Stomach

Author: Leandro Alegsa Creation date: November 13, 2022

The stomach is part of the digestive system. It is an internal organ between the esophagus and the small intestines. The stomach is the third stage in the digestive process. It holds food after ingestion. Food in the stomach then passes through to the small intestine where most of the food's nutrition is absorbed.

The stomach contains hydrochloric acid made by stomach cells. As a useful side-effect, the acid kills bacteria in the food. However, its main function is to help the protein-digesting enzymes called proteases work. In general, the food is churned, squeezed and mixed. This prepares it for digestion. The partially digested food is called chyme.

The stomach acid may cause problems. It can make peptic ulcers worse. It can also cause heartburn—pain in the chest when acid from the stomach refluxes (backs up into) the oesophagus. These can usually be treated by neutralizing the acid. Vomiting occurs when food goes back up the oesophagus and out the mouth. Vomiting can be a symptom of disease.

Stomachs of other mammals work differently to human stomachs. Ruminants, for example, have a stomach with multiple compartments. In that system, food goes through the first part of the digestive system twice, and the work of digesting the grass is done by bacteria.

Other words for stomach include belly and tummy.

Anatomical drawing of the human stomach and the surrounding structures, the liver has been drawn to the right and upwards

Here the large mesh between the stomach and the transverse colon has been cut and the stomach lifted so that its back and the pancreas behind it can be seen

Biological significance

Pre-digestion

In most animals and humans, the actual digestion and absorption takes place in the intestine or, more specifically, in the small intestine. In the stomach, mainly proteins are pre-digested into so-called polypeptide chains. The enzymes necessary for protein digestion, pepsin and cathepsin, have maximum activity due to the low pH value. They are secreted by the cells of the stomach as inactive precursors (pepsinogen) and are only activated by contact with the stomach acid. Fats pass the stomach largely unhindered, but they are softened (liquefied) by peristalsis. Carbohydrates pass the stomach just as unhindered, since the enzymatic cleavage of the α(1-4)-glycoside bond of amylose by α-amylase (from saliva) is inhibited again by the acidic pH in the stomach.

Killing bacteria

The acidic gastric juice effectively prevents the survival of most bacteria and thus protects against infections. For a long time, the doctrine was that in the extremely acidic environment of the human stomach, the growth of microorganisms was in principle not possible. However, with the discovery of Helicobacter pylori in the gastric mucosa in 1983, this view had to be revised. From then on, H. pylori was considered to be the only exception - a bacterium that is able to settle below the protective mucus layer. In 2005/2006, however, scientists at Stanford University were able to prove that the flora (cf. intestinal flora) is considerably more complex than previously assumed: They discovered a total of 128 additional bacterial species colonizing the stomach, including a relative of the extremophilic genus Deinococcus. A total of about ten percent of the identified microorganisms differed significantly genetically from all previously known bacteria.

Incorrectly, the pH value prevailing in the stomach is often compared with concentrated acid. The gastric juice also has a very low pH value of 0.8 to 1.5, but the concentration of hydrochloric acid is lower. However, this low pH value only applies when the stomach is empty. With food intake, the value increases and can rise to 4.5 to 6.5 depending on the buffering capacity of the food pulp.

Regular further digestion

The stomach makes it possible for living beings to get by with a few larger meals per day. It is able to adjust its wall tension to the respective amount of contents (accommodation). The gatekeeper (pylorus) ensures that all substances remain in the stomach for a sufficiently long time and are sufficiently mixed with the digestive juices. The food pulp is then slowly and evenly fed into the intestine.

Structure in humans and animals with monoclavicular stomachs

Anatomy

Seen from the outside, the stomach is divided into the anterior upper surface (paries anterior) and the posterior lower surface (paries posterior), the major curvature (curvatura major), to which the omentum majus is partially attached, and the minor curvature (curvatura minor), to which the omentum minus is attached.

Macroscopically, it is subdivided into the following areas:

The pars cardiaca (also cardia or cardia, "entrance to the stomach") - with the ostium cardiacum ("mouth of the stomach") - represents the transition between the oesophagus and the stomach.
The fundus ventriculi (or fundus gastricus, fornix ventriculi, fornix gastricus, "base of the stomach", "dome of the stomach", "vault of the stomach") lies below the diaphragm to the left and cranial of the entrance to the stomach and is filled with air swallowed during food intake. On the X-ray it appears as a gastric bubble.
The corpus ventriculi (or corpus gastricum, "stomach corpus", "stomach body") makes up the largest portion of the stomach.
The pars pylorica is narrower than the corpus, the part closer to the duodenum, the canalis pyloricus ("gateway canal"), being even narrower than the more distant part, the antrum pyloricum ("gateway cavity"), also called the vestibulum pyloricum.
The pylorus ("gatekeeper") is the connection to the duodenum via the ostium pyloricum ("gatekeeper's mouth"). It appears from the outside as a hard constriction and forms the sphincter muscle (Musculus sphincter pylori) towards the inside.

