ANATOMY

2.  To sustain life body needs a continual supply of water, electrolytes and nutrients.
 This function is served by the gastrointestinal or the so called digestive system.
 The digestive system comprises gastrointestinal tract (GIT) and accessory organs of
digestion like teeth, tongue, salivary glands, liver and exocrine part of pancreas.

3.  Gastrointestinal tract also known as alimentary canal is basically a muscular tube
extending from the mouth to the anus.
 At either end the lumen (opening) is continuous with external environment .
 It measures about 10 metres (30 feet) and comprises following parts:
MOUTH
 Mouth is loosely used term to denote the external opening and for the cavity
(empty space) it leads to.
 The cavity containing anterior two-thirds of tongue and teeth is the mouth cavity or
oral cavity or Buccal cavity.
 Tongue, in the digestive system, plays two important roles:
 Tells the taste of food and
 Helps in chewing and swallowing of the food.
 Teeth are accessory organ/ helpful organ of digestion which help in chewing the
food.

4. FUNCTION OF MOUTH

5. SUNLINGUAL
GLAND
SUBMANDIBULAR
GLAND
PAROTID GLAND

6. PHARYNX
 The pharynx is a median passage that is common to the gastrointestinal and
respiratory systems.
 Nasopharynx : Not part of the digestive system, it leads to the lungs.
 Oropharynx : Which is posterior to oral cavity.
 Laryngopharynx : It is below the oropharynx and is connected to the oesophagus.

7. OESOPHAGUS
 It is a fibromuscular tube about 25 cm long.
 Its junction to the pharynx is called upper oesophageal sphincter.
 Its junction with the stomach is called as lower oesophageal sphincter.

8. STOMACH
 It is a hollow muscular bag connected to the oesophagus at its upper end and to the
duodenum at the lower end.
SMALL INTESTINE
 It is a long tubular structure which can be divided into three parts
SMALL
INTESTINE
ILEUM
JEJUNUM
DUODENUM

9. DUODENUM
 Duodenum is the first part of small intestine.
 It is C-Shaped and measures about 25 cm in length.
JEJUNUM
 Jejunum is the middle part of the small intestine, is about 25 metres long.
ILEUM
 Ileum is the last part of small intestine, is about 3.5 metres long.

10. LARGE INTESTINE
 It arches around and encloses the coils of the small intestine and tends to be more
fixed than the small intestine.
 It is divided in to following parts,
LARGE
INTESTINE ANAL
CANAL
RECTUM
COLON
CAECUM
AND
APPENDIX

12. PERITONEUM
 Peritoneum refers to the serous membrane which lines the walls of abdominal
cavity (parietal layer) and also the viscera (internal organs of the body) contained
in abdominal and pelvic cavities (visceral layer).
 The space between the parietal and visceral layers of peritoneum is called the
peritoneal cavity.
NOTE
The serous membrane, or serosal
membrane, is a thin membrane
that lines the internal body cavities
and organs such as the heart, lungs,
and abdominal cavity.

13. FUNCTIONS OF PERITONEUM
 It is a serous membrane which enables the abdominal contents to glide over each
other without friction.
 It forms a partial or complete covering for the abdominal organs.
 It forms ligaments (flexible fibrous connective tissue) and mesenteries (fold of
peritoneum) which help to keep the organs in position.

14. STRUCTURAL CHARACTERISTICS OF GIT WALL
 Different parts of the gastrointestinal tract are specialized for carrying out different
functions particularly digestion and absorption, but the basic structural characteristics of
the wall of whole gastrointestinal tract are similar.
 The intestinal wall from inside to outwards consists of following layers,
LAYERS
OF GIT
WALL
SEROSA
LAYER
MUSCLE
COAT
SUBMUCOSA
LAYER
MUCOSA
LAYER

16. MUCOSA LAYER
 It is innermost coat consisting of three layers,
Surface epithelium
 Lining the luminal surface consists of epithelial cells which vary in type from
simple squamous to tall columnar depending upon the function of the part of GIT.
Lamina propria
 Is composed of loose connective tissue which contains numerous glands, small
blood vessels, lymphatics and nerve fibres.
Muscularis mucosa
 Is composed of two thin layers of smooth muscle fibres which help in localised
movements of the mucosa.
SUBMUCOSA
 This refers to the layer of connective tissue present outside the mucosa.
 It contains blood vessels, lymphatics and a network of nerve fibres and nerve cells
called submucosal nerve plexus (Meissner’s plexus)

17. MUSCLE COAT
 It is formed by a thick layer of smooth muscle fibres surrounding the submucosa.
 The smooth muscle fibres are arranged in two layers,
 Ciliary muscle fibres form the inner layer, and
 Longitudinal muscle fibres form the outer layer.
 In between the circular and longitudinal muscle fibres is present an extensive
network of nerve cells and fibres named Auerbach’s plexus (Myenteric plexus).
SEROSA
 This is the outermost layer consisting of a layer of connective tissue.
 This layer helps in the attachment of gut to the surrounding structures.

