The stomach is known to house newly eaten foods and begin chemical breakdown using a mixture of hydrochloric acid, potassium chloride, and sodium chloride. Due to the presence of hydrochloric acid, the stomach's environment has a low pH of around 1.5 - 3.5. This means the digestive fluid mixture is highly acidic and works to dissolve the food we eat. Typically, the action of gravity combined with muscular rings in the digestive tract keeps this acidic mixture in the stomach, though sometimes these functions falter. When eating, food passes from our mouth to our stomach via a long muscular tube called the esophagus. At the end of the esophagus, a muscular ring called the lower esophageal sphincter (LES) opens and closes to allow food to enter our stomach and gas to exit it. This sphincter may inappropriately relax, allowing contents of the acidic stomach environment to back up into the esophageal tube, causing the signs and symptoms associated with heartburn. Over time, this sphincter may also become weak, causing more frequent bouts of heartburn. Frequent or severe heartburn may indicate gastroesophageal reflux disease (GERD). Heartburn is more strongly felt when lying down or bending over due to the change in your center of gravity. If your lower esophageal sphincter is already weakened or opened, laying down or bending over easily allows the flow of stomach acid to back up into the esophagus. Heartburn is also a common complaint during pregnancy. This occurs because of the increased concentration of the hormone progesterone, which can relax the LES. Problematic relaxation of the LES may be seen in patients who smoke or are overweight. A hiatal hernia may also be the cause of heartburn. In this condition, the stomach enlarges and bulges outward from the diaphragmatic hole where the lower esophageal sphincter is usually found. A higher LES and larger stomach can increase the risk of acidic backflow. Finally, certain medications, such as anti-inflammatory or pain relievers, can further relax the digestive muscles.