The shape and position of the stomach is not constant, but rather depends on its filling and the position of the person. The pars cardiaca, however, is relatively strongly attached by connective tissue, while the pylorus, which is only attached to the ligamentum hepatoduodenale, lowers when the contents of the stomach are larger, causing the stomach to form the typical hook shape. In women the stomach is usually deeper and steeper than in men.

See also: Blood vessels: Truncus coeliacus and gastric lymph nodes

Histology

Microscopically, the stomach consists of:

the gastric mucosa (tunica mucosa gastrica), which consists of the laminae epithelialis, propria and muscularis mucosae. It is divided into numerous fields (areae gastricae) by depressions (foveolae gastricae), with a large number of glands (glandulae gastricae).
a layer of connective tissue (tela submucosa) with blood vessels
a muscular layer (tunica muscularis gastrica) of smooth muscle consisting of the fibrae obliquae, the stratum circulare and the stratum longitudinale
a coating of the tunica serosa (peritoneum)

For the secretion of hormones and other secretions in its mucous membrane, the stomach has different types of cells, which are localized in typical places of the stomach, in particular

in corpus and fundus: accessory cells (form mucus), principal cells (form pepsinogen), parietal or vestibular cells (form hydrochloric acid and intrinsic factor),
in the antrum: G-cells (produce gastrin).

Embryology

The stomach arises as a spindle-shaped extension of the foregut (the anterior segment of the primitive intestinal tube of the embryo). This is attached to the dorsal and ventral body walls, respectively, by two mesenteries (mesogastrium dorsale and ventrale). The stomach, which initially stands in the longitudinal axis of the embryo, widens on the dorsal side to form the curvatura major and curves on the ventral side to form the flat concave curvatura minor.

The final shape and position of the stomach is formed by different growth of the stomach wall sections. The changes in position are also referred to as "gastric twists" (not to be confused with the disease gastric torsion), although in the process not passive twists but complicated remodelling processes of the stomach anlage take place. The so-called "1st gastric rotation" can be imagined as a twisting around the longitudinal axis to the left by 140 degrees. This causes the large curvature of the stomach to move from the dorsal side to ventrolateral (left ventral), and the small curvature of the stomach to move correspondingly to the right dorsolateral (lateral dorsal). The "2nd gastric rotation" can be rewritten as a rotation around the vertical axis by 90 degrees to the left. This causes the entrance of the stomach to move to the left and the pylorus to move to the right. The "3rd gastric rotation" is again about the longitudinal axis to the right by about 45 degrees. The curvatura major now points to the left and caudoventrally, the curvatura minor to the right and craniodorsally.

gastric blood supply

The stomach is supplied arterially via the coeliac trunk. This short vascular trunk branches off directly from the aorta into three main branches: the common hepatic artery, the sinistral gastric artery and the splenic artery. The stomach is supplied directly by the arteria gastrica sinistra or indirectly by further branches of the other two. The upper part of the small curvature receives its oxygenated blood directly from the Arteria gastrica sinistra. It converges with the gastrica dextra artery, which, originating in the common hepatic artery, supplies the lower part of the small curvature. Both run along the stomach in the lesser reticulum (omentum minus), where they meander along the right side of the stomach. Also originating in the common hepatic artery, the gastroomental artery dextra supplies the lower half of the greater curvature. It converges with the gastroomental sinistra artery, which in turn arises from the splenic artery. Both run at the stomach in the great web (omentum majus). In addition, the splenic artery gives off several gastric arteries, which are responsible for the supply of the fundus, and a posterior gastric artery, which provides blood flow to the posterior wall of the stomach.

Macroscopic structure of the stomach. (1) corpus, (2) fundus, (3) anterior wall of stomach, (4) major curvature, (5) minor curvature, (6) cardia, (9) sphincter pylori, (10) antrum, (11) canalis pyloricus, (12) incisura angularis, (13) gastric groove, (14) mucosal folds (by fenestration).

Microscopic preparation of normal gastric mucosa

Questions and Answers

Q: What is the stomach?

A: The stomach is part of the digestive system. It is an internal organ between the esophagus and small intestines.

Q: What is the function of hydrochloric acid in the stomach?

A: Hydrochloric acid made by stomach cells has a few functions. As a useful side-effect, it kills bacteria in food, but its main purpose is to help protein-digesting enzymes called proteases work.

Q: How does food move through the digestive process?

A: Food moves through the digestive process by being held in the stomach after ingestion, then passing into the small intestine where most of its nutrition is absorbed. Partially digested food (chyme) then passes out of the stomach and into other parts of digestion.

Q: What are some problems that can occur with too much stomach acid?

A: Too much stomach acid can make peptic ulcers worse and cause heartburn—pain in chest when acid from stomach refluxes back up into oesophagus. These can usually be treated by neutralizing the acid. Vomiting may also occur when food goes back up oesophagus and out mouth, which can be a symptom of disease.

Q: How do ruminants' digestive systems differ from humans'?

A: Ruminants have a multi-compartmented stomach where food goes through first part of digestion twice, and digesting grass is done by bacteria instead of human digestion processes like churning, squeezing and mixing for preparation for digestion as seen in humans.

Q: Are there any other words for "stomach"?

A: Yes, other words for "stomach" include belly and tummy