1. FUNCTIONAL ANATOMY OF STOMACH

Stomach is a hollow muscular organ situated just below diaphragm on the left side in abdominal cavity.

• Volume of empty stomach is 50 mL.
• Under normal conditions, stomach can expand to accommodate 1 to 1.5 L of solids and liquids.
• It is capable of expanding still further up to 4 liters.

Parts of Stomach

In humans, stomach has four parts:

1. Cardiac region
2. Fundus
3. Body or corpus
4. Pyloric region

1. Cardiac Region

• Cardiac region or cardiac end of stomach is the upper part of stomach where esophagus opens.
• This opening is guarded by a sphincter called cardiac sphincter.
• Sphincter is a circular smooth muscle which surrounds and closes a tube or an opening in the body.

2. Fundus

Fundus is a small dome-shaped structure. It is elevated above the level of esophageal opening.

3. Body or Corpus

• Body or corpus of stomach is the largest part of stomach forming about 75 to 80% of the whole stomach.
• It extends from just below the fundus up to pyloric region.

4. Pyloric Region

• Pyloric region has two parts:
  • Antrum
  • Pyloric canal
• Body of the stomach ends in antrum.
• The junction between body and antrum is marked by an angular notch called incisura angularis.
• Antrum is continued as a narrow canal called pyloric canal or pyloric end or pylorus.
• Pyloric canal opens into first part of small intestine called duodenum.
• Opening of pyloric canal is guarded by sphincter called pyloric sphincter.
• It opens towards duodenum.

Curvatures of Stomach

1. Lesser curvature on right side.
2. Greater curvature on left side.

2. STRUCTURE OF STOMACH WALL

Stomach wall is formed by four layers of structures:

1. Outer serous layer formed by peritoneum.
2. Muscular layer made up of three layers of smooth muscle:
   • Inner oblique
   • Middle circular
   • Outer longitudinal
3. Submucosal layer formed by areolar tissue, blood vessels and lymph vessels.
4. Inner mucous layer lined by mucus-secreting columnar epithelial cells.

Gastric glands are situated in the inner mucous layer.

Inner surface of mucous layer is covered by 2 mm thick mucus.

3. GLANDS OF STOMACH

Glands of stomach or gastric glands are tubular structures made up of different types of cells.

Gastric glands open into stomach cavity through gastric pits.

Classification of Gastric Glands

Gastric glands are classified into three types depending upon their situation:

1. Fundic glands situated in body and fundus of stomach.
2. Pyloric glands present in pyloric part of stomach.
3. Cardiac glands located in cardiac region of stomach.

Fundic Glands

• Fundic glands or oxyntic glands are considered as typical gastric glands.
• Fundic glands are long and tubular glands.
• Each fundic gland has three parts:
  • Body
  • Neck
  • Base

Cells Present in Fundic Glands

1. Chief cells or pepsinogen cells
2. Parietal cells or oxyntic cells
3. Mucus neck cells
4. Enterochromaffin cells (EC cells) or Kulchitsky cells
5. Enterochromaffin-like cells (ECL cells)
6. Enteroendocrine cells
7. Stem cells

• Stem cells divide and replace other cells in gastric glands.

Parietal Cells

• Parietal cells are different from other cells of gland because of presence of canaliculi.
• Canaliculi are singular secretory canaliculus into lumen of canaliculus.
• Parietal cells empty their secretions directly into lumen of gland through canaliculi.

Pyloric Glands

• Pyloric glands of stomach are short and tortuous in nature.
• Pyloric glands are formed by G cells, mucus cells, EC cells and ECL cells.

Cardiac Glands

• Cardiac glands of stomach are also short and tortuous in structure with many mucus cells.
• EC cells and ECL cells are also present in cardiac glands.

Secretory Functions of Cells in Gastric Glands

Vasoactive intestinal polypeptide is also secreted by nerve endings in stomach.

4. NERVE SUPPLY TO STOMACH

• Stomach is innervated by fibers of both divisions of autonomic nervous system.
• It receives sympathetic fibers through splanchnic nerve.
• Parasympathetic fibers to stomach pass through branches of anterior and posterior vagal trunks.

