The lower esophageal sphincter (LES). This regulates the food to leave the stomach to the intestine. I had a lot of other things wrong that caused me to have the roux-en-y. I had bad acid reflux that was causing stomach acid to go to my lungs. I had a botched nissen fundoplication surgery 2 years ago. I had another hiatal hernia (came back after repaired when I had the fundoplication), I had a very rare condition called chilaiditi syndrome. I was in the ER at least once a month for the past few years with extreme pain. My surgery was March 15th and 4 hours which resulted in me losing the bottom 80% of my stomach, had some of my intestine removed, hernia fixed, and the roux-en-y. I was 205 before surgery and I now weigh 170 lbs. I have stopped losing weight. Due to losing the majority of my stomach I have to eat small amounts throughout the day. I was told when you have just the roux-en-y you also have to do that while it heals. I no longer have reflux and my digestive system is now working as it should. I'm grateful that i found this surgeon and I am feeling better. If you feel comfortable with this surgeon and you've done your research then you should have it done. Let me know if you have any other questions and I wish you the best.