Physiology of the Stomach

Fig 1.0 - Overview of the stomach in the gastrointestinal tract

Fig 1.0 – Overview of the stomach in the gastrointestinal tract

The stomach is a ‘J’ shaped organ of the gastrointestinal tract, directly inferior to the diaphragm. In the GI tract, it connects the oesophagus to the duodenum.

Its position and size varies continuously, with the diaphragm pushing it inferiorly during inspiration and pulling superiorly on expiration. The stomach is very distensible to enable it to accommodate food, where the digestion of starch, proteins and triglycerides begins.

In this article we will look at the physiological processes that occur in the stomach, and any clinical correlations.

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Functions of the Stomach

As a GI organ, the stomach fulfills many functions, all associated with the storage and digestion of food:

  • Mixes saliva, food and gastric juice to form chyme.
  • Reservoir for food before release into the small intestine.
  • Secretes gastric juice which can kill bacteria, pepsin to digest proteins, intrinsic factor to absorb vitamin B12 and lipase to digest triglycerides.
  • Secretes gastrin into blood.

Histology of the Stomach

The mucosa of the stomach consists of three main layers:

  • Epithelial layer – The surface of the mucosa consists of a layer of simple columnar epithelial cells called surface mucous cells.
  • Lamina propria – A layer of thin connective tissue.
  • Muscularis mucosae – A thin layer of smooth muscle.
Fig 1.1 - The layers of the stomach wall.

Fig 1.1 – The layers of the stomach wall.

In the mucosa, epithelial cells extend down into the lamina propria to form gastric glands.

These glands secrete many important substances (see ‘Gastric glands’, below).

Underneath the mucosa lies the submucosa, muscularis externa, and the serosa:

  • The submucosa is composed of areolar connective tissue.
  •  The muscularis externa has three layers of smooth muscle; an outer longitudinal layer, a middle circular layer and an inner oblique layer.
  • The serosa covering the stomach is composed of simple squamous epithelium and areolar connective tissue. It is fused with the visceral peritoneum with overlays the stomach

At the lesser curvature of the stomach, the a double layer of peritoneum extends upward to the liver as the lesser omentum. At the greater curvature of the stomach, another double layer of peritoneum continues downward as the greater omentum, draping over the intestines.


Gastric Glands

Fig 1.2 - The exocrine cells of a gastric gland.

Fig 1.2 – The exocrine cells of a gastric gland.

The gastric glands are, by definition, exocrine glands. They contain four main cell types, each with a different secretion:

  • Mucous neck cells secrete mucus.
  • Parietal cells produce intrinsic factor and HCl.
  • Chief cells secrete pepsinogen and lipase.
  • G cells secrete gastrin.

Gastrin is arguably the most important secretion, influencing the majority of the functions of the stomach.

Its functions include

  • Stimulate parietal cells to secrete HCl and chief cells to secrete pepsinogen.
  • Contracts lower oesophageal sphincter.
  • Increases stomach motility.
  • Relaxes pyloric sphincter.

Digestion

One of the major functions of the stomach is to begin the digestion of food. Once the food bolus reaches the stomach, peristaltic waves macerate the food, mixing it with gastric secretions to form a solution known as chyme.

Fig 1. 3 - The peristaltic ejection waves of the stomach

Fig 1. 3 – The peristaltic ejection waves of the stomach

As digestion proceeds, more vigorous mixing waves begin in the body of the stomach, with the waves intensifying in the pylorus.

The pyloric sphincter, which is normally closed, allows a small volume of chyme to pass through into the duodenum.

Most of the chyme however is forced back into the stomach body for further mixing. A further peristaltic wave forces the chyme towards the pyloric sphincter.

Enzymatic digestion of proteins begins in the stomach. The main enzyme involved is pepsin, secreted by chief cells. Pepsin breaks peptide bonds between amino acids, breaking down a protein chain into smaller peptide fragments. Pepsin is most effective an acidic environment, which the stomach provides. This enzyme is secreted as the inactive pepsinogen so as to prevent the digestion of proteins that produce it.

Only a small amount of digested nutrients are absorbed in the stomach, because its epithelial cells are relatively impermeable. Mucous cells absorb some water, ions, alcohol and short chain fatty acids.


Gastric Acid Secretion in the Stomach

Maintaining an acidic environment is crucial to the proper functioning of the stomach. Gastric acid is synthesised and secreted by the parietal cells of the stomach.

The parietal cells have a unique structure.  Canaliculi (invaginations of the cell walll) are present on the lumenal surface of the cell. These invaginations play a role in the secretion of gastric acid:

  1. In the mitochondria of the parietal cell, water is split into H+ and OH-
  2. OH- combines with CO2 to form HCO3-, which leaves the cell in exchange for a Cl- ion.
  3. The H+ is pumped into the lumen of the gastric gland by proton pumps on the canaliculi.
  4. Cl- ions diffuse into the gastric lumen from the parietal cell. They combine with H+ to from HCl.

The sight, smell and taste of food initiates reflexes that stimulate parasympathetic neurons to release acetylcholine. Acetylcholine and gastrin stimulate parietal cells to secrete more HCl. They also cause the release of histamine, which acts synergistically to enhance the effects of acetylcholine and gastrin.

Fig 1.4 - Mechanism of gastric acid secretion from the parietal cells.

Fig 1.4 – Mechanism of gastric acid secretion from the parietal cells.


Clinical Relevance: Physiological Disorders of the Stomach

Stomach Ulcers

Fig 1.5 - A peptic ulcer.

Fig 1.5 – A peptic ulcer.

An ulcer is an erosion in the mucosal layer of the stomach .There are two types of ulcer involving the stomach:

  • Gastric ulcers are lesions of the stomach mucosa.
  • Peptic ulcers are lesions of the mucosa of the pyloric canal or duodenum.

Ulcers are often associated with the bacterium Helicobacter pylori.

Individuals with an ulcer generally have a high volume of gastric acid secretion, which overwhelms the bicarbonate buffer and reduces the protective nature of the mucous lining. H. pylori erodes the mucous lining resulting in inflammation, rendering the mucosa vulnerable to gastric acid and pepsin. If an ulcer erodes into the gastric arteries, it can cause severe bleeding

Achlorhydria

Achlorhydria is a condition where gastric acid production is absent. It has various causes, including hypothyroidism, H. pylori infection, and autoimmune disorders.

As the stomach pH is no longer low, pepsin cannot work optimally. The higher pH also fails to prevent bacterial replication, and pathogen overgrowth is a common finding. Both these factors cause symptoms similar to gastro-oesophagal reflux disease. Treatment focuses on addressing the underlying cause

 

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