Stomach

In human anatomy, the stomach is a bean-shaped muscular organ located behind the lower ribs. It is part of the digestive system that breaks down foods into substances the body can use for energy. The stomach receives food from the esophagus, the tube that connects the mouth to the stomach. Using a combination of acids and muscular contractions, the stomach churns food until it is of the proper consistency to transfer to the small intestine, the next organ of the digestive system. The stomach can hold more than a quart of food at a time, storing it until it can digest it all.

The stomach is notably susceptible to developing a variety of conditions and disorders that can cause symptoms such as discomfort and pain. These conditions include indigestion, gastroesophageal reflux disease, gastric ulcer, gastritis, stomach bleeding, and stomach cancer. Individuals can cultivate good stomach health by eating fresh vegetables and fruits instead of fatty foods, maintaining a healthy weight, drinking enough water every day, and consuming sufficient dietary fiber for ease of digestion.

Background

In humans and many other animals, food that is chewed by teeth in the mouth and swallowed into the esophagus is deposited in the stomach, the first organ of the digestive system that breaks down food for usable energy. Many animal stomachs contain colonies of microbes, or microscopic organisms, which help to eliminate potentially harmful bacteria in food and then digest the food to send on to the small intestine. These microbes work in conjunction with the hydrochloric acid that is naturally produced in the stomach to eliminate foodborne bacteria that might otherwise colonize in the stomach and make the host sick. The acid is corrosive enough to burn human skin. It is prevented from damaging the stomach itself by a thick layer of mucus that coats the stomach walls.

Stomach acidity has played a key role in the successful evolution of humans and other animals with stomachs. Biologists believe organisms that eat higher on the food chain—for example, scavengers such as vultures that regularly eat carrion, or still-decaying animal flesh—have the most caustic stomach acid to protect them from becoming lethally ill from the harmful microbes contained in their food. Animals are more likely to become ill from foodborne microbes if they eat prey that is biologically similar to themselves. A vulture, for instance, would be more likely to contract a deadly pathogen, or harmful infectious agent, from eating another vulture than from eating an insect. Nonetheless, because of their corrosive stomach acid, vultures do not become ill from eating carrion.

Organisms that regularly eat lower on the food chain, such as plant-eating herbivores, house less acid in their stomachs. This is because the risks of contracting foodborne illnesses are significantly lower when eating plants or other foods that are biologically dissimilar to the predator. Herbivore stomachs still contain some degree of acid for eliminating dangerous bacteria that may enter the body through food, but a regular diet of organisms that are lower on the food chain such as plants will present the least risk of ingesting potentially dangerous bacteria.

Humans are omnivores, meaning they regularly eat both plants and animals, but their stomachs contain levels of acid that are more comparable to carrion consumers such as vultures than to various other omnivores that are lower on the food chain. Some biologists have suggested that humans retained this high stomach acidity over millions of years of evolving because, during some epoch of human history, it was vital for early humans to feed on carrion. Biologists have also theorized that human stomachs retained their high acidity throughout their evolution simply to guard against pathogens, regardless of where on the food chain people ate.

High stomach acidity and beneficial stomach microbes, therefore, have proven to be essential components of human survival, both for their ability to ward off dangerous pathogens acquired from food and for digesting the usable elements of that food. Infants and the elderly are more susceptible to intestinal infections than any other group of humans because their levels of stomach acid are lower. This is due simply to the weakened immune systems associated with the young and old.

Healthy adult humans should retain normal levels of stomach acid for most of their lives. Sometimes, however, people develop medical problems that can be treated only with methods that require permanent reductions in stomach acid volumes. Esophagitis, for instance, is the inflammation of the esophagus caused by stomach acids backing up into it. One treatment for this is proton-pump inhibitors, drugs that reduce the stomach's acid production. Individuals exposed to this treatment are advised to avoid consuming foods that carry high pathogen risks. These include any sort of raw or undercooked meats and sometimes all meat.

Stomach acid was also an important factor in helping people in the nineteenth-century United States learn more about human digestion. In 1822 in Michigan, a fur trapper accidentally shot nineteen-year-old Alexis St. Martin. Army physician William Beaumont treated St. Martin but expected he would die soon, as his lung was exposed through the gunshot wound. However, after several surgeries over a period of months, St. Martin recovered but was left with a fistula in his side, a hole penetrating his upper abdomen and into his stomach.

Beaumont saw this fistula as an opportunity to learn exactly how the human stomach digested food. For years, Beaumont noted everything St. Martin ate and then observed the biological activities he could see through the fistula. He learned more about stomach acid by sending samples from inside the fistula to chemistry labs. Beaumont eventually concluded that it was stomach acid that was primarily responsible for digesting food. This fact opposed the popular theory at the time that the stomach digested food chiefly by violently grinding it down into smaller components. Later physicians learned from Beaumont's studies and started to experiment with fistulas in laboratory animals. Further studies in successive centuries eventually revealed the information that doctors in the twenty-first century know about the human stomach.

Overview

The human stomach is a bean-shaped muscular sac located in the upper left abdomen, behind the lower ribs. The stomach is just one organ of the human body's complex digestive system. This system starts in the mouth, where food is broken down into smaller pieces through chewing and made softer by mixing with saliva. The food then moves into the throat, or pharynx, and descends into the esophagus, the tube that connects the throat to the stomach. The end of the esophagus, just above the stomach, is called the lower esophageal sphincter. It is a muscular valve that opens to allow food into the stomach and closes to prevent food or stomach acid from backing up into the esophagus.

