Stomach | Physiology

                             Stomach

Stomach

 This article discusses the structure and secretion of the stomach. Here, learn about its different functions and more.

Keywords: Stomach | stomach parts | Gastric gland | Gastric juice | Phase of gastric secretion | Stomach functions | Gastrin | Postprandial Alkaline tide |Regulation of HCl secretion|

Table of contents :

1. Introduction

2. Parts of the stomach

3. Structure of  the stomach

4. Secretion of the stomach

5. Control of gastric secretion

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                                                 Introduction

The stomach is present in the left hypochondrium. It is continuous above the esophagus by the cardiac orifice and below, connected to the first part of the duodenum through the pyloric orifice. (The word stomach comes from the Latin Stomachus, which originates in the Greek word "Stomachos." ultimately from "stoma, "meaning mouth.)

Gastric and gastro words are related to stomach, and both- are derived from the Greek word Gaster, meaning 'belly.' 

The stomach can be divided into three parts:

(Another additional part is the Cardium –the part of the stomach around the Cardiac orifice.) 

1. The fundus, the uppermost part of the stomach, lies above the cardiac opening level and close to the left diaphragm.

2. The body is the portion of the stomach in the lower end of the fundus to the angular notch level, also known as incisura angularis, in the lesser curvature.

3. The pyloric part is the lowermost part of the stomach distal to the body of the stomach and is divided into two parts: The proximal antrum and the distal pyloric canal.

 

 

 Structure of  stomach: The stomach contains the usual four layers of the gastrointestinal tract, from outside to inside 

1. Serous layer 2. Outer longitudinal muscle layer and inner circular muscle layer 3. Submucosa, and 4. Mucous membrane. 

One characteristic feature of the stomach is its three muscular layers: outer longitudinal, middle circular, and inner oblique muscle layers. The inner oblique muscle layer is arranged in a fan-shaped manner. Another characteristic feature of the stomach is its thick mucous membrane, covered by a thick visible mucus layer. The thick mucous membrane is folded. These folds enable the abdomen to increase in size when food enters the stomach.

Gastric glands: They are present in the stomach mucosa, and depending upon the position of the glands, they are of three types:

1. Cardiac

2. Fundic and

3. Pyloric glands.

            

Cardiac and pyloric glands secrete mucus, but fundic glands secrete gastric juice.

Fundic or acid-secreting glands are straight or tubular glands that open at the depressed part of the mucosa called the gastric pit. Each gland has four types of cells :

1. Isthmus cells,

2. Neck cells,

3. Oxyntic cells and

4. Chief cells.

Isthmus and neck cells secret mucin. Neck cells cause regeneration of the epithelium and the gland.

Chief cells are the central cells of the gland, and they secrete pepsinogen. They are also known as peptic cells.

 Oxyntic cells, also known as parietal cells, secrete HCl acid. The lining membrane of these cells contains many H⁺ -K⁺ ATPase. These H⁺ -K⁺ ATPase secrete H+ into the gastric glands' lumen. These hydrogen ions combine with Cl—and form HCl acid.

G cells are present in the pyloric glands. They are triangular cells and secrete gastrin, which stimulates acid secretion from oxyntic cells by the paracrine effect.                   

Other secretions having an effect on gastric juice secretions:

1.D cells secret somatostatin and have an inhibitory effect on the G cells.

 2. Mast cells and enterochromaffin-like cells( ECL) secret histamine.

 3. Argentaffin cells secrete serotonin.

 Secretion of the stomach is known as a gastric secretion or gastric juice:

 Daily secretion is 2.5 liters.

 Gastric juice is highly acidic.

 Composition of gastric juice:

 99.45% water, and the rest 0.55% is solids

 Organic constituents are mucin, pepsinogen, and intrinsic factor. gastric lipase, lysozyme, Carbonic anhydrase etc.

Electrolyte constituents are sodium, potassium magnesium ions, bicarbonate phosphate sulfate in ionic form, and HCL present as H⁺ and Cl^- ions.

Gastric mucus is of two types:

Visible mucus and Soluble mucus.

Surface epithelial cells secrete large amounts of visible mucus and bicarbonate ions. This bicarbonate mixes with mucus, making it alkaline. It helps form the gastric mucosal barrier, is slippery, and prevents physical trauma from solid food and acids. This visible mucus is secreted in the resting stomach and increases after food intake.

