Stomach

The stomach is a muscular, hollow organ in the upper gastrointestinal tract of humans and many other animals, including several invertebrates. It plays a key role in the digestive system, helping to break down food so our bodies can use it. After we eat, food moves from the esophagus into the stomach, where it mixes with special liquids that help break it apart.

In the stomach, food is mixed with digestive enzymes and gastric acid. These help turn the food into a softer, more liquid form so it can move on to the next part of our digestive system. The stomach is very important for keeping our bodies healthy by helping us digest what we eat.

The stomach is located between the esophagus and the small intestine. A special valve called the pyloric sphincter controls when food leaves the stomach and enters the duodenum, the first part of the small intestine. From there, muscles help push the food along the rest of the intestines through a process called peristalsis.

Structure

In the human digestive system, the stomach sits between the esophagus and the duodenum, which is the first part of the small intestine. It is found in the upper left area of the abdominal cavity. The stomach can stretch to hold about one litre of food when it’s not full.

The human stomach has four main parts: the cardia, fundus, body, and pylorus. The cardia is where food enters from the esophagus. The fundus is the upper curved part. The body is the main central area, and the pylorus connects the stomach to the duodenum. The stomach’s walls help break down food for digestion.

Development

In the early stages of a human embryo's growth, part of it touches the yolk sac. As the embryo develops during the third week, it starts to wrap around the yolk sac. This wrapped part becomes the basis for the adult digestive system. The yolk sac is surrounded by a network of tiny blood vessels that later form the main arteries supplying the digestive system: the celiac artery, superior mesenteric artery, and inferior mesenteric artery. These arteries help define different parts of the digestive system.

As the stomach forms and turns, certain tissues around it also turn and create spaces. These tissues eventually become layers that protect the stomach and other organs, supplying them with blood and nerves. These layers are part of a larger structure called the peritoneum, which helps keep everything in place and working properly.

Function

Digestion

Further information: Human digestive system

See also: Gastric acid

In the human digestive system, food that has been chewed enters the stomach through a tube called the esophagus. The stomach releases special substances that help break down the food. These include enzymes that break apart proteins and a strong acid that helps kill tiny germs and creates the right conditions for the enzymes to work. The stomach's muscles mix and churn the food, turning it into a thick liquid called chyme. This chyme then moves slowly into the small intestine, where nutrients from the food are taken up by the body.

Mechanical digestion

When food first enters the stomach, waves of mixing begin. These waves gently at first, then become stronger, helping to blend the food with stomach juices. The stomach has a narrow opening that only lets small pieces of food and liquids pass through at a time. This careful release ensures the small intestine can handle the food bit by bit. The stomach continues to mix the remaining food until more can pass through.

Chemical digestion

The top part of the stomach, called the fundus, holds food and gases made during digestion. While food sits here, some digestion continues before mixing with the rest of the stomach contents. The stomach also helps break down proteins using acid and special enzymes. It can even produce a enzyme that helps with fat digestion.

The stomach finishes emptying its contents into the small intestine within a few hours after a meal. Different foods take different times to leave the stomach, with fatty foods staying the longest.

Absorption

Though most absorption happens in the small intestine, the stomach can absorb a few small substances, such as water when the body is thirsty, some medicines, certain acids from drinks, and a tiny bit of vitamins.

The stomach also makes a important substance needed for absorbing a vital vitamin that helps red blood cells and the nervous system work properly.

Control of secretion and motility

The release of food from the stomach into the small intestine is carefully controlled. The stomach coordinates this process using both nerves and special chemicals called hormones. When the small intestine is busy digesting food, the stomach holds back the food until it’s ready.

Other

The stomach plays a role in sensing the nutritional value of foods and can influence how the brain perceives taste. It also shares some interesting connections with the thyroid gland, an organ in the neck that helps regulate many body processes.

Chyme from the stomach is slowly released into the duodenum through coordinated peristalsis and opening of the pyloric sphincter. The movement and the flow of chemicals into the stomach are controlled by both the autonomic nervous system and by the various digestive hormones of the digestive system:

Clinical significance

Diseases

Doctors can look at the stomach using special pictures called radiographs, often with a liquid called barium swallow. They can also use a tiny camera called an endoscope. To see how well the stomach empties, they may do a special test called a gastric emptying study.

Many problems like stomach sores and inflammation happen because of a tiny germ called Helicobacter pylori. This can also be linked to a serious sickness called stomach cancer.

The noises we hear from our stomach are actually coming from the intestines, not the stomach itself.

Surgery

Doctors sometimes change the stomach to help people lose weight. They might put a band around part of the stomach to make it smaller, or they might change the shape of the stomach.

In very serious cases, doctors might need to remove part or all of the stomach. This is called a gastrectomy. They might also wrap a part of the stomach around the pipe that connects the throat to the stomach to help with certain problems.

Etymology

The word "stomach" comes from an old Greek word, "stomachos," which means "mouth." Words like "gastro" and "gastric" also come from Greek words meaning "belly."

Other animals

The stomach looks and works a bit differently in various animals. Most animals have a stomach that curves to the left before connecting to the next part of the digestive system. However, some animals like lampreys, hagfishes, chimaeras, lungfishes, and certain teleost fish do not have stomachs at all. In these animals, food passes directly from the esophagus to the intestine.

In many animals, the stomach has special areas with different types of glands. For example, ruminants such as cows have a special four-chambered stomach. The first three chambers work differently from the main stomach chamber, called the abomasum.

Birds and crocodilians also have unique stomach structures. They have a narrow tube called the proventriculus that connects to a muscular part called the gizzard, which helps grind up food.

Insects have a part called a crop, and their stomach is known as the midgut.

Additional images

Image showing the celiac artery and its branches

A human stomach showing the many [folds](/wiki/Gastric_folds) ([rugae](/wiki/Rugae))

High-quality image of the stomach