The stomach stores food and begins protein digestion.

• This J-shaped chamber has a fundus, body, and antrum. Its terminal part has a pyloric sphincter.

• The most important function of the stomach is to store food. Most of this occurs in the body of the stomach.

• It also secretes HCl to begin protein digestion.

• The mixing movements of the stomach produces chyme.

• Most of this mixing occurs in the antrum. • The stomach accommodate a twenty-fold

increase in volume by receptive relaxation.

Gastric filling involves receptive relaxation

• Every time bolus reaches the stomach, it relaxes slightly. So stomach size can be increased 20 times with little changes in tension in it’s wall. The reflex relaxation of the stomach as it is receiving food is called receptive relaxation which is mediated by vagus nerve.

Gastric storage and mixing• Food is stored in stomach

body.• Basic electrical rhythym (BER)

occurs continuously and it may initiate contraction (peristaltic movement). This peristaltic movement spreads over fundus and body to the antrum. In antrum these peristaltic waves (contraction) become strong and it helps in mixing the food, while they are weak in the fundus and body of the stomach so these region act as storage of food before it is mixed with gastric juice.

body

antrum

Gastric mixing (cont.)

• Each time strong antral contraction occurs, it pushes food forward toward the pyloric sphincter which is closed so food returns back. Such movements will mix the food with digestive food and converting food into semi-fluid called chyme.

• The strong peristaltic contractions force chyme to pass through pyloric sphincter into duodenum. The amount of chyme passing depends on the strength of contractions which are controlled by signals from stomach it’s self and the duodenum.

Gastric emptying

Direction ofmovementof peristalticcontraction

Pyloric sphincter

Duodenum

Duodenal factors that influence gastric emptying• The duodenum can reduce gastric emptying by reducing gastric

peristaltic contractions.• Presence fat, acid, hypertonic chyme and distension stimulate certain

receptors in the duodenum which in turn send neural and hormonal signals to reduce gastric motility and thus reduce emptying.

***Neural signals are mediated through both intrinsic and autonomic systems (entrogastric reflex).

*** hormones secreted from duodenum that reduce gastric contractions and thus emptying are cholycystokinin and secretin.

Note: Peristaltic action in the stomach drives chyme from the stomach into the small intestine. Distension of the stomach increases gastric motility. Signaling by the vagus nerve and the hormone gastrin increases motility.

Types of gastric glands and cells• Gastric cells1. mucous neck cells

(secrete mucus) 2. surface epithelial cells

(secrete mucus) 3. chief cells (secrete

pepsinogen) 4. G cells (secrete gastrin -

in antrum) 5. parietal (oxyntic) cells

(secrete HCl and intrinsic factor)

6. D cells (secrete somatostatin)

7. Entrochromaffin like cells (secrete histamin)

• Gastric glands1. The oxyntic glands -body

and fundus

2. Pyloric glands - antrum region

Table 15-3 (2), p. 480

In oxyntic mucosa

Gastricpit

Gastricgland

Surface epithelial cells

Mucosa cells

Chief cells

Parietal cells

Enterochomaffin-like (ECL) cells

In pyloric gland area

G cells

D cells

GASTRIC SECRETION

•approx. 2 L gastric juice per day •secretion is isotonic, pH 2-3 (in stomach lumen)

Functions of gastric secretion

1. Intrinsic Factor: - from parietal cells, for vitamin B12 uptake in lower small intestine ( terminal ileum).

- It’s absence leads to pernicious anemia.

2 .Mucus: protects gastric epithelium. It adheres to gastric surface and prevents H+ and pepsin eroding the mucosa; failure of this process leads to gastric ulceration.

GASTRIC SECRETION (cont.)

