What is the stomach?
The stomach is a part of the gastrointestinal tract that begins at the mouth and continues though the esophagus, stomach, small intestine, large intestine and rectum and ends in the anus.
The stomach is the only part of the GI tract that is not a long tube. Rather, it is a hollow, muscular, J-shaped reservoir that receives ingested food from the esophagus, retains it and passes it along to the small intestine. The average adult stomach can hold as much as 1.5 liters when fully expanded.
According to Robert D. Richards, Jr., MD, a board-certified gastroenterologist with Gastroenterology Associates of Central Virginia, the stomach is like a crumpled bag that expands and accepts large or small meals in a process called receptive relaxation.
The stomach’s role in the digestive process
The stomach serves three essential functions: (1) it acts as a storage reservoir for food, (2) it grinds and churns food into small particles and (3) it releases these particles in a controlled manner into the small intestine for further digestion and absorption.
“The real breakdown and absorption is in the small intestine,” says Dr. Richards. “The stomach initiates the digestive process as it secretes acid that sterilizes food and kills bacteria.”
Parts of the stomach
The anatomical sections of the stomach are the cardia, fundus, body and antrum, along with the end portion, a muscular valve called the pylorus.
The cardia is a short section near the top of the stomach where it meets the esophagus. It is the opening that allows food to pass down into the stomach.
The fundus is the upper part of the stomach, next to the cardia. Its main function is to send a signal to the brain to stop eating.
The body of the stomach is the midportion, and it is involved with the reservoir function. Most of the stomach's parietal and chief cells are located in the body. Parietal cells secrete hydrochloric acid and intrinsic factor, which is essential for the proper absorption of vitamin B-12.
This production of hydrochloric acid is the primary secretory function of the stomach. It begins to break everything down and is helpful in killing bacteria or infectious agents.
Chief cells ultimately make pepsin, a potent enzyme for the predigestion of protein. This process occurs as the chief cells secrete pepsin in the form of pepsinogen, an inactive enzyme. The hydrochloric acid then converts the pepsinogen into the active enzyme pepsin.
This is the lower portion of the stomach where the food is mixed with gastric juices. The antrum is the motor of the stomach that grinds and smashes food into smaller pieces in a churning cycle that lasts up to four hours after a meal.
Here, the stomach also contains G cells that secrete a hormone called gastrin that travels by blood to regulate acid secretion by cells of the upper portions of the stomach. Gastrin is the most potent stimulator of acid secretion.
The pylorus is the end portion of the stomach, located at the junction of the stomach and small intestine. It acts as a valve that regulates the emptying of the stomach contents.
The pylorus usually stays closed but opens periodically to let a small amount of food into the duodenum, the first part of the small intestine.This food is called chyme — a thick, semifluid mass of partially digested food.
“The small intestine wants to receive babylike food with the consistency of applesauce,” says Dr. Richards.
Why does my stomach “growl”?
No, it isn’t expressing itself. It’s a normal — although sometimes embarrassing — process called "borborygmi." The rumbling sounds come from the GI tract as the pylorus valve opens and a mixture of air and fluid pushes chyme down through the small intestine.
Layers of the stomach wall
There are four layers to the stomach. The mucosa is the innermost later. It is secretory in nature and is in contact with the lumen (the open cavity that fills the stomach).
Beneath the mucosa is the submucosa, a layer of dense, connective tissue that provides structural support and blood supply. The mucosa and submucosa are responsible for acid and digestive enzyme secretion, intrinsic factor release and digestive hormone release (such as gastrin).
The muscularis propria is the muscle layer. It grinds food into smaller particles and controls the release of these particles into the duodenum. As the muscle relaxes, the stomach distends and stores food. Contraction of the muscle is essential for grinding, mixing and delivering the gastric contents to the small intestine.
The outside layer is the serosa, which is a thin, strong layer of connective tissue that covers the muscle layer. The serosa does not aid in the digestive process, but its slippery surface protects the stomach from friction.
