The stomach serves as the storage tank where proteins are broken down and the food that we eat is converted into chyme. It lies in the upper left quadrant, hidden by the diaphragm and the liver. Although both ends of the stomach are relatively fixed, what’s in between is quite movable. The shape of the stomach of a stout person is different from the shape of the stomach of a tall person.
Gross Anatomy of the Stomach
An empty stomach only has a volume of about 50 ml. However, when the stomach is full or very distended, it has the capacity to hold about 4 litres of food and may extend as far away as the pelvis! When the stomach is empty, it collapses on itself, forming folds called rugae.
The stomach is divided into major regions according to its location and function. The cardiac region or what sometimes known as cardia is the portion where the food from the esophagus enters the stomach. The fundus region is shaped like a dome, located beneath the diaphragm and is the part that bulges. The middle portion of the stomach is known as the body and is continuous inferiorly with the pyloric region. The pyloric antrum is the more superior and wider part of the pyloric region and as it narrows down, it forms the region known as the pyloric canal that terminates at the pylorus. The pyloric sphincter is responsible for controlling gastric emptying. The literal translation of pylorus is gatekeeper.
Microscopic Anatomy of the Stomach
The stomach walls, like all other parts and accessories of the alimentary canal, have four tunics. However, what sets the stomach apart is that the mucosa and muscularis is modified to serve the specific roles that the stomach plays. In addition to the usual longitudinal and circular layers of the smooth muscles, the muscularis externa of the stomach contains a layer of smooth muscle that runs obliquely. This physical addition to the muscularis allows the stomach to not only move the food along the GI tract but to also physically break it to smaller fragments by churning and mixing and pummelling the food.
The epithelial lining of the stomach mucosa is composed entirely of goblet cells. These goblet cells synthesize a two-layer coat that is alkaline nature. Gastric pits is scattered all over the otherwise smooth lining of the mucosa. These gastric pits lead to the gastric glands which creates the gastric juices.
Secretory Cells in the Stomach
Mucous neck cells are named so because they can be found in the neck regions of the gastric glands. Little is known on the exact function of the acidic mucus that it secretes.
Parietal cells are the ones responsible for secreting the all important hydrochloric acid and intrinsic factor. They are found in the middle region of the gastric glands dispersed among the chief cells.
Chief cells are responsible in producing pepsinogen, an inactive form of protein-digesting enzyme. When the chief cells are activated, the pepsinogen that they release are activated by the HCl. With this activation, pepsin is now present. Pepsin then is the one that catalyzes the conversion of the remaining pepsinogen to pepsin. The chief cells also secrete lipase, but the amounts are insignificant.
Enteroendocrine Cells are the ones that secrete a number of chemical messengers. Of the various ones that they release, serotonin and histamine act as local paracrines; somatostatin act as hormones which diffuses by way of the blood capillaries and then influences several digestive system organs.