The stomach is the most dilated part of the digestive tube and lies between the esophagus and the small intestine (duodenum). It is a sac-like organ that is generally pear-shaped, being broader on the left side and narrower on the right. The stomach serves as a connection between the esophagus and the intestine and can change considerably in size and shape depending on how full or empty it is.

Regions of the Stomach

The stomach is divided into four main regions: the cardia, fundus, body, and pyloric region. The cardia is the region where the esophagus opens into the stomach. It is located on the upper border of the stomach near the left end of the lesser curvature. 

Relations to Surrounding Structures

The stomach is closely related to several surrounding organs. The fundus is in contact with the diaphragm, spleen, and left lobe of the liver. 

The pylorus lies against the quadrate lobe of the liver and the gallbladder and continues directly into the duodenum. The greater curvature gives attachment to the greater omentum and is related to the transverse colon. Behind the stomach are the pancreas, the left kidney, and the left suprarenal gland. The pancreas is separated from the stomach by the bursa omentalis. 

The stomach rests on a group of structures collectively referred to as the stomach bed, which includes the diaphragm, spleen, pancreas, left kidney, left suprarenal gland, and portions of the colon.

Size and Changes in Shape

The size and shape of the stomach vary greatly depending on its degree of filling. An empty, contracted stomach may appear quite small and resemble a loop of intestine. When greatly distended, the greater curvature may extend downward as far as the navel. During distension, the stomach may also rotate slightly, causing the greater curvature to move into a more anterior position.

The average stomach measures approximately 25-30 cm in length and 12-14 cm in width. The wall is usually 2-3 mm thick, although this varies with the degree of distension.

The oblique layer is best developed in the fundus and body. Its fibers run obliquely across the stomach wall and intermingle with the circular fibers.

The presence of these three muscle layers allows the stomach not only to move food along the digestive tract but also to vigorously churn and mechanically break it down.

Mucous Membrane

When the stomach is empty or partially contracted, the mucous membrane forms numerous folds called rugae. These folds are mainly longitudinal but are crossed by transverse folds, creating small areas known as gastric areas. As the stomach fills and expands, these folds gradually disappear.

At the pylorus, the mucous membrane forms the pyloric valve, which covers the pyloric sphincter.

The gastric mucosa varies in color from light gray to reddish, depending on its blood supply. During digestion, when blood flow increases, superficial veins may become visible.

The surface of the mucosa contains numerous tiny openings called gastric pits, which lead into the gastric glands.

Gastric Glands and Secretory Cells

The gastric glands produce gastric juice and contain several specialized cell types.

The surface epithelium is composed of mucus-secreting cells that produce a protective alkaline mucus coating. The glands of the cardia and pyloric region also contain many mucus-secreting cells.

Parietal cells are found mainly in the glands of the fundus and body. They secrete: hydrochloric acid (HCl) and intrinsic factor.

Hydrochloric acid creates the highly acidic environment of the stomach, activates pepsin, kills many ingested bacteria, and helps denature proteins. Intrinsic factor is necessary for the absorption of vitamin B₁₂ in the small intestine.

Chief cells secrete pepsinogen, the inactive precursor of pepsin. Hydrochloric acid converts pepsinogen into active pepsin.

Mucous neck cells are located in the upper portions of the gastric glands and secrete a thin acidic mucus.

Enteroendocrine cells release hormones into the surrounding tissue. These include gastrin, which is produced mainly by specialized G cells.

Gastric Secretion

The production of gastric juice is controlled by both the nervous and endocrine systems. Signals from the brain, stomach, and small intestine can either stimulate or inhibit gastric secretion. For this reason, gastric secretion is divided into three phases: the cephalic phase, gastric phase, and intestinal phase. Although described separately, these phases can overlap and occur at the same time.

Cephalic Phase

The cephalic phase occurs before food enters the stomach and is relatively brief. It is triggered by the sight, smell, taste, or even thought of food.  

Sensory receptors send signals to the brain, which then stimulates the stomach to begin secreting gastric juice in preparation for digestion. This response is a conditioned reflex and occurs only when a person desires or enjoys the food. Factors such as depression or loss of appetite can suppress this phase.

