophilic end repels other fat molecules, preventing them from combining together. These coated fat molecules become a particle known as micelles. Once the fat molecules are converted into micelles, the lipase enzymes from the pancreas begin to break down the fat molecules into fatty acids and monoglycerides. These substances are able to pass through the wall of the small intestine, where they enter intestinal epithelial cells and then to the lymphatic system and finally to the bloodstream.