Stomach

1. Overview of the Stomach

One of the functions of the stomach is to store food that will enter through the esophagus. Remember, the esophagus is the tube that connects your mouth and your throat to your stomach and carries food down to your stomach after it's been chewed and swallowed.

After food enters, the stomach will help to chemically and mechanically break it down. Although a minimal amount of carbohydrate digestion occurs in the mouth, chemical digestion really gets underway in the stomach. An expansion of the alimentary canal that lies immediately inferior to the esophagus, the stomach links the esophagus to the first part of the small intestine (the duodenum) and is relatively fixed in place at its esophageal and duodenal ends. In between, however, it can be a highly active structure, contracting and continually changing position and size. These contractions provide mechanical assistance to digestion.

think about it

The empty stomach is only about the size of your fist but can stretch to hold as much as 4 liters of food and fluid, or more than 75 times its empty volume, and then return to its resting size when empty.

reflect

Although you might think that the size of a person’s stomach is related to how much food that individual consumes, body weight does not correlate with stomach size. Rather, when you eat greater quantities of food—such as at holiday dinner—you stretch the stomach more than when you eat less.

Popular culture tends to refer to the stomach as the location where all digestion takes place. Of course, this is not true. An important function of the stomach is to serve as a temporary holding chamber. You can ingest a meal far more quickly than it can be digested and absorbed by the small intestine. Thus, the stomach holds food and parses only small amounts into the small intestine at a time. Foods are not processed in the order they are eaten; rather, they are mixed together with digestive juices in the stomach until they are converted into chyme, which is released into the small intestine.

As you will see in the sections that follow, the stomach plays several important roles in chemical digestion, including the continued digestion of carbohydrates and the initial digestion of proteins and triglycerides. Little if any nutrient absorption occurs in the stomach, with the exception of the negligible amount of nutrients in alcohol.

2. Structure of the Stomach

There are four main regions in the stomach: the cardia, fundus, body, and pylorus. The cardia (or cardiac region) is the point where the esophagus connects to the stomach via the lower esophageal sphincter and through which food passes into the stomach. Located below the diaphragm, above and to the left of the cardia, is the dome-shaped fundus. Below the fundus is the body, the main part of the stomach that is primarily responsible for mechanical and chemical digestion in the stomach. The funnel-shaped pylorus connects the stomach to the duodenum. The wider end of the funnel, the pyloric antrum, connects to the body of the stomach. The narrower end is called the pyloric canal, which connects to the duodenum. The smooth muscle pyloric sphincter is located at this latter point of connection and controls stomach emptying. In the absence of food, the stomach deflates inward, and its mucosa and submucosa fall into a large fold called a ruga.

The convex lateral surface of the stomach is called the greater curvature; the concave medial border is the lesser curvature. The stomach is held in place by the lesser omentum, which extends from the liver to the lesser curvature, and the greater omentum, which runs from the greater curvature to the posterior abdominal wall.

terms to know

Stomach

The alimentary canal organ that contributes to chemical and mechanical digestion of food from the esophagus before releasing it, as chyme, to the small intestine.

Cardia

(also, cardiac region) The part of the stomach surrounding the cardiac orifice (esophageal hiatus).

Fundus

The dome-shaped region of the stomach above and to the left of the cardia.

Body

The mid-portion of the stomach.

Pylorus

The lower, funnel-shaped part of the stomach that is continuous with the duodenum.

Pyloric Sphincter

The sphincter that controls stomach emptying.

Ruga

The fold of alimentary canal mucosa and submucosa in the empty stomach and other organs.

2a. Histology

The wall of the stomach is made of the same four layers as most of the rest of the alimentary canal but with adaptations to the mucosa and muscularis for the unique functions of this organ. In addition to the typical circular and longitudinal smooth muscle layers, the muscularis has an inner oblique smooth muscle layer. As a result, in addition to moving food through the canal, the stomach can vigorously churn food, mechanically breaking it down into smaller particles.

The stomach mucosa’s epithelial lining consists only of surface mucus cells, which secrete a protective coat of alkaline mucus. A vast number of gastric pits dot the surface of the epithelium, giving it the appearance of a well-used pincushion, and mark the entry to each gastric gland, which secretes a complex digestive fluid referred to as gastric juice.

Although the walls of the gastric pits are primarily made up of mucus cells, the gastric glands are made up of different types of cells. The glands of the cardia and pylorus are primarily composed of mucus-secreting cells. Cells that make up the pyloric antrum secrete mucus and a number of hormones, including the majority of the stimulatory hormone, gastrin. The much larger glands of the fundus and body of the stomach, the site of most chemical digestion, produce most of the gastric secretions. These glands are made up of a variety of secretory cells. These include parietal cells, chief cells, mucous neck cells, and enteroendocrine cells.

