# Stomach --- The **stomach** is a J-shaped [[organ]] that makes up part of the [[gastrointestinal tract|GI tract]]. This is where most of the [[digestion|chemical digestion]] of food takes place, as well as much of the [[digestion|mechanical digestion]]. When the stomach is done with it, the material inside it is called "chyme". ![[stomach.png]] ## Anatomy The stomach is connected to the [[esophagus]] by the cardiac [[sphincter]], and to the [[duodenum]] by the pyloric [[sphincter]]. Generally speaking the stomach can hold about a gallon of material. On the inner-most layer of the stomach has ridge-like features called **rugae**. These rugae let the stomach expand when it fills up with food. Once the chyme leaves the stomach shrinks back down again and the rugae reform. Like all of the [[gastrointestinal tract|GI tract]], the stomach wall is comprised of four layers, the innermost mucosa, the submucosa, the muscularis and the serosa. The **muscularis** of the stomach is composed of three layers, the **oblique layer** that is the innermost, the middle **circular layer** and the outermost **longitudinal** layer. These muscles contract in order to perform [[digestion#Mechanical and Chemical Digestion|mechanical digestion]], mixing and churning and smushing the food around. In the walls of the stomach are pits called [[gastric gland|gastric glands]], which contain specialized cells that produce a mixture called [[gastric gland|gastric juice]], which helps protect the stomach all as well as perform [[digestion|chemical digestion]]. The stomach wall has two types of [[afferent nervous system|sensory neurons]]; mechanoreceptors can sense the amount of stretch the stomach has, and chemoreceptors that can sense changes in [[osmosis|osmolarity]] and [[pH]]. Both of these play a role in initiating the [[digestion|gastric phase]] of digestion. ## Physiology The [[digestion|mechanical digestion]] performed by the stomach is done by the three layers of the muscularis in [[peristalsis]]. The strong muscles of the stomach begin contracting in the cephalic and gastric phases of [[digestion]], and are inhibited by during the intestinal phase. There are a bunch of hormones that act on the stomach at various points of the digestion process: - [[cholecystokinin]] - [[gastric gland|gastrin]] When the chyme is ready to leave the stomach, the stomach squeezes it towards the pyloric sphincter. The pyloric sphincter only lets a small amount through at a time (~30mL) and the rest is washed back into the stomach, further mixing it. During chemical digestion, proteins are broken down by [[protease]] and lipids are broken down by [[lipase]]; both of which[^1] are secreted by [[gastric gland|chief cells]] in the gastric glands. (Carbohydrates are digested in the mouth and the small intestine.) [^1]: Technically, chief cells secrete [[protease|pepsinogen]], which becomes pepsin when in contact with [[stomach acid|HCl]]. The stomach also has a somewhat limited ability to break down [[alcohol]], which means less is able to reach the bloodstream. The more it breaks down alcohol in the stomach, the less you get drunk. ## Problems with the Stomach - [[stomach cancer]] - [[GERD]] - [[hiatal hernia]] %%this line intentionally left blank%%