3 .HCl - from parietal (oxyntic) cells• Function of Gastric acid

-to kill micro-organisms -to activate pepsinogen (cleaved to form pepsin)-breaks down connective tissue in food -denatures protein

4. Pepsinogen -from chief (peptic) cells

-conversion to pepsin (active enzyme) requires low pH -pepsin initiates protein digestion; optimum pH 1-2

Autocatalysis

Digestion

Protein

Peptide fragments

Gastriclumen

HCI

Pepsinogen Pepsin

Pepsinogen is secreted as inactive enzyme. It is activated by HCl forming pepsin. Once pepsin is formed it can activates more pepsinogen (autocatalytic process). Pepsin is working in acidic media and it’s work stops if pH is increased. It breaks down proteins into smaller fragments.

Mucosal Protection of gastric epithelium

• Gastric mucosa (surface) is covered by mucous layer. This mucus is secreted by surface epithelial cells and mucous cells. It protects gastric mucosa against

1. Mechanical injury.2. Self-digestion by pepsin.3. HCl- it is alkaline

(contains bicarbonates ions).

Parietal cells secret HCl and intrinsic factor

Multiple regulatory pathways influence the parietal and chief cells.

– Through both local reflex and vagal reflexes, the acetylcholine (a neurotransmitter) is released from the intrinsic nerve plexuses stimulates these cells. Also Ach stimulates G and ECL.

**** G cells secrets hormone Gasrtin**** ECL releases Histamin

• Gasrin is secreted into blood from G cells in response to:- Protein products in the stomach- AchGastrin carried by blood to fundus and body of the stomach. There it

stimulates both parietal and chief cells.*** Gastrin also stimulates secretion of histamin from ECL and histamin in

turn stimulates parietal cells for hydrochloric acid release. Gastrin is the main factor for stimulation HCl secretion during meal digestion

in the stomach.

– Somatostatin released from D cells in response to high acidity. It inhibits in negative-feedback mechanism the secretions from parietal cells, G cells and ECL.

histamine

ECL cell

Other facts about stomach activity include:• Gastric secretion gradually decreases as food empties from the

stomach into the intestine. The presence of protein is withdrawn from the stomach as the meal enters the small intestine.

• The stomach lining is protected from gastric secretions by the gastric mucosal barrier. The mucosal membrane is almost impermeable to hydrogen ions.

• Carbohydrate digestion continues in the body of the stomach. Proteins digestion begins in the antrum. Salivary amylase continues to work in the internal mass of food. Food is not mixed with gastric secretions in the body of the stomach.

• The stomach absorbs alcohol and aspirin, but does not absorb food.

Phases of gastric secretion• The control of gastric secretion

involves three phases.I. Cephalic phase occurs before

food enters stomach. Stimuli from the head increase the secretion of hydrochloric acid and pepsinogen.

** Thinking about, tasting, chewing, smelling and swallowing food increase gastric secretion through impulses carried by the vagus nerve:

a. Directly stimulation of parietal cells and

b. Indirectly stimulation of G cells

Phases of gastric secretion (cont.)

II. the gastric phase occurs while food is in stomach

Stimuli acting in the stomach itself, for example protein, distension, caffeine and alcohol stimulate chemreceptors in the stomach and this stimulation will stimulate both intrinsic nerves plexus

(enteric) and extrinsic nerves (vagus) .

***Also paracrine (local) hormones (histamine) and endocrine hormones (gastrin) are released which are stimulants for more gastric juice secretion.

III. The intestinal phase: occurs after food enters small intestine

-inhibitory effec of secretin and CCK decrease the flow of gastric juices.

Table 15-4, p. 483

Table 15-5, p. 484

What Inhibits HCl Secretion?

• Somatostatin released by Gastric D cells is the central inhibitory mechanism on acid production.– Directly inhibits parietal cell acid production, inhibits ECL

histamine release, and inhibits release of gastrin from G cells.– Somatostatin release is stimulated by Gastrin and neural inputs

in D cells in the fundus, and by low pH in D cells in the antrum

• Secretin from duodenal cells plays the main role in inhibiting acid secretion after the entry of fat and acid into the duodenum

• Cholecystokinin (CCK) also inhibits acid secretion.

Secretion from other parts of GIT

Esophygeal secretion: entirely mucus and it’s function is; to prevent mucosal excoriation by newly entering food. To protect the lower esophagus from the effect of refluxed gastric juice.