The liver is the next link in the digestive process. It produces bile, which is essential to the digestion of fats. Bile is stored in the gall bladder prior to its release into the small intestine. It contains no enzymes. It acts as a detergent to emulsify fats (break down large fat globules into smaller droplets). Emulsification of fats exposes a greater surface area to the action of pancreatic lipase.
The liver helps regulate blood glucose levels and produces urea. Glucose and other monosaccharides absorbed during digestion are delivered to the liver via the hepatic portal vein. Glucose-rich blood is processed by the liver, which converts excess glucose to glycogen for storage. If the blood has a low glucose concentration, the liver converts glycogen into glucose and releases it into the blood, restoring blood glucose levels to normal. In addition, the liver synthesizes glucose from noncarbohydrate precursors via the process of gluconeogenesis. Glycogen metabolism is under both hormonal and nervous control.
Pancreatic lipase is produced and secreted by the pancreas, which is also responsible for manufacturing amylase (for starch digestion), trypsin, and chymotrypsin (for protein digestion). Unlike pepsin, these enzymes have a pH optimum in the alkaline range. The necessary alkaline environment is created by the release of large quantities of bicarbonate ion (HCO3-) by the pancreas along with the digestive enzymes. This bicarbonate neutralizes the acidity of the chyme released into the duodenum from the stomach.
Pancreatic proteases like trypsin are produced and stored in an inactive form called zymogen. The zymogen is activated after secretion when it is cut by a protease. This prevents damage to the pancreatic tissues that could occur if proteases were synthesized in their active state.