From Encyclopedia Jr, free information reference for Kids
Chyme is the liquid substance found in the stomach before passing the Pyloric valve and entering the duodenum. It comes from the mechanical and chemical break down of a bolus and consists of partially digested food, water, hydrochloric acid, and various digestive enzymes. Chyme slowly passes through the pyloric sphincter and into the duodenum where the extraction of nutrients begins. Depending on the quantity and contents of the meal, the stomach will digest the food into chyme anywhere between 40 minutes and a few hours.
Chyme is acidic, as it is a very low pH, around 2, that of the stomach. To counteract this, the duodenum secretes a hormone which causes the gall bladder to contract, releasing bile (an alkali) into the duodenum, and also for the pancreas to secrete large amounts of sodium bicarbonate, to neutralize the pH to 7 before reaching the ileum. The reason why the duodenum is not affected is because of the neutralizing actions of the sodium bicarbonate and bile, and the duodenum is protected by a thick layer of mucus.