To understand exactly what happens during and after gastric bypass surgery, it's helpful to know a little about the human digestive tract. The stomach is located in the upper abdomen, under the lower ribs. When you swallow food, it moves from your mouth to your esophagus and ultimately lands in the sac-shaped stomach. There, strong stomach acids begin the digestive process. It takes nearly three hours for the food to liquefy and then move into the first section of your small intestine, known as the duodenum. In this section, pancreatic juices and bile speed digestion and the majority of the body's nourishment and calories are absorbed. Food is then moved to the middle section of the small intestine, the jejunum, then on to the final section of the small intestine, the ileum. In these sections, the absorption of calories and nutrients take place on a smaller scale. From the small intestine, any undigested food is passed to the large intestine, where it remains until it is eliminated.
Gastric bypass surgery works by altering this digestive process in two ways. It decreases the size of the stomach and causes food to bypass part of the small intestine. These two steps result in the patient feeling fuller more quickly and absorbing fewer calories.
The two gastric bypass surgeries currently in use are the Roux-en-Y gastric bypass and the extensive gastric bypass, or biliopancreatic diversion. The latter of these two surgeries can help a patient lose weight, but it carries of high risk of nutritional deficiencies and a higher risk of death than Roux-en-Y surgery. For this reason, surgeons don't use it as often as they use the Roux-en-Y bypass, which is the most common gastric bypass surgery in the United States.
The Roux-en-Y gastric bypass derives its name from the rearrangement of the small intestines into a Y-shaped configuration. One part of this Y-shape is referred to as a Roux limb. It moves food from the new upper stomach pouch into the small intestine, thus bypassing the lower stomach, the duodenum, and the first portion of the jejunum in order to reduce absorption.