5. FUNCTIONS OF STOMACH

1. Mechanical Function

i. Storage Function

• Food is stored in stomach for a long period, i.e. for 3 to 4 hours and emptied into intestine slowly.
• Maximum capacity of stomach is up to 1.5 L.
• Slow emptying of stomach provides enough time for proper digestion and absorption of food substances in small intestine.

ii. Formation of Chyme

Peristaltic movements of stomach mix the bolus with gastric juice and convert it into a semisolid known as chyme.

2. Digestive Function

Refer functions of gastric juice.

3. Protective Function

Refer functions of gastric juice.

4. Hemopoietic Function

Refer functions of gastric juice.

5. Excretory Function

Many substances like toxins, alkaloids and metals are excreted through gastric juice.

6. PROPERTIES AND COMPOSITION OF GASTRIC JUICE

Gastric juice is the mixture of secretions from different gastric glands.

Properties of Gastric Juice

Composition of Gastric Juice

• Gastric juice contains 99.5% water.
• It contains 0.5% solids.
• Solids are organic and inorganic substances.

7. FUNCTIONS OF GASTRIC JUICE

Digestive Function

Gastric juice acts mainly on proteins.

Proteolytic enzymes of gastric juice are pepsin and rennin.

Gastric juice also contains other enzymes like gastric lipase, gelatinase, urease and gastric amylase.

Pepsin

• Pepsin is secreted as inactive pepsinogen.
• Pepsinogen is converted into pepsin by hydrochloric acid secreted by parietal cells.
• Optimum pH for activation of pepsinogen is below 6.
• Pepsin converts proteins into proteoses, peptones and polypeptides.
• Pepsin also causes curdling and digestion of milk (casein).

Digestive Enzymes of Gastric Juice

Gastric Lipase

• Gastric lipase is a weak lipolytic enzyme.
• It needs acidic medium with pH between 4 and 5 for its action.
• It becomes inactive if pH falls below 2.5.
• It acts on tributyrin (triglyceride in butter fat) and hydrolyzes it into fatty acids and glycerol.

Other Enzymes of Gastric Juice

• Gastric amylase — degrades starch, but its action is insignificant.
• Gelatinase — degrades gelatin and collagen into peptides.
• Urease — acts on urea and produces ammonia.
• Rennin — curdles milk and is present in animals only.

8. HEMOPOIETIC FUNCTION

Intrinsic factor of Castle secreted by parietal cells of gastric glands plays an important role in erythropoiesis.

• It is necessary for absorption of vitamin B12.
• Vitamin B12 is an important factor during maturation of erythrocytes.
• Absence of intrinsic factor in gastric juice causes deficiency of vitamin B12 leading to pernicious anemia.

9. PROTECTIVE FUNCTION: FUNCTION OF MUCUS

Mucus is a mucoprotein secreted by mucus neck cells of gastric glands and surface mucus cells in fundus, body and other parts of stomach.

Mucus protects gastric wall by the following ways:

1. Mucus protects stomach wall from irritation or mechanical injury, by virtue of its high viscosity.
2. It prevents digestive action of pepsin on wall of stomach, particularly gastric mucosa.
3. Mucus protects gastric mucosa from hydrochloric acid of gastric juice because of its alkaline nature and acid-combining power.

10. FUNCTIONS OF HYDROCHLORIC ACID

• HCl kills some bacteria that enter the stomach along with food substances.
• It activates pepsinogen into pepsin.
• HCl provides acidic medium necessary for the action of hormones.

11. SECRETION OF PEPSINOGEN

• Pepsinogen is synthesized from amino acids in the ribosomes attached to endoplasmic reticulum in chief cells.
• Pepsinogen molecules are packed into zymogen granules by Golgi apparatus.
• When zymogen granule is secreted into stomach from chief cells, the granule is dissolved and pepsinogen is released into gastric juice.
• Pepsinogen is activated into pepsin by hydrochloric acid.

12. SECRETION OF HYDROCHLORIC ACID

• Hydrochloric acid secretion is an active process that takes place in the canaliculi of parietal cells in gastric glands.
• Energy for this is derived from oxidation of glucose.
• In parietal cells, carbon dioxide is formed from metabolic activity and is also derived from blood.
• Carbon dioxide combines with water to form carbonic acid in the presence of carbonic anhydrase.
• Carbonic acid is unstable and immediately splits into hydrogen ion and bicarbonate ion.
• Hydrogen ion is actively pumped into canaliculus of parietal cell.
• Chloride ion is also pumped into canaliculus actively.
• Hydrogen ion combines with chloride ion to form hydrochloric acid.
• To compensate for loss of chloride ion, bicarbonate from parietal cell enters the blood and combines with sodium to form sodium bicarbonate.

