When you eat or drink, food or liquid travels from the mouth through a hollow tube called the esophagus into your stomach.
In your stomach, acids and enzymes break down the food. From the stomach, the food enters the small intestine, another hollow organ that functions to digest and absorb certain parts of the food.
After the remaining food leaves the small intestine, it enters the colon. The colon has almost nothing to do with digestion. Its main function is to remove water from the stool and store it so that you can have a bowel movement and clear the colon of waste.
During the upper endoscopy, your doctor will be looking only at the upper digestive tract. This includes the esophagus, stomach and the first part of the small intestine, which is called the duodenum.