The stomach is a pouch-like organ that lies in the upper left part of the abdominal cavity, just under the diaphragm and next to the liver.
The stomach has five functions:
- digestion of food
- secretion of gastric juices as well as mucus which helps to coat its lining, preventing erosion by gastric juices
- secretion of gastric hormones
- regulation of the rate at which partially digested food is delivered to the small intestine
- absorption of small quantities of water and dissolved substances. It does not absorb very well, except in the case of alcohol. Consumption of alcohol on an empty stomach will quickly increase blood levels.
The major regions of the stomach include:
- the fundus or the body
- the pyloric canal
The pyloric sphincter(located at the end of the pyloric canal) helps regulate the rate at which gastric contents are delivered to the small intestine.
Rugae are thick accordion-like folds within the stomach that allow it to expand during a large meal. This remains permanent when stretched too often during frequent overeating.
The stomach has three layers of muscles that lie in three directions:
The vagus nerve (X) of the PNS (parasympathetic nervous system) innervates stomach action. Many drugs and surgical procedures interfere with the function of the vagus nerve and consequently, gastric secretion and motility.
The stomach contains gastric glands in the mucous membrane. These glands contain three types of cells which secrete different substances, collectively known as gastric juices:
- mucus cells secrete mucus to protect the lining from the potential destructiveness of the acidic gastric juices
- chief cells secrete digestive enzymes, mainly pepsinogen
- parietal cells secrete hydrochloric acid (HCl) and intrinsic factor (essential for Vitamin B12 absorption). HCl lowers the pH of the gastric juice, kills microorganisms, breaks down the cell walls and connective tissues in food, and activates the exzymatic secretions of the chief cells. HCl also converts pepsinogen to pepsin, which begins the digestion of proteins to polypeptides.
There are three phases of digestion in the stomach:
- Cephalic phase prepares the stomach for incoming food and lasts very briefly. Sight, smell, taste, or thought can stimulate secretions that can reach a rate of about 500 mph. This stage is directed by the CNS and, under the control of the vagus nerve, parasympathetic fibers innervate the cells of the gastric juices.
- Gastric phase begins with the arrival of food, causing the secretion of the hormone gastrin, which is released along with many acids and enzymes. This phase may last for several hours.
- Intestinal phase begins with the entrance of chyme into the small intestine, causing the secretion of many hormones including secretin, CCK (cholecystokinin), and GIP (gastric inhibitory peptide).