Functions of the Stomach

 

The mucosa of the stomach contains simple tubular glands. The glandular secretions differ in different regions of the stomach. The most characteristic secretion is derived from the glands in the fundus and body of the stomach

 

Glands in the fundus and body of the stomach

Mucous neck cells    -       mucus

Parietal cells        -          acid

Chief cells           -        pepsin

ECL cells             -           histamine

 

Functions of the stomach

•        Food is stored body and    antrum

•        Formation of chyme

•        Chemical break down

•        Mechanical break down

•        Release food in a controlled steady rate.

•        Digestion of protein

•        Secretion of intrinsic factor

•        Gastric acid secretion.

•        Conversion of dietary iron (Fe⁺³) to absorbable iron (Fe⁺²)

 

Constituents of gastric juice

•        Cations ® Na⁺, K⁺, Mg⁺⁺, H⁺  

   (pH approximately 1.0)

•        Anions® HCO3, Cl - , HPO4- -  , SO4^-

•        Pepsins

•        Lipase

•        Mucus

•        Intrinsic factor

 

•        Intrinsic factor is Essential for vit. B₁₂ absorption, secreted by parietal cells along with HCl, Pernicious anemia is an autoimmune disease. autoantibodies are formed against parietal cells. a megaloblastic type of anemia and achlorhydria

Functions of HCl

1.Kill ingested bacteria by HCl

2.aids protein digestion

3.stimulates the flow of bile& pancreatic juice

4. Dietary iron is in the form of Fe⁺⁺⁺.  HCl permits Fe⁺⁺⁺ to form soluble complexes with ascorbic acid and other substances.

Then Fe⁺⁺⁺ is converted to Fe⁺⁺& is the absorbable form.

    Fe⁺⁺⁺                   Fe⁺⁺.

                gastric acid

 

Gastric acid secretion

There are three primary stimuli of gastric secretion

1. Gastrin

2. Acetylcholine

3. Histamine

 

1.Gastrin ®  G cells antrum blood

2.Acetylecholine ® postganglionic cholinergic nerve endings

                                                                                   

3.Histamine® enterochromaffin-like cell (ECL cell)

Receptors for the stimulus are

•        Histamine ® H2 receptor

•        Acetylcholine ® M3 - muscarinic receptors

•        Gastrin ® CCK-B receptors

Hypoglycaemia via brain & vagal efferents stimulate acid & pepsin secretion.

Alcohol & Caffeine act directly on mucosa.

•        At rest® tubular vesicular structures & H⁺ –K⁺ ATPase is inactive.

•        In active state®Microvilli projects out –surface area increased.

 ­ H⁺ K⁺ ATPase molecules at surface.

 

Regulation of G.I secretion

•        Cephalic phase

•        Gastric phase

•        Intestinal phase

•        Cephalic phase

    Presence of food in the mouth reflexley stimulate G.A secretion.

 

•        Inborn reflex

•         An alimentary conditioned reflex that become established early in life

 

Cephalic

•        influence responsible for 1/3-1/2 of acid secreted during a   normal meal.

 

Gastric phase

•        Presence of food in the stomach accelerates the increase in gastric secretion.

•        1.Local reflex arc

   2. Gastrin

    3.vagus

 

•        release of somatostatin, inhibits both G and ECL cells as well as secretion by parietal cells themselves.

•        A key mechanism of termination of the meal on reaching the small intestine.

 

–      Secretin ®secretion increased by products of protein digestion and by Acid bathing mucosa of upper small intestine.

•        Acid®® Secretin®® (-) Acid, an example of feedback control.

Gastric mucosal barrier

•        Gastric acid can irritate the mucosa

•        Pepsin can auto digest the mucosa

•        As a protective measure it is secreted as a proenzyme.

•        To protect the mucosa from acid & pepsin gastric mucosal barrier is present

•        The mucosal barrier helps to create a pH gradient between the lumen and the epithelial cell.

 

Mucus - Glycoprotein, forms a coating over the cells.It  is a flexible gel. Surface mucus cells, mucus neck cells

HCO3^- - Trapped in mucus gel

•        Trefoil peptides- these are acid resistant and presence in mucosa provides resistance to autodigestion. These are secreted by the mucosa.

•        It stabilizes the mucus-bicarbonate layer

 

Substances which disrupt the barrier

   Ethanol

                          Vinegar

                           Bile salts

                Aspirin & other NSAIDS® inhibit                                        Prostaglandin synthesis

(Prostaglandin is a stimulus for mucus secretion