Gastric Phase

The gastric phase begins when food enters the stomach and usually lasts for 3-4 hours.  

As food accumulates in the stomach, it causes distension of the stomach wall, activating stretch receptors. This stimulates parasympathetic neurons to release acetylcholine, which increases gastric secretion.  

Partially digested proteins, caffeine, and an increase in pH stimulate enteroendocrine G cells to release gastrin. Gastrin increases the production of hydrochloric acid (HCl) by parietal cells. HCl creates the acidic environment needed to convert pepsinogen into pepsin and supports protein digestion. Gastrin also stimulates vigorous contractions of the stomach's smooth muscle.  

To prevent excessive acidity, the stomach has protective feedback mechanisms. When the pH becomes too low, stomach cells reduce HCl secretion and increase mucus production.

Intestinal Phase

The intestinal phase begins when partially digested food enters the duodenum.  

Initially, intestinal mucosal cells release intestinal (enteric) gastrin, which briefly stimulates additional gastric secretion. However, as the duodenum becomes distended with chyme, the enterogastric reflex inhibits further gastric activity.  

One effect of this reflex is closure of the pyloric sphincter, which slows the movement of additional chyme into the duodenum until the intestine is ready to process it.

Protection of the Stomach: The Mucosal Barrier

The stomach is regularly exposed to highly acidic gastric juice and digestive enzymes capable of breaking down proteins. Since these substances could potentially damage the stomach itself, the organ is protected by a mucosal barrier.

Several mechanisms contribute to this protection:

• A thick layer of bicarbonate-rich mucus covers the stomach wall. This mucus forms a physical barrier and helps neutralize acid.

• The epithelial cells of the stomach are connected by tight junctions, preventing gastric juice from penetrating deeper tissues.

• Stem cells located near the gastric pits rapidly replace damaged epithelial cells.

As a result of this continuous renewal process, the surface epithelium of the stomach is completely replaced every 3-6 days.

Digestive Functions of the Stomach

The stomach participates in nearly all digestive processes except ingestion and defecation. Although most nutrient absorption occurs in the small intestine, the stomach can absorb certain substances, including alcohol and aspirin.

Mechanical Digestion

Shortly after food enters the stomach, rhythmic contractions called mixing waves begin. These waves occur approximately every 20 seconds.

A mixing wave is a specialized form of peristalsis that blends food with gastric juice, softening it and producing a semi-liquid mixture called chyme. The waves begin gently but become progressively stronger as they move from the body of the stomach toward the pylorus.

The pylorus acts as a filter and normally contains about 30 mL of chyme. Only liquids and very small food particles are allowed to pass through the partially closed pyloric sphincter.

During gastric emptying, mixing waves force about 3 mL of chyme at a time through the pyloric sphincter and into the duodenum. Most of the chyme is pushed back into the body of the stomach, where further mixing occurs. This cycle repeats until the stomach has emptied its contents.

The rate of gastric emptying is controlled by both the stomach and the duodenum. When chyme enters the duodenum, receptors are activated that inhibit gastric secretion and slow the release of additional chyme. This prevents the small intestine from being overloaded.

Chemical Digestion

The fundus serves as a storage area for undigested food and gases released during digestion. Food may remain in the fundus for some time before being mixed with chyme.

While food is stored there, salivary amylase continues to digest carbohydrates. Once the food mixes with the acidic chyme, salivary amylase becomes inactive. At the same time, the acidic environment activates lingual lipase, which begins breaking down triglycerides into free fatty acids and mono- and diglycerides.

The digestion of proteins begins in the stomach through the combined actions of hydrochloric acid (HCl) and pepsin. HCl creates the acidic conditions necessary for converting pepsinogen into pepsin, while pepsin initiates the breakdown of proteins.

Intrinsic Factor

One of the stomach's most important functions is the production of intrinsic factor.

Intrinsic factor is required for the absorption of vitamin B₁₂ in the small intestine. Vitamin B₁₂ is essential for the formation of mature red blood cells and for normal neurological function.

Without intrinsic factor, vitamin B₁₂ cannot be absorbed effectively.