The Stomach: Essential to the Digestive System

The esophagus connects to the stomach, allowing food to pass into it through an opening called the cardia. The cardial notch, found at the top of the cardia, opens and closes to prevent food and drink from coming back up into the esophagus. The pylorus is a muscular, funnel-shaped valve that fills up with air when swallowing occurs. Its wider end, the pyloric antrum, connects to the stomach. The narrower end of the pylorus, the pyloric canal, connects to the duodenum, the opening of the small intestine. The fundus is the part of any hollow organ that is farthest from the opening; in the case of the stomach, this is the upper left side, near the cardia.

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The walls consists of four layers of connective tissue, blood vessels, nerves, and muscle fibers. The inner lining, called the mucosa, secretes gastric juices through special glands buried in the lining. Beneath the mucosa, a supporting layer called the submucosa covers the muscle layer, which is responsible for the contractions that occur during digestion. The serosa is the stomach’s thin, outer layer of cells. Large, vertical folds line the stomach, allowing fluids to flow easily to the duodenum at the lower end of the stomach.

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Food mixes with saliva in the mouth and the digestive break-down process. A person swallows their food, and from this point on, the eating process is involuntary. The chewed food enters the esophagus and contractions called peristalsis push it down into the stomach. It stores food until digestion begins. Gastric juices made up of enzymes and gastric acids break down the food into a substance called chyme, which is easier for the body to transport. This semifluid paste passes from the stomach to the small intestines, where the majority of digestion takes place.

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The gastric glands found in the lining of the stomach’s fundus secrete parietal cells. These cells are responsible for producing the gastric acids needed for digestion. By age two, acid secretion in the stomach reaches adult levels. Smell, taste, and the thought of food initiate these secretions. Gastric acids are highly corrosive, equaling the pH levels found in battery acid. The stomach must produce a new layer of mucus every two weeks to protect itself from these powerful gastric acids, which could eat through the stomach’s lining if the mucus wasn’t there to protect it.

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Contrary to misconceptions, stomach size does not correlate with the weight of an individual. The stomach stretches according to the amount of food eaten. After digestion, the stomach returns to its normal size. When empty, the stomach is about the size of an adult fist. The actual size of the stomach doesn’t change according to the amount of food an individual eats, but its capacity does. If an individual repeatedly eats large meals, no matter what size they are, the capacity of his or her stomach increases.

The body signals the brain when the stomach is empty, which sends the body messages that it needs food. However, the digestive system has its own brain, called the enteric nervous system, located within the walls of the tubular-shaped digestive tract. Unlike the main brain, the ENS only controls digestive functions. Contrary to previously held theories, researchers now believe that gastrointestinal issues such as irritable bowel syndrome and constipation cause messages to travel to the ENS, triggering mood changes and resulting in anxiety or depression.

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Digestive diseases affect between 60 and 70 million people in the U.S. alone, according to the National Institutes of Health. These issues may affect just one part of the digestive tract or several parts. Common symptoms associated with the stomach include indigestion and heartburn and over-the-counter and home remedies can usually treat them sufficiently. Sometimes, however, these issues indicate a more serious condition, such as sores in the lining -- peptic ulcers -- or GERD, gastroesophageal reflux disease. Other digestive disorders include hernias, ulcerative colitis, Crohn’s disease, infections, and cancers.

Physicians use several methods for detecting and diagnosing digestive and stomach disorders. A complete physical examination, and sometimes, a psychological evaluation, help the physician determine the need for any further testing. Endoscopy uses a flexible viewing to examine internal structures such as the stomach, the esophagus, and part of the small intestine. Doctors may also suggest X-rays, MRIs, CAT scans and ultrasounds. Reflux-related and acid-related testing allow the physician to determine the cause of recurring indigestion, acid-reflux issues, or more serious gastrointestinal problems.

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Indigestion, stomach aches, constipation, and other common digestive problems are usually the result of foods that are hard to digest or that provoke a food intolerance. Overeating and binging on foods high in sugar and fat may cause the gut’s microbial ecosystem to shift significantly, setting off a wide array of digestive troubles. Yogurt’s probiotic benefits can ease the system and help with regularity. Bananas are low in acid, and they can relieve indigestion. Peppermint tea has an antispasmodic effect on muscles and eases stomach pain, while ginger tea relieves nausea.

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Myths about the body and its complex functions have fascinated humans since time began. Despite medical knowledge achieved through centuries of extensive scientific research, myths about the stomach and its functions have survived. One of the most common is that spicy food and stress cause ulcers. While they may exacerbate the problem, neither food nor stress is actually to blame for the development of an ulcer. A bacteria that lives in the stomach, called Helicobacter pylori, is the actual cause. Another myth is that a growling or gurgling stomach is a sign of hunger. The truth is, these sounds could also be the last bit of a meal working through the peristalsis stage of digestion.

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