Parietal cells—Located primarily in the middle region of the gastric glands are parietal cells, which are among the most highly differentiated of the body’s epithelial cells. These relatively large cells produce both hydrochloric acid (HCl) and intrinsic factor. HCl is responsible for the high acidity (pH 1.5 to 3.5) of the stomach contents and is needed to activate the protein-digesting enzyme, pepsin. The acidity also kills much of the bacteria you ingest with food and helps to denature proteins, making them more available for enzymatic digestion. Intrinsic factor is a glycoprotein necessary for the absorption of vitamin B₁₂ in the small intestine.

Chief cells—Located primarily in the basal regions of gastric glands are chief cells, which secrete pepsinogen, the inactive proenzyme form of pepsin. HCl is necessary for the conversion of pepsinogen to pepsin.

Mucous neck cells—Gastric glands in the upper part of the stomach contain mucous neck cells that secrete thin, acidic mucus that is much different from the mucus secreted by the goblet cells of the surface epithelium. The role of this mucus is not currently known.

Enteroendocrine cells—Finally, enteroendocrine cells found in the gastric glands secrete various hormones into the interstitial fluid of the lamina propria. These include gastrin, which is released mainly by enteroendocrine G cells.

terms to know

Gastric Pit

A narrow channel formed by the epithelial lining of the stomach mucosa.

Gastric Gland

The gland in the stomach mucosal epithelium that produces gastric juice.

Gastrin

A hormone secreted by the enteroendocrine cells that stimulates the secretion of gastric acid by the parietal cells of the stomach mucosa.

Parietal Cell

A gastric gland cell that secretes hydrochloric acid and intrinsic factor.

Hydrochloric Acid (HCl)

A digestive acid secreted by parietal cells in the stomach.

Intrinsic Factor

A glycoprotein required for vitamin B₁₂ absorption in the small intestine.

Chief Cell

A gastric gland cell that secretes pepsinogen.

Pepsinogen

The inactive proenzyme form of pepsin secreted by chief cells and converted to pepsin by HCl in the stomach.

Mucous Neck Cell

A gastric gland cell that secretes a uniquely acidic mucus.

3. Digestive Functions of the Stomach

The stomach participates in virtually all digestive activities with the exception of ingestion and defecation. Although almost all absorption takes place in the small intestine, the stomach does absorb some drugs, such as alcohol and aspirin.

Within a few moments after food enters your stomach, mixing waves begin to occur at intervals of approximately 20 seconds. A mixing wave is a unique type of peristalsis that mixes and softens the food with gastric juices to create chyme. The initial mixing waves are relatively gentle, but these are followed by more intense waves, starting at the body of the stomach and increasing in force as they reach the pylorus. It is fair to say that long before your sushi exits through the pyloric sphincter, it bears little resemblance to the sushi you ate.

The pylorus, which holds around 30 mL (1 fluid ounce) of chyme, acts as a filter, permitting only liquids and small food particles to pass through the mostly, but not fully, closed pyloric sphincter. In a process called gastric emptying, rhythmic mixing waves force about 3 mL of chyme at a time through the pyloric sphincter and into the duodenum. Release of a greater amount of chyme at one time would overwhelm the capacity of the small intestine to handle it. The rest of the chyme is pushed back into the body of the stomach, where it continues mixing. This process is repeated when the next mixing waves force more chyme into the duodenum.

Gastric emptying is regulated by both the stomach and the duodenum. The presence of chyme in the duodenum activates receptors that inhibit gastric secretion. This prevents additional chyme from being released by the stomach before the duodenum is ready to process it.

The fundus plays an important role because it stores both undigested food and gases that are released during the process of chemical digestion. Food may sit in the fundus of the stomach for a while before being mixed with the chyme. While the food is in the fundus, the digestive activities of salivary amylase continue until the food begins mixing with the acidic chyme. Ultimately, mixing waves incorporate this food with the chyme, the acidity of which inactivates salivary amylase and activates lingual lipase. Lingual lipase then begins breaking down triglycerides into free fatty acids as well as mono- and diglycerides.

The breakdown of protein begins in the stomach through the actions of HCl and the enzyme pepsin.