 Soluble mucus is secreted by neck cells. It is not secreted in resting conditions, but secretion starts when food enters the stomach. Mucin is the essential chemical constituent of mucus. This is a glycoprotein composed of monomers. Each has a protein core surrounded by carbohydrate chains that protect it from proteolytic digestion. Mucin secretion is enhanced by vagal stimulation, food in the stomach, and prostaglandins.

 Hydrochloric acid( HCl): The average concentration of HCl is 40-60 equivalents per liter, and a maximum concentration of 160 equivalents per liter is possible. Secreted hydrochloric acid is diluted in the stomach. 

In the oxyntic cell, water breaks down into H⁺ and  OH^-. OH^- H⁺ is pumped out of the cell into the lumen by H⁺ -K⁺ATPase, i.e., 'Proton pump.' One potassium ion enters the cell in exchange for one hydrogen ion. Hydrogen ions re-enter the gastric lumen through a potassium channel against their concentration gradient. This Potassium ion is subsequently pumped back into the parietal cells by the H⁺ -K⁺ -ATPase.

Inside the parietal cell, carbon dioxide comes from cell metabolism and reacts with water to form carbonic acid under the influence of the carbonic anhydrase enzyme. The carbonic anhydrase enzyme is a catalyst. This carbonic acid breaks down into H and HCO₃^-. This H ⁺ neutralizes the toxic OH^-  radical of the first reaction. The HCO₃^- passes out through the basolateral membrane in exchange for a Cl^- ion through a Cl^- / HCO₃^-  exchanger. This causes an increase in the cell concentration of Cl^- ions. These ions come to the luminal membrane and pass into the gastric lumen through a Cl-channel. Finally, hydrochloric acid secures the H + and Cl- into the lumen.

 Postprandial alkaline tide from the above reactions is evident, especially after a meal when hydrochloric acid is produced in large amounts, HCO3- passes to the blood in equally large quantities of excess HCO₃^-  ions in the blood leading to a transit alkalemia. The bicarbonate ions in the blood are filtered through the kidney, and bicarbonate ions pass into the urine, making it alkaline. This alkaline reaction of blood and urine after meals is the postprandial alkaline tide.

 Regulation of hydrochloric acid secretion

 Acetylcholine, histamine, and gastrin regulate HCl acid secretion. The cell membrane has a specific receptor for them.

 Acetylcholine is secreted from the vagal cholinergic nerve endings. It binds to the M₃ muscarinic receptor, opening a Ca++ channel and causing a rise in intracellular Ca⁺⁺ concentration. Ach( acetylcholine) acting through the M₃ muscarinic receptor in the parietal cell stimulates H⁺ -K⁺ATPase.

 Gastrin is a less powerful stimulant of hydrochloric acid secretion. Gastrin is secreted from G cells present in the pyloric antrum. It increases intracellular calcium ion concentration through the phosphor inositol pathway.

Histamine is secreted from enterochromaffin-like cells( ECL). Present in gastric glands. Histamine acts on the H₂ receptor of the parietal cell. In addition, histamine activates adenyl cyclase through a Gs protein, which causes the breakdown of ATP into cAMP.This increases cAMP concentration and activates cAMP-dependent protein kinase a, ultimately increasing hydrochloric acid secretion.

 Acetylcholine, Histamine, and Gastrin are independent stimulants of hydrochloride acid secretion, and they potentiate each other's actions.

The parasympathetic nerve is a potent stimulator of HCl acid secretion. Mode of action:

By stimulation:

Directly stimulates M₃ muscarinic receptors by releasing acetylcholine,

Indirectly stimulates gastrin release via GRP, stimulating histamine formation by  ECL cells.  

By inhibition :

 Inhibition of somatostatin secretion from D cells. This removes the inhibitory effect of somatostatin on hydrochloric acid.

 

 Pepsinogen is secreted from the chief or peptic cells of the stomach. It is a proenzyme converted into pepsin, the active form due to acidic media around pH 2, an autocatalytic process.

 Pepsin is a proteolytic enzyme, an endopeptidase—it acts in the midportion of the molecules and not terminally. It causes the cleavage of peptide bonds involving aromatic amino acids and produces proteases and peptones. It digests about 20% of ingested protein.

