The pharynx and esophagus
The pharynx (throat) is the transition area from the mouth to the esophagus. From the pharynx there are two paths that the food bolus can take; 1) the wrong path, which is down the windpipe into the lungs, or 2) the correct path into the esophagus and then the stomach. The act of swallowing is a complex process that closes the windpipe (to protect our lungs) and moves food into the esophagus. This process is mostly automatic (reflex) but it is also partially under our direct control.
Once it enters the esophagus, food is moved down the esophagus and into our stomach. The esophagus is a muscular tube that contracts in a synchronized fashion (peristalsis) to move food down towards the stomach. While the muscles behind the food product contract, the muscles ahead of the food relax, causing the forward propulsion of the food. Peristalsis is the main mechanism by which food moves through our digestive system.
Once the food approaches the stomach, a muscular valve (the lower esophageal sphincter) relaxes and lets the food pass into the stomach. This sphincter has the important function of closing the stomach so no food or stomach acid reenters the esophagus (and therefore avoiding heartburn or regurgitation).
The stomach and small intestine
From glands that line the stomach, acid and enzymes are secreted that continue the breakdown process of the food. The stomach muscles further mix the food. At the end of this process, the food you placed in your mouth has been transformed to a thick creamy fluid called chyme.
This thick fluid is then pushed into the duodenum (the first part of the small intestine). With the help of enzymes from the pancreas and bile from the liver, further breakdown of the food occurs in the small intestine.
The small intestine has three segments. The first segment is the duodenum where further breakdown of the food takes place. The next two parts of the small intestine (jejunum and ileum) are mostly responsible for the absorption of nutrients from the processed food into the bloodstream through the walls of the intestine.
After the small intestine, the leftover waste leaves the upper gastrointestinal tract (upper GI tract) which is made up of everything above the large intestine, and moves into the large intestine or colon (the beginning of the lower GI tract).