A Simple Guide to Digestion of Food in the Stomach

About 85% of its mass lies to the left of the midline of the body.

The main parts of the stomach are:

1. Cardiac orifice

This is the opening from the esophagus.

It is guarded by the cardiac sphincter.

The stratified epithelium of the esophagus undergoes transition into the columnar epithelium of the stomach.

2. Body of the stomach

This is the main portion of the stomach.

3. Fundus

This is the dilated upper part of the stomach lying to the left of the cardiac sphincter.

4. Pyloric Portion

This portion is the end of the stomach and consists of:

A pyloric antrum which is a dilated portion set from the body of the stomach

b. by the incura angularis a slightly constricted area.

c. pyloric sphincter here which is made up of circular muscle fibers

d. which encircle a narrow pyloric canal

5. Pyloric orifice

This is the opening into the duodenum of the small intestine

The stomach is made up of 4 layer or coats:

1. Mucosa is the innermost coat.

When the stomach is empty, the contracted stomach has its mucosa thrown into many longitudinal folds called rugae which disappear when the food distend the stomach.

The surface epithelium consists of simple columnar epithelial cells which contain many invaginations or gastric pits at the bottom of which the gastric glands open.

Goblet cells and villi are missing.

2. Submuosa consists of the areolar connective tissue containing the blood vessels, lymphatics and nerves.

It is situated between the mucosa and the muscular coat binding them together.

3. Muscular coat consists of 3 layers:

a. outer longitudinal

b. middle circular

c. inner oblique

4. Serosa is the outermost coat of the stomach consist of a thin layer of connective tissue which forms part of the greater omentum

The gastric glands are simple or branched tubular glands with a narrow neck which opens into a gastric pit in the mucosa.

They are confined to the mucosa.

There are about 30 to 40 millions of such glands.

4 types of cells are found:

1. chief or zymogenic cells which secrete the pepsin.
Pepsin cannot function without the aid of hydrochloric acid.

2. parietal cells which are triangular in shape and peripherally located, most in the neck region of the stomach.

It secretes hydrochloric acid.

3. mucous neck glands which lie in the neck region between parietal cells.

It produces mucin.

4. argentaffine cells which occur singly and produce the anti anemia or intrinsic factor.

The pepsin decomposes the proteins into metaproteins, then to proteoses and finally to peptones.

The action of the gastric juices and the muscular movements of the stomach (churning) convert the food into semi liquid chyme, an acid substance which allows the pyloric sphincter to open allowing the chyme to be propelled into the small intestine.

This process usually takes about 4 hours for a meal to be digested by the stomach juices.

The stomach parietal cells also contain endocrine cells which produces gastrin.

The motility of the stomach is controlled by the autonomic nervous system via 2 plexuses in the stomach wall.

The stomach is surrounded by parasympathetic (stimulant) and orthosympathetic (inhibitor) plexuses (networks of blood vessels and nerves in the anterior gastric, posterior, superior and inferior, celiac and myenteric) which regulate both the secretions activity and the motor (motion) activity of its muscles.

Solids can only flow into the duodenum after they become suspended as particles of 0.3 mm diameter.

Emptying rate for carbohydrate is faster than protein. This is followed by fats.

TABLE OF CONTENT

Chapter 1 The Stomach

Chapter 2 Digestion in the Stomach

Chapter 3 Gastrin

Chapter 4 Pepsin

Chapter 5 Gastric Acid

Chapter 6 Intrinsic Factor