Food enters the stomach from the esophagus, through the lower esophageal sphincter (aka, cardiac valve). The stomach is where the digestive breakdown of food really begins. It operates like a food mixer, churning the food bolus to a pulp called chyme, and releasing numerous chemicals such as digestive hormones, enzymes and gastric juices which help to break down food molecules in the chyme into small particles for absorption into the bloodstream. An empty stomach has a volume of approximately 50 ml. But typically after a meal, its capacity expands to about 1 liter of food, and may expand to hold as much as 4 liters. The chyme slowly exits the stomach via the pyloric sphincter or valve and passes into the duodenum – the first segment of the small intestine – where digestion continues.
Anatomy of The Stomach
Shaped like the letter “j”, the stomach is a large stretchy bag situated in the middle of the chest behind the liver, between the esophagus and the first section of the small intestine (the duodenum).
It is subdivided into 4 regions each with different cells and gastric functions:
(1) the Cardiac region, where the contents of the esophagus empty into the stomach through the lower esophageal or cardiac sphincter;
(2) the Fundus, an expanded area curving up above the esophageal opening;
(3) the Body, the central and largest region;, and
(4) the Pylorus, the narrow end of the stomach that joins the small intestine at the pyloric sphincter.
Like the cardiac sphincter, the pyloric sphincter is a ring of muscle that regulates the movement of food into and out of the stomach.
Functions of The Stomach
Although it is a very complex organ which performs a wide variety of digestive actions, the stomach has 3 main functions:
- It stores the food bolus we swallow. This allows us to eat a large number of food calories in a relatively short time and then digest it over a longer period. Without the stomach’s storage capacity, we would need to eat very small amounts of food continuously throughout the day, because the small intestine digests food very slowly. Gastric bypass induces weight loss by reducing the size of the stomach, so that less food (and thus fewer calories) can be eaten. See Bariatric Surgery Guide.
- It breaks down large fat and protein molecules in food, so they can be absorbed in the small intestine. To do this, the stomach releases a number of powerful gastric juices containing hydrochloric acid and other digestive enzymes. In addition to breaking down food, these acidic juices (PH 1-3) also kill bacteria in the food. For easier digestion, powerful muscles in the stomach wall churn the food into a paste of porridge-like consistency, called chyme. This churning action also ensures that the secreted gastric acids and enzymes are thoroughly mixed with the food.
- It empties the partially digested chyme into the duodenum (the first segment of the small intestine) at a manageable speed, through the the pyloric sphincter. While the intestine is full and still digesting food, the stomach acts as storage area for food.
The absorption of food and water by the stomach is negligible, but iron and highly fat-soluble substances like alcohol are absorbed directly. Finger-like folds called villi give the stomach a huge surface both to absorb food and secrete digestive juices.
Special cells secrete a protective coating called mucus, on the stomach walls to prevent damage from gastric acids. Originally it was thought that peptic ulcers were caused by an erosion of this mucus lining by these acids. However recent research indicates that these ulcers are caused largely by the spread of a type of bacteria called Helicobacter pylori bacterium into the gastric walls.
Food typically takes 4-5 hours to pass through the stomach into the duodenum, the first part of the small intestine.