13. REGULATION OF GASTRIC SECRETION

Regulation of gastric secretion and intestinal secretion is studied by some experimental procedures.

Methods of Study

1. Pavlov Pouch

• Pavlov pouch is a small part of stomach that is incompletely separated from the main portion and made into a small bag-like pouch.
• A small part of muscular coat called isthmus is retained.
• Isthmus connects the two parts.

Nerve Supply of Pavlov Pouch

• Pavlov pouch receives parasympathetic (vagus) nerve fibers through isthmus.
• It receives sympathetic fibers through blood vessels.

Use of Pavlov Pouch

• Used to demonstrate different phases of gastric secretion, particularly cephalic phase.
• Used to demonstrate role of vagus in cephalic phase.

2. Farrell and Ivy Pouch

Farrell and Ivy pouch is prepared by removing the part of Pavlov pouch from stomach and transplanting it in the subcutaneous tissue of abdominal wall or thoracic wall in the same animal.

It is used for experimental purpose when new blood vessels are developed.

This pouch is useful to study the role of hormones during gastric and intestinal phases of gastric secretion.

3. Sham Feeding

Sham feeding means the false feeding. It is another experimental procedure devised by Pavlov to demonstrate regulation of gastric secretion.

Procedure for Sham Feeding

1. A hole is made in the neck of an anesthetized dog.
2. Esophagus is transversely cut.
3. The cut ends are drawn out through a hole in the neck.
4. When the dog eats food, it comes out through the upper end of esophagus.
5. Dog has the satisfaction of eating food. It is called sham feeding.

This experimental procedure is supported by preparation of Pavlov pouch with a fistula from the stomach.

The fistula opens to exterior and it is used to observe gastric secretion.

14. PHASES OF GASTRIC SECRETION

Gastric juice is secreted in three phases:

1. Cephalic phase
2. Gastric phase
3. Intestinal phase

In human beings, a fourth phase called interdigestive phase exists.

All the phases are regulated by neural mechanism or hormonal mechanism or both.

I. Cephalic Phase

Secretion of gastric juice by stimuli arising from head region (cephalus) is called cephalic phase.

• This phase is regulated by nervous mechanism.
• During this phase 30% of total amount of gastric juice is secreted.
• During this phase gastric secretion occurs even without the presence of food in stomach.
• Quantity of juice is less but it is rich in enzymes and hydrochloric acid.
• Nervous mechanism regulates cephalic phase through reflex action.

Reflex Actions Involved in Cephalic Phase

• Unconditioned reflex
• Conditioned reflex

Unconditioned Reflex

Unconditioned reflex is the inborn reflex.

When food is placed in mouth, it induces salivary secretion. Simultaneously, gastric secretion also occurs.

Stages of Reflex Action

1. Presence of food in mouth stimulates taste buds and other receptors in mouth.
2. Sensory impulses from mouth pass via afferent nerve fibers of glossopharyngeal and facial nerves to appetite center present in amygdala and hypothalamus.
3. From here, efferent impulses pass through dorsal nucleus of vagus and vagal efferent nerve fibers to walls of stomach.
4. Acetylcholine is secreted at vagal efferent nerve endings and stimulates gastric glands to increase secretion.

Conditioned Reflex

Conditioned reflex is the reflex acquired by previous experience.

• Presence of food in mouth is not necessary to elicit this reflex.
• Sight, smell, hearing or thought of food which induce salivary secretion also induce gastric secretion.

Stages of Conditioned Reflex

1. Impulses from special sensory organs pass through afferent fibers of neural circuits to cerebral cortex.
2. From cerebral cortex impulses pass through dorsal nucleus of vagus and vagal efferents and reach stomach wall.
3. Vagal nerve endings secrete acetylcholine.
4. Acetylcholine stimulates gastric glands to increase secretion.

II. Gastric Phase

Secretion of gastric juice when food enters the stomach is called gastric phase.