The stomach begins the work of the digestive system by performing the initial internal breakdown of consumed food. The stomach secretes its acid to kill any potentially harmful bacteria in the food and break down the food itself. The organ also periodically contracts to dissolve the food even further. Food usually spends between four and six hours digesting in the stomach.

The stomach also stores food in addition to digesting it. The organ can expand to hold up to one quart, or one liter, of food at a time. The stomach holds this food until it is all digested into a kind of liquid paste called chyme. The chyme then passes through the pyloric sphincter at the bottom of the stomach and enters the duodenum, the first part of the small intestine. Food nutrients are absorbed in the rest of the small intestine. Food waste that cannot be used by the body continues through the digestive tract, passing into the large intestine and rectum before being expelled through the anus.

The human stomach is prone to developing a multitude of conditions and disorders that can cause discomfort and pain. The most common stomach condition experienced by people is dyspepsia, also called stomach upset or indigestion. Dyspepsia is manifested as uncomfortable feelings of fullness either during or after a meal or as pain in the stomach. Dyspepsia is the collective name for the symptoms experienced because of another, independent stomach problem. These problems range from harmless to serious.

Many people suffer from gastroesophageal reflux, for instance. This is a condition in which acid and other contents of the stomach back up through the lower esophageal sphincter into the esophagus. This can cause intense, but generally not lethal, chest pain called heartburn. The condition is caused most commonly by consuming spicy foods, chocolate, alcohol, or caffeine. Dietary changes are sometimes recommended to avoid heartburn. If incidents of gastroesophageal reflux recur, however, a person may be diagnosed with gastroesophageal reflux disease, sometimes called GERD or acid reflux disease. Chronic GERD can cause people major discomfort. It is treated usually with dietary changes, antacid tablets, or, more rarely, proton-pump inhibitors.

Another painful stomach condition is a gastric ulcer, or stomach ulcer. This is an erosion in the lining of the stomach caused usually by a bacterial infection or through long-term use of anti-inflammatory drugs such as aspirin or ibuprofen. Ulcers cause people great pain when they are exposed to stomach acid. Gastric ulcers are treated with medication. Similarly, people may also suffer from gastritis, a condition involving the inflammation of the stomach lining. Medications such as antibiotics or antacids may be used to quell gastritis's symptoms of stomach pain, nausea, and vomiting.

Stomach cancer is one of the most serious stomach conditions a person can develop. Stomach cancer appears as a tumor inside the stomach. It can be caused by long-term smoking and by regularly consuming diets high in processed foods, such as smoked or salted meats. As with most cancers, stomach cancer is treated with a combination of medications, radiation therapy, and chemotherapy. Stomach cancer was a leading cause of death among Americans in the early twentieth century. This was partially due to the lack of adequate refrigeration at the time, which forced butchers to preserve meat through smoking and curing. Both processes chemically change meat by adding to it cancer-causing agents called carcinogens. Widespread availability of refrigeration and freezing has made stomach cancer a rare occurrence in the United States in the twenty-first century.

People can take various steps to nurture good stomach health. For instance, upset stomach can be avoided by eating smaller meals and by refraining from consuming carbonated beverages or potentially inflammatory foods with heavy spices. People can aid their stomach health more generally by quitting smoking, losing weight, becoming mildly physically active after eating to help digestion, and drinking at least eight glasses of water a day. Eating the right amount of dietary fiber can help promote healthy digestion and control a person's appetite by making one feel full longer. Individuals can take probiotic medications to help them maintain the level of beneficial microbes in the stomach; this is especially important with the presence of gastrointestinal disorders such as acid reflux disease.

Bibliography

Beasley, DeAnna E., et al. "The Evolution of Stomach Acidity and Its Relevance to the Human Microbiome." PloS One, vol. 10, no. 7, 2015, www.ncbi.nlm.nih.gov/pmc/articles/PMC4519257/. Accessed 21 June 2017.

Bradford, Alina. "Stomach: Facts, Functions & Diseases." Live Science, 1 Sept. 2015, www.livescience.com/52046-stomach-facts-functions-diseases.html. Accessed 21 June 2017.

Choi, Charles Q. "Why the Platypus Will Never Have a Stomach." Live Science, 3 Dec. 2013, www.livescience.com/41661-why-platypus-wont-regain-stomach.html. Accessed 21 June 2017.

"The Digestive System & How It Works." National Institute of Diabetes and Digestive and Kidney Diseases, Sept. 2013, www.niddk.nih.gov/health-information/digestive-diseases/digestive-system-how-it-works. Accessed 21 June 2017.

Ghose, Tia. "Man with Hole in Stomach Revolutionized Medicine." Live Science, 24 Apr. 2013, www.livescience.com/28996-hole-in-stomach-revealed-digestion.html. Accessed 21 June 2017.

Hoffman, Matthew. "Picture of the Stomach." WebMD, www.webmd.com/digestive-disorders/picture-of-the-stomach#1. Accessed 21 June 2017.

Lashner, Bret. "9 Amazing, Weird Facts about Your Gut." Cleveland Clinic, 29 Mar. 2016, health.clevelandclinic.org/2016/03/9-amazing-weird-facts-gut/. Accessed 21 June 2017.

"Your Digestive System." WebMD, www.webmd.com/heartburn-gerd/your-digestive-system#1. Accessed 21 June 2017.

Zimmerman, Kim Ann. "Digestive System: Facts, Function & Diseases." Live Science, 11 Mar. 2016, www.livescience.com/22367-digestive-system.html. Accessed 21 June 2017.