In the Roux-en-Y gastric bypass, the rest of the stomach is sectioned off from the new pouch but left intact. It still has a blood supply and drains any small amount of fluid through the small intestine, or duodenum. Because it is not used, it shrinks in size and stays dormant.
In the sleeve gastrectomy, approximately 75% of the stomach is permanently removed leaving a smaller banana shaped stomach that holds approximately ½ cup of food per meal.