• This phase is regulated by both nervous and hormonal mechanisms.
• Gastric juice secreted during this phase is rich in pepsinogen and hydrochloric acid.
• During gastric phase 60% of total amount of gastric juice is secreted.

Mechanisms Involved in Gastric Phase

1. Nervous mechanism
2. Hormonal mechanism

Nervous Mechanism

Nervous mechanism controls secretion during gastric phase through local myenteric reflex and vagovagal reflex.

Local Myenteric Reflex

Local myenteric reflex is elicited by stimulation of myenteric nerve plexus in stomach wall.

After entering stomach, food particles stimulate the local nerve plexus.

Vagovagal Reflex

Vagovagal reflex is a reflex in which both afferent and efferent vagal fibers are involved.

• Presence of food in stomach stimulates sensory (afferent) nerve endings of vagus.
• Sensory impulses are transmitted to brainstem via sensory fibers of vagus.
• Brainstem sends efferent impulses back to stomach through motor fibers of vagus.
• These cause secretion of gastric juice.
• Since both afferent and efferent impulses pass via vagus, this reflex is called vagovagal reflex.

Hormonal Mechanism: Gastrin

• Gastrin is a gastrointestinal hormone called gastrin.
• It is secreted by G cells present in pyloric glands of stomach.
• Small amount of gastrin is also secreted in mucosa of upper small intestine.
• Gastrin is a polypeptide containing G14, G17 or G34 amino acids.
• Gastrin is released when food enters stomach.
• Nerve endings release neurotransmitter called gastrin-releasing peptide which stimulates G cells to secrete gastrin.

Actions of Gastrin on Gastric Secretion

Gastrin stimulates secretion of pepsinogen and hydrochloric acid by gastric glands.

III. Intestinal Phase

Intestinal phase is the secretion of gastric juice when chyme enters the intestine.

• When chyme enters the intestine initially gastric secretion increases and later it stops.
• Intestinal phase of gastric secretion is under both nervous and hormonal control.

Initial Stage of Intestinal Phase

• Chyme entering intestine stimulates duodenal mucosa to release gastrin which is transported to stomach through blood.
• Gastrin increases gastric secretion.
• During this phase 10% of total amount of gastric juice is secreted.

Later Stage of Intestinal Phase

After the initial increase, there is decrease or complete stoppage of secretion of gastric juice.

Factors responsible for inhibition:

1. Enterogastric reflex.
2. GI hormones.

Enterogastric Reflex

It is a reflex that inhibits the secretion and movements of stomach due to distention or irritation of intestinal mucosa.

It is mediated by myenteric nerve (Auerbach’s) plexus and vagus.

GI Hormones

Presence of chyme in intestine stimulates secretion of many GI hormones from intestinal mucosa and other structures.

Some of these hormones inhibit gastric secretion and some inhibit gastric motility also.

GI hormones which inhibit gastric secretion:

1. Secretin — secreted by presence of acid chyme in intestine.
2. Cholecystokinin — secreted by presence of chyme containing fats and amino acids in intestine.
3. Gastric inhibitory peptide (GIP) — secreted by presence of chyme containing glucose and fats in intestine.
4. Vasoactive intestinal polypeptide (VIP) — secreted by presence of acidic chyme in intestine.
5. Peptide YY — secreted by presence of fatty chyme in intestine.

In addition, pancreas also secretes a hormone called somatostatin during intestinal phase. It also inhibits gastric secretion.

IV. Interdigestive Phase

Secretion of small amount of gastric juice in between meals or during period of fasting is called interdigestive phase.

Gastric secretion during this phase is mainly due to hormones like gastrin.

This phase of gastric secretion is demonstrated by Farrell and Ivy pouch.

15. SCHEMATIC REGULATION OF GASTRIC SECRETION

16. COLLECTION OF GASTRIC JUICE

In human beings, gastric juice is collected by using Ryle tube which is made of rubber or plastic.

• Ryle tube is passed through nostril or mouth and through esophagus into stomach.
• A line is marked in the tube.
• Entrance of tip of tube into stomach is indicated when this line comes near the mouth.
• Contents of stomach are collected by means of aspiration.

17. GASTRIC FUNCTION TESTS: GASTRIC ANALYSIS

For analysis, gastric juice is collected from patient only in the morning.

Analysis of gastric juice is done for diagnosis of ulcer and other disorders of stomach.

