Pyloric Sphincter Definition
The pyloric sphincter is a small piece of smooth visceral muscle that acts as a valve and regulates the flow of partially digested food from the stomach to the duodenum. The opening and closing of the sphincter is controlled by peristaltic waves produced by the stomach during the digestion process. At rest, the sphincter remains partially open to allow the free flow of water into the duodenum; however it immediately closes in the presence of food to allow digestion to occur.
Location of the Pyloric Sphincter
The human stomach is separated into four different sections, as shown in the diagram below. The cardia is found between the esophagus and the entrance to the stomach, the fundus is the upper part of the stomach, and the main body is where most of the initial digestion occurs. At the base is the pylorus, which is funnel shaped and narrows down to around 1 inch in diameter. The pyloric sphincter is located at the junction between the pylorus and the duodenum of the small intestine.
Function of the Pyloric Sphincter
During the consumption of food, the stomach receives signals to start producing gastric secretions to start the digestion process. When food enters the stomach, it is immediately mixed with these secretions to form a thick acidic liquid called chyme. The pyloric sphincter remains closed during this time, while the stomach attempts systematic mixing through peristaltic waves. One of the functions of the sphincter is to regulate the amount of chyme released into the duodenum, which allows for maximal absorption of nutrients. Another major function of the pyloric sphincter is to prevent the regurgitation of chyme and bile. Regurgitation of chyme or bile can cause stomach inflammation and discomfort for the patient.
Physiological Regulation of the Pyloric Sphincter
A complicated network of physiological signals controls the opening and closing of the pyloric sphincter. These signals originate from either the stomach or the duodenum.
1. In the fundus, there are a group of cells that generate a constant basic electrical rhythm (BER), which consists of slow depolarizations that move along the entire stomach. While undergoing digestion, these base electrical rhythm waves are able to excite the action potentials in the smooth muscle and cause strong peristaltic waves to mix the food and gastric juices. The waves push the partially digested food towards the pylorus and force open the pyloric sphincter to allow a few milliliters of chyme through at a time.
2. The volume of food in the stomach also affects the pyloric sphincter. When a large volume is present, it stretches the stomach walls, which in turn produces strong peristaltic waves and inhibits the pyloric sphincter from closing abruptly, allowing larger volumes of chyme to enter the duodenum.
3. Some foods trigger the release of certain hormones to enhance the rate of stomach emptying. Meats promote the release of gastrin from the stomach lining which speeds up the opening of the pyloric sphincter. Carbohydrate rich foods speed up the rate of stomach emptying, while protein rich food promotes a much slower rate, and food high in fat causes the lowest rate.
4. The pH of chyme is also another regulating factor. When there is more acidity in the stomach, the pyloric sphincter opens, and when the acidic contents move into the duodenum, acidity levels rise and cause the sphincter to close immediately.
5. Increased osmotic pressure in the stomach also causes expulsion of chyme into the duodenum.
1. High acidity and high levels of fat or protein products in the duodenum cause an enterogastric reflex which passes through the duodenal wall via the nerves and promotes the closure of the pyloric sphincter to stop chyme from entering.
2. Enterogastrones also affect the activity of the pyloric sphincter. These are hormones that promote the closure of the sphincter and reduce stomach mixing. Cholecystokinin (CCK) is released when there is a high level of fat in the duodenum and inhibits the effect of gastrin released in the stomach. Another hormone called gastric inhibitory peptide (GIP) can also affect it in the same way, but to a lesser degree.
Pyloric Sphincter Disorders
Pyloric stenosis occurs when the sphincter muscle becomes abnormally large or thick, restricting normal chyme flow from the stomach. This disease is mostly prevalent in newborn children and referred to as infantile hypertrophic pyloric stenosis. It is characterized by severe projectile vomiting after eating, and vomit may contain traces of blood. The child may lose weight and exhibit constant hunger and dehydration due to malnutrition. The cause of pyloric stenosis is still unknown but scientists believe it is linked to genetics. Parents who had this disease as a child are at higher risk (20%) of having children with the same disorder. Currently, surgery is the only option to reverse the effects of the enlarged muscle. Adults are also at risk of this disease but the narrowing is usually due to scarring from cancers, growths, inflammation or ulcers.
Pylorospasms occur when the muscle fibers of the sphincter contract prematurely and fail to relax causing inflammation. This leads to obstruction to the flow of chyme to the duodenum and can cause the stomach to be overly full which leads to vomiting and pain after eating. This disorder can be treated with muscle relaxing drugs.