The stomach is a muscular organ located on the left side of the upper abdomen. The stomach receives food from the esophagus. As food reaches the end of the esophagus, it enters the stomach through a muscular valve called the lower esophageal sphincter. In most vertebrates, the stomach is located between the esophagus and the small intestine. It secretes digestive enzymes and gastric acid to aid in food digestion.
The stomach secretes acid and enzymes that digest food. Ridges of muscle tissue called rugae line the stomach. The stomach muscles contract periodically, churning food to enhance digestion. The pyloric sphincter is a muscular valve that opens to allow food to pass from the stomach to the small intestine.
The stomach is a bean-shaped sack located behind the lower ribs. Once food hits the stomach, sphincters at the opening of the stomach and the exit into the small intestine close. The lining of the stomach then secretes hydrochloric acids and enzymes that break down the food so that it can continue on its journey through the digestive system, according to the Cleveland Clinic. As it secretes acid and enzymes, the stomach muscles contract in a process called peristalsis to mix the food with the acid and enzymes.
A bolus (a small rounded mass of chewed up food) enters the stomach through the esophagus via the lower esophageal sphincter. The stomach releases proteases (protein-digesting enzymes such as pepsin) and hydrochloric acid, which kills or inhibits bacteria and provides the acidic pH of two for the proteases to work. Food is churned by the stomach through muscular contractions of the wall called peristalsis – reducing the volume of the fundus, before looping around the fundus and the body of stomach as the boluses are converted into chyme (partially digested food). Chyme slowly passes through the pyloric sphincter and into the duodenum of the small intestine, where the extraction of nutrients begins. Depending on the quantity and contents of the meal, the stomach will digest the food into chyme anywhere between forty minutes and a few hours. The average human stomach can comfortably hold about a litre of food.
Gastric juice in the stomach also contains pepsinogen. Hydrochloric acid activates this inactive form of enzyme into the active form, pepsin. Pepsin breaks down proteins into polypeptides.
Although the absorption is mainly a function of the small intestine, some absorption of certain small molecules nevertheless does occur in the stomach through its lining. This includes:
- • Water, if the body is dehydrated
- • Medication, like aspirin
- • Amino acids
- • 10–20% of ingested ethanol (e.g. from alcoholic beverages)
- • Caffeine
- • To a small extent water-soluble vitamins (most are absorbed in the small intestine)
The parietal cells of the stomach are responsible for producing intrinsic factor, which is necessary for the absorption of vitamin B12. B12 is used in cellular metabolism and is necessary for the production of red blood cells, and the functioning of the nervous system.
Conditions and diseases:
The stomach can have many different conditions and diseases that can cause pain, discomfort, digestion problems and even death.
One of the most common stomach problems is upset stomach or dyspepsia. “Dyspepsia is a term used to describe one or more symptoms, including a feeling of fullness during a meal, uncomfortable fullness after a meal, and burning or pain in the upper abdomen,” told Live Science.
Upper abdominal pain, indigestion and heartburn affects around 25 percent of the population each year. The cause can be as benign as food intolerance or gastroesophageal reflux disease (GERD), or as serious as an ulcer or cancer.
Stomach ulcers are breaks in the lining of the stomach, caused by certain medications and too much acid in the stomach. These breaks are exposed to the acids in the stomach, causing pain. In most cases, ulcers can easily be treated with medication.