Its numerous digestive functions notwithstanding, there is only one stomach function necessary to life: the production of intrinsic factor. The intestinal absorption of vitamin B₁₂, which is necessary for both the production of mature red blood cells and normal neurological functioning, cannot occur without intrinsic factor.

EXAMPLE

People who undergo total gastrectomy (stomach removal)—for life-threatening stomach cancer, can survive with minimal digestive dysfunction if they receive vitamin B₁₂ injections.

The contents of the stomach are completely emptied into the duodenum within 2 to 4 hours after you eat a meal. Different types of food take different amounts of time to process. Foods heavy in carbohydrates empty fastest, followed by high-protein foods. Meals with a high triglyceride content remain in the stomach the longest. Since enzymes in the small intestine digest fats slowly, food can stay in the stomach for 6 hours or longer when the duodenum is processing fatty chyme. However, note that this is still a fraction of the 24 to 72 hours that full digestion typically takes from start to finish.

IN CONTEXT

Bariatric Surgery

Bariatric surgeries are weight-loss surgeries that are typically done when an individual has serious weight-related health issues or when diet and exercise have not been sufficient to achieve weight loss. There are several different types of procedures that involve modifying the gastrointestinal system to reduce the amount of food that can be eaten and/or limiting how much of the digested food can be absorbed (Mayo Clinic, 2022).

Gastric banding is an example of a bariatric surgery in which a small pouch is created in the stomach. This reduces the size of the stomach that can be used for digestion. Because the stomach is smaller, an individual who had this surgery will feel full after consuming less food.

terms to know

Mixing Wave

A unique type of peristalsis that occurs in the stomach.

Gastric Emptying

The process by which mixing waves gradually cause the release of chyme into the duodenum.

3a. Gastric Secretion

The secretion of gastric juice is controlled by both nerves and hormones. Stimuli in the brain, stomach, and small intestine activate or inhibit gastric juice production. This is why the three phases of gastric secretion are called the cephalic, gastric, and intestinal phases. However, once gastric secretion begins, all three phases can occur simultaneously.

The cephalic phase (reflex phase) of gastric secretion, which is relatively brief, takes place before food enters the stomach. The smell, taste, sight, or thought of food triggers this phase.

EXAMPLE

When you bring a piece of sushi to your lips, impulses from receptors in your taste buds or the nose are relayed to your brain, which returns signals that increase gastric secretion to prepare your stomach for digestion.

This enhanced secretion is a conditioned reflex, meaning it occurs only if you like or want a particular food. Depression and loss of appetite can suppress the cephalic reflex.

The gastric phase of secretion lasts 3 to 4 hours and is set in motion by local neural and hormonal mechanisms triggered by the entry of food into the stomach.

EXAMPLE

When the sushi reaches the stomach, it creates distention that activates the stretch receptors. This stimulates parasympathetic neurons to release acetylcholine, which then provokes increased secretion of gastric juice.

Partially digested proteins, caffeine, and rising pH stimulate the release of gastrin from enteroendocrine G cells, which in turn induces parietal cells to increase their production of HCl. HCl is needed to create an acidic environment for the conversion of pepsinogen to pepsin, and protein digestion. Additionally, the release of gastrin activates vigorous smooth muscle contractions. However, it should be noted that the stomach does have a natural means of avoiding excessive acid secretion and potential heartburn. Whenever pH levels drop too low, cells in the stomach react by suspending HCl secretion and increasing mucous secretions.

The intestinal phase of gastric secretion has both excitatory and inhibitory elements. The duodenum has a major role in regulating the stomach and its emptying. When partially digested food fills the duodenum, intestinal mucosal cells release a hormone called intestinal (enteric) gastrin, which further excites gastric juice secretion. This stimulatory activity is brief, however, because when the intestine distends with chyme, the enterogastric reflex inhibits secretion. One of the effects of this reflex is to close the pyloric sphincter, which blocks additional chyme from entering the duodenum.

IN CONTEXT

GLP-1 Receptor Agonists

GLP-1 receptor agonists, such as semaglutide and liraglutide, have been showing promise for treating type 2 diabetes and, more recently, obesity. GLP-1 receptor agonists are a class of non-insulin injectable drugs that impact the gut hormone incretin, which works to increase insulin secretion in response to meals. Furthermore, they are able to slow down digestion, such as by slowing gastric emptying.

terms to know

Cephalic Phase

(also, reflex phase) The initial phase of gastric secretion that occurs before food enters the stomach.

Gastric Phase

The phase of gastric secretion that begins when food enters the stomach.

Intestinal Phase

The phase of gastric secretion that begins when chyme enters the intestine.