Gastric Juice is Analyzed for

1. Measurement of peptic activity.
2. Measurement of gastric acidity:
   • Total acid
   • Free acid (hydrochloric acid)

Fractional Test Meal (FTM): Fractional Gastric Analysis

Fractional analysis is the common method of gastric analysis.

• It is carried out with fractional test meal.
• After overnight fasting, gastric juice is collected.
• Patient takes a small test meal called fractional test meal.
• After ingestion of test meal, gastric juice is collected at every 15th minute for a period of 2½ hours.
• All the samples are analyzed for peptic activity and acidity.

18. APPLIED PHYSIOLOGY: GASTRIC DISORDERS

I. Gastritis

Gastritis is the inflammation of gastric mucosa.

• It may be acute or chronic.
• Acute gastritis is characterized by inflammation of superficial layers of mucous membrane and infiltration with leukocytes, mostly neutrophils.
• Chronic gastritis involves inflammation of deeper layers and infiltration with mononuclear cells.
• It results in atrophy of gastric mucosa, with loss of chief cells and parietal cells of gastric glands.
• Therefore, secretion of gastric juice decreases.

Causes of Gastritis

1. Infection with bacterium Helicobacter pylori
2. Excess consumption of alcohol
3. Smoking
4. Excess administration of aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs)
5. Trauma by nasogastric tubes
6. Repeated exposure to radiation (rare)
7. Autoimmune diseases

Features of Gastritis

Features of gastritis are nonspecific.

• Abdominal pain or upset felt as a diffused burning sensation.
• Often referred to epigastric pain.
• Nausea
• Vomiting
• Anorexia (loss of appetite)
• Indigestion
• Discomfort or feeling of fullness in epigastric region
• Belching

II. Gastric Atrophy

Gastric atrophy is the condition in which the muscles of stomach shrink and become weak.

Gastric glands also shrink, resulting in deficiency of gastric juice.

Cause of Gastric Atrophy

• Chronic gastritis called chronic atrophic gastritis.
• There is atrophy of gastric mucosa including loss of gastric glands.
• Autoimmune atrophic gastritis also causes gastric atrophy.

Features of Gastric Atrophy

• Generally, gastric atrophy does not cause any noticeable symptom.
• However, it may lead to achlorhydria (absence of hydrochloric acid in gastric juice) and pernicious anemia.
• Some patients develop gastric cancer.

III. Peptic Ulcer

Ulcer means erosion of the surface of any organ due to shedding or sloughing of necrotic tissue which lines the organ.

Peptic ulcer means an ulcer in stomach or duodenum, caused by digestive action of gastric juice.

• Peptic ulcer found in stomach is called gastric ulcer.
• Peptic ulcer found in duodenum is called duodenal ulcer.

Causes of Peptic Ulcer

1. Increased peptic activity
2. Hyperacidity of gastric juice
3. Reduced alkalinity of duodenal content
4. Decreased mucin content in gastric juice
5. Constant physical or emotional stress
6. Food with excess spices or smoking
7. Long-term use of NSAIDs
8. Chronic inflammation due to Helicobacter pylori

Features of Peptic Ulcer

• Most common feature is severe burning pain in epigastric region.
• In gastric ulcer, pain occurs while eating or drinking.
• In duodenal ulcer, pain is felt 1 or 2 hours after food intake and during night.

Other Features of Peptic Ulcer

1. Nausea
2. Vomiting
3. Hematemesis (vomiting blood)
4. Heartburn (burning pain in chest due to regurgitation of acid from stomach into esophagus)
5. Anorexia (loss of appetite)
6. Loss of weight

IV. Zollinger-Ellison Syndrome

Zollinger-Ellison syndrome is a gastric disorder characterized by hypersecretion of gastric acid, diarrhea and peptic ulcer.

Cause of Zollinger-Ellison Syndrome

This syndrome is caused by gastrinomas.

• Gastrinomas are neuroendocrine tumors in stomach.
• Tumors secrete large quantity of gastrin which increases hydrochloric acid secretion in stomach.
• Gastrinomas are found in duodenum and pancreas also.

Features of Zollinger-Ellison Syndrome

1. Abdominal pain
2. Diarrhea — frequent and watery, loose bowel movements
3. Difficulty in eating
4. Occasional hematemesis