The stomach is the food storage organ in the human gastro-intestinal system. It is located in the left upper quadrant of the abdomen, below the diaphragm. The organ somewhat resembles the English letter ‘J’ and has two openings at either end. AT the top, it is connected with the esophagus and to the bottom; it opens to duodenum-the first part of the small intestine.
# Lower Esophageal Sphincter
The opening between esophagus and stomach is guarded by a sphincter. This works as a physiological sphincter and is made up of several factors. First there are the smooth muscles of the esophagus. Then there is the right crus of the diaphragm which hooks around the esophagus. Then there is the oblique angle between the two organs and also the oblique fibers of stomach, which completes this physiological sphincter into a working one.
Lower esophageal sphincter can become dysfunctional in two ways. It may become either too constricted or too relaxed. When the sphincter is too relaxed it is called Gastro Esophageal Reflex Disease, which is quite common. Then the stomach contents escape into esophagus and the acids can easily damage the wall of esophagus. This can cause cardiac burn, ulcers and stricter formation. The constricted form of dysfunction is called Achalasia Cardia where swallowing is impaired and leads to development of an enlarged esophagus called mega-esophagus.
# Pyloric Sphincter
The lower opening of the stomach is guarded by the pyloric sphincter. Unlike the previous sphincter this is an anatomical sphincter as well as a physiological one. There are number of factors which forms this sphincter a valuable one. First, this is the port which releases chyme-the contents of stomach-to the small intestine. If the chime was allowed to be released in a larger volume, then the intestines would not be able to do all the digestion and absorption.
# Structure of Stomach
Stomach is a muscular bag which is made up of smooth muscle and also connective tissue. It can be distended to accompany food. The structure can again be divided into anatomical and histological structures. Histology is what we see under a microscope. Anatomy is what we see with our naked eye.
# Anatomical Structure
Anatomically, the stomach can be divided into five parts. The part immediately next to the lower esophageal sphincter is the cardia. And then there is the fundus. These two areas form the upper part of the stomach. Then the middle part is made up of the body and the body ends in the pyloric and antrum areas. Pylorus is the most distant part of the stomach which lies next to the pyloric sphincter.
The histological structure and thereby the function is different with each part of the stomach. The fundus and the body are rich in acid and pepsinogen secretions while the pyloric area mainly secretes mucous. Then there are also gastrin secreting G cells in the pyloric area.
# Histological Structure
The whole gastro-intestinal system shares some common histological structure with few variations. In stomach the main variation is that there are three muscle layers instead of two. Other than that it is almost the same as the rest of the tract.
There are three muscle layers on the outside. From inside out these layers are the oblique, circular and longitudinal. Inside the muscle layer there is the sub-mucosa with all the blood vessels. Innermost, there is the mucosa with the epithelial layer, which is a simple cubicle in the stomach.
# Functions
The main function of stomach is to store the food as they pass through the digestive system. Other than that there are other functions. The stomach is a distendable organ so that it can accommodate a large volume and still compress when not needed. The smooth muscles have given the organ this ability.
Another use of the muscle layer is to mix the chime and break it into pieces so that the digestion is easy and rapid. It mixes food with mucous and acids.
The stomach secretes hormones such as gastrin, enzymes like pepsinogen which become pepsin and initiate the protein digestion and acids. It also secret a large amount of mucous which protects both stomach and the first part of the small intestine from erosion. Otherwise, those structures would have been digested by acids and pepsin.
The upper part of the stomach is responsible for the secretion of acids and pepsinogens while the lower part secretes gastrin and mucous. Other than these main functions, stomach also plays an important role in vitamin B12 absorption by secreting the intrinsic factor. It also plays a role in the absorption of iron and various drugs.
# Gastrin
Gastrin is perhaps the most important hormone in the gastro-intestinal system. It is being secreted by the G-cells of the lower part of the stomach. The secretion is under tight regulation and the regulation can be divided into three parts, cephalic, local and intestinal phases. The hormone is a polypeptide and it is responsible for the most actions of stomach.
Gastrin increases the acid secretion of the stomach parietal cells, it also increases the secretion of mucous so that more acid cannot harm the stomach wall. It also increases the gastric mobility.