Conditions that affect the stomach
Gastritis occurs when the mucosa layer of the stomach lining becomes inflamed. Symptoms may include epigastric (above the stomach) abdominal pain, nausea, vomiting or bloating.
The most common causes of gastritis are Helicobacter pylori (H. pylori) infection andnonsteroidal anti-inflammatory drugs.
H. pylori is a rod-shaped bacteria that lives in the mucus layer above the stomach lining. It is less common in developed countries like the United States because it is spread by contaminated food or water.
Treatment for gastritis may include antibiotics for an H. pylori infection, an acid-suppression medication such as a proton pump inhibitor or discontinuing the use of NSAIDs like aspirin and ibuprofen.
Peptic ulcer disease
Peptic ulcer disease occurs when the stomach acid and pepsin successfully penetrate the stomach or intestinal lining causing ulcers to develop.
The disease is defined as a break in the mucosa layer of the stomach lining or the duodenum. Ulcers can cause abdominal pain, nausea and, in some cases, bleeding.Occasionally the ulceration can go through the full thickness of the stomach or duodenum and cause a perforation in the stomach or intestinal lining.
The two most common causes of ulcers are H. pylori infection and the use of NSAIDs.
“H. pylori is a huge cause of peptic disease,” says Dr. Richards. “By finding it and eradicating it, we can greatly reduce the risk of developing an ulcer or getting another.”
The arrival of proton pump inhibitors has revolutionized ulcer treatment and has almost eliminated the need for surgery. The healing process also includes stopping NSAIDs and eradicating H. pylori infections with antibiotics.
“Proton pump inhibitors work dramatically well at healing ulcers and preventing them if you have high risk,” says Dr. Richards.
One of the most common stomach myths has to do with ulcer formation. For the record, daily stress will not cause ulcers. Also, hot and spicy foods and beverages will not cause ulcers, but they may irritate an existing one.
Gastric cancer, or stomach cancer, is one of the most common forms of cancer. The disease was the leading cause of cancer death worldwide until being overtaken by lung cancer.
H. pylori infection is a risk factor for stomach cancer. It also runs in families independent of H. pylori infection, which suggests a genetic predisposition to gastric cancer.
Surgery, often combined with chemotherapy, is the primary method of treatment for stomach cancer.
Gastroparesis, or stomach paralysis, is a condition where the stomach empties slowly. Its symptoms can include nausea, vomiting or feeling full after eating a small amount.
Normal stomach emptying is a complex process that coordinates the gut’s nervous system, the pacemaker cells of the stomach (called the interstitial cells of Cajal) and the smooth muscle that lines the stomach. With gastroparesis, this coordination is disrupted because the number of pacemaker cells is reduced.
Gastroparesis is most commonly associated with diabetes or past stomach surgery, but its cause may be unknown. It can be diagnosed using a gastric emptying scan — a radiology test that measures stomach emptying. Treatment includes avoiding foods that delay gastric emptying, such as fat and fiber. Eating smaller, more frequent meals is also recommended.
Can eating less shrink your stomach?
Contrary to popular belief, reducing portion size will not cause one's stomach to shrink. Rather, reducing food intake affects the levels of hormones such as leptin and ghrelin — known as the appetite hormone — causing the appetite to decrease.
Remember the stomach during recovery from activity
The reason fat and fiber cause the stomach to empty slower is because fiber requires more churning, and fat must be processed more carefully in the small intestine.
For a faster recovery after athletic training, drink fluids and take carbohydrates with a little protein and no fat. Try to postpone eating foods like peanut butter.
Maintaining a healthy stomach
Our stomachs are complex reservoirs that hold our food, break it down and regulate its continuation along the GI tract. As with many intricate systems, the stomach has a sensitive nature. So pay attention to the signs it gives and seek assistance from a gastrointestinal specialist if you notice something irregular.
Robert D. Richards, Jr., MD, with Gastroenterology Associates of Central Virginia
Mayo Clinic (http://www.mayoclinic.org/)
American College of Gastroenterology (http://gi.org/)