 Intrinsic factor is secreted from gastric mucosa's oxyntic cells and ⁴hydrochloric acid. It combines with Vitamin B_12,  also known as the' extrinsic factor of Castle.' Intrinsic and extrinsic factors are highly resistant to digestive degradation, and in the terminal ileum, vitamin B₁₂ is absorbed, and intrinsic factors are reabsorbed. The deficiency of B₁₂ leads to pernicious anemia, an autoimmune disease that may be due to gastrectomy.

                             Control of gastric secretion

 Gastric secretion is controlled by

 Neural mechanism and

 Chemical mechanism.

  The phase of gastric secretion

 1. Cephalic phase

 2. Gastric phase

 3. Intestinal phase and

 4. Inter digestive phase 

 Cephalic phase  (Cephalos means brain): This phase starts within 5 minutes after stimulation and lasts 2 hours; the secretion rate is between 250 and 750 ml per hour. The secretion contains acid and pepsin and is not affected by the chemical nature of food. Gastric secretion in the cephalic phase increases if a person is mentally relaxed.

This phase is due to the vagus nerve, which carries impulses from the brain via conditioned and unconditioned reflexes.

Conditioned reflexes initiated by sight, smell, and thought of food. It may occur in response to the nonphysiological conditioned stimulus.

 Unconditioned reflexes are initiated by food present in the mouth, mastication, and deglutition.

Gastric phase

In this phase, gastric secretion is stimulated by food in the stomach. The mechanism of action of this phase is neural and chemical.

                                           Neural mechanism

The neural mechanism involves two reflexes - 1. long vasovagal reflexes and 2. short reflexes.

In the long vasovagal reflex, impulse starts due to a stretch of the stomach wall. This travels to the brain, where efferent vagal fibers return to the stomach and stimulate gastric secretion.

In short reflexes, the afferent and efferent nerves are located entirely within the stomach wall. A Stretch of the stomach wall is a stimulus for gastric secretion.

Chemical stimulation of this phase is mediated through the hormone gastrin. Gastrin secretion is stimulated by digested products of protein. The phase starts within 15 minutes and lasts as long as the food is in the stomach. The rate of secretion is about 70 ml/hour. The composition of the secretion depends on the nature of the food. For example, protein food rich in tryptophan and phenylalanine stimulates the increased secretion of hydrochloric acid and pepsin.

 When pH is below 3, D cells are stimulated to release somatostatin, inhibiting gastric secretion. This negative feedback prevents excessive acidity in the gastric lumen.

 Intestinal phase: initially, it is stimulated but later depressed. Gastric chyme entering the duodenum is less acidic (pH is more than 3), gastric secretion is stimulated, but later on, when incoming gastric chyme becomes strongly acidic( pH more than 2), gastric secretion is inhibited. Secretion in this phase starts 2 to 3 hours after stimulation and lasts for 8 to 10 hours, and the secretion rate is 60 ml per hour. Gastric secretion is stimulated in this phase by gastrin liberated from G cells in the duodenum and upper jejunum in response to peptides and amino acids in the chyme. Another hormone, enterotoxin, also stimulates gastric secretion in this phase.

 Later in this phase, various factors reflexly inhibit gastric secretion and motility.

The acid in the duodenum causes neural inhibition of gastric secretion. In addition, hyperosmolarity, hyper-acidity, and digested products of lipids in the duodenum inhibit gastric secretion in this phase.

The interdigestive phase is when there are no stimulatory effects from the brain, stomach, and intestine. The basal secretion increases in abnormal conditions, e.g., duodenal ulcer.

Movements of the stomach

The fundus and body move due to the 'basic electrical rhythm.'

Movements of Pylorus

Emptying of stomach

Movements of the pyloric sphincter.

The parasympathetic nerve vagus causes an increase in movements and relaxes the sphincter. Therefore, when the tone is high, it induces relaxation; when it is low, the vagus causes contraction.

Sympathetic nerves inhibit gastric movements and constrict the sphincter.

 Functions of stomach

Storage and digestion of food.

Release of food to the intestine in a regulated pattern.

Secrets hydrochloric acid, mucus, digestive enzymes, intrinsic factors.

Absorption of water, drugs, vitamins, and alcohol.

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