Physiology for MRCEM Primary

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Gastro-intestinal physiology


[box type=”download”]  Overview of functions (storage, digestion, chyme regulation + intrinsic factor)  Role of the chief and parietal cells  The crucial role of the proton (H-K-ATPase) pump in hydrogen ion management  Protective mucosal barrier mechanisms within the gastric lumen  Secretion patterns (cephalic, gastric, intestinal) and their stimulant pathways  Chyme production and its effects upon the pyloric sphincter  Factors affecting gastric emptying (duodenal luminal pH, fats, distension) [/box]

The main functions of the stomach are to store food temporarily to chemically and mechanically digest food using acids, enzymes and movements, to regulate the release of the resulting chyme into the small intestine, and to secrete an intrinsic factor which is essential for the absorption of vitamin B12.
Within the mucosa it has secretory cells that line the gastric glands or pits.
When empty, the stomach has a volume of about 50 mL; however, when fully distended, its 4 L.
Proteins in the food are broken down into polypeptides by enzymes called pepsins.
Inactive pepsinogens from the chief cells in the gastric mucosa are activated to pepsins by the acid environment in the stomach.
The hydrochloric acid is produced by parietal cells.
This concentration of H+ ions is achieved using a protein known as the proton pump or the H+K+ ATPase protein utilizing ATP.
The gastric mucosa is protected by an alkaline, mucin-rich fluid secreted by the gastric glands.
In addition, local mediators, such as prostaglandins, are released when the mucosa is irritated, and these increase the thickness of the mucous layer and stimulate the production of bicarbonate which neutralizes the acid.

Control of gastric secretions

Three phases: cephalic, gastric and intestinal.

The cephalic phase is brought about by the sight, smell, taste and mastication of food.
Acid secretion is stimulated by the activation of the vagus and its actions on the enteric plexus.
Postganglionic parasympathetic fibres in the myenteric plexus cause the release of acetylcholine (ACh) and stimulate the release of gastric juices from the gastric glands.
Vagal stimulation also causes the release of a hormone called gastrin from cells in the antrum of the stomach called G-cells.
Gastrin is secreted into the bloodstream and, when it reaches the gastric glands, it stimulates the release of acid and pepsinogens.
Both vagal activity and gastrin also stimulate the release of histamine from mast cells, which, in turn, acts on parietal cells to produce more acid.

The gastric phase of secretion of acid, pepsinogen and mucus (When food arrives in the stomach).
The main stimuli – the distension of the stomach and the chemical composition of the food.
Mechanoreceptors in the stomach wall are stretched and set up local myenteric reflexes and also longer vagovagal reflexes.
Both cause the release of ACh which stimulates the release of gastrin, histamine and, in turn, acid, enzymes and mucus.
Stimulation of the vagus also releases a specific peptide, gastrin-releasing peptide (GRP), which mainly acts directly on the G-cells to release gastrin.
Whole proteins do not affect gastric secretions directly, but their breakdown products, such as peptides and free amino acids, do so by directly stimulating gastrin secretion.
A low pH (more acid) inhibits gastrin secretion; therefore, when the stomach is empty or after food has entered it and acid has been secreted for some time, there is an inhibition of acid production.
Thus, gastric acid secretion is self-regulating.
The gastric phase normally lasts for about 3 h and the food becomes a sludge-like – chyme.
The chyme enters the first part of the small intestine, the duodenum, via pyloric sphincter.
The presence of chyme in the pyloric antrum distends it and causes antral contractions and opening of the sphincter.
The rate at which the stomach empties depends on the volume in the antrum and the fall in the pH of the chyme, both leading to an increase in emptying.
However, distension of the duodenum, the presence of fats and a decrease in pH in the duodenal lumen all cause an inhibition of gastric emptying.
This ensures a precise supply of chyme to the intestines.