18. INNERVATION (supply of nerves) OF THE GIT
 The innervation of the gastrointestinal tract includes intrinsic and extrinsic system.
INTRINSIC INNERVATION
 The intrinsic nervous system also called as enteric nervous system consists of nerve
cells and fibres which originate and are located in the intestinal wall itself.
 The enteric nervous system is composed mainly of two plexuses, such as Myenteric
plexus and Meissner’s plexus.
 Myenteric plexus or Auerbach’s plexus is present in between the circular and
longitudinal muscle fibres of muscular coat of the GIT.
 Meissner’s plexus or submucosal plexus is present in the submucosal layer. It
controls the secretory activity and blood flow to the gut.
NOTE
 The Myenteric plexus and Meissner’s plexus are interconnected with each other
and are under the control of parasympathetic and sympathetic components of
extrinsic nervous system.

19. EXTRINSIC INNERVATION
 The extrinsic system of nerves supplying the gut consists of the parasympathetic
and sympathetic components of autonomic nervous system.
Parasympathetic innervation
 The parasympathetic supply to the gut is made up of cranial and sacral divisions.
 The cranial parasympathetic fibres originate in medulla, come through vagus and
supply the oesophagus, stomach, small intestine, pancreas and first half of large
intestine.
 The sacral parasympathetic fibres originate in sacral spinal cord, pass through
pelvic nerves to hypogastric (pelvic) ganglion as a postganglionic fibre, supply of
distal half of large intestine and rectum.
 The main function of parasympathetic innervation is increase in gastrointestinal
motility and secretory activity.

21. Sympathetic innervation
 The sympathetic fibres to gut arise from eighth thoracic (T8) to second lumbar (L2)
spinal segments. The sympathetics innervate all portions of the gastrointestinal tract
rather than being more extensively supplied to portions near the oral cavity and
anus.
Functions of sympathetic innervation
Sympathetic stimulation causes,
 Vasoconstriction
 Inhibition of motility in the gut.
NOTE
 Most of the effects of sympathetic stimulation are opposite to that of
parasympathetic stimulation.

24. GROSS ANATOMY
General features
 Stomach is a J-shaped hollow muscular bag connected to the oesophagus at its
upper end and to the duodenum at the lower end.
 The volume of stomach is 1200-1500 ml, but its capacity is greater than 3000 ml.
 The stomach has two curvatures. The concavity of the right inner curve is called
lesser curvature, and the convexity of the left outer curve is the greater
curvature.
Anatomical relations of stomach
 Anteriorly the stomach is related to left lobe of liver and anterior abdominal wall.
 Posteriorly the structures related to stomach include: abdominal aorta, pancreas,
spleen, left kidney and left adrenal gland.
 Superiorly the stomach is related to diaphragm, oesophagus and left lobe of liver.
 Inferiorly the stomach lies on transverse colon and small intestine.
 To the left side of stomach lie diaphragm and spleen.
 To the right side of stomach are situated liver and duodenum.

26.  The stomach can be divided into five anatomic regions
 Cardiac is the narrow conical portion of the stomach immediately distal to the
gastroesophageal junction.
 Fundus is the dome-shaped proximal portion of the stomach.
 Body or corpus is the main part of the stomach that extends upto the incisura
angularis.
 Pyloric antrum extends from the incisura angularis to the pyloric canal.
 Pyloric canal or pylorus is the distal-most one inch long tubular part of stomach.

27. The gastric wall consist of mucosa, submucosa, muscular coat and serosa.
GASTRIC MUCOSA
Gross features:
 The inner surface of stomach exhibits coarse rugae. These infoldings of mucosa
and submucosa are most prominent in the proximal stomach.
 The delicate texture of the mucosa is punctured by millions of gastric foveolae or
pits, leading to the mucosal glands.
Histological features:
 Gastric mucosa comprises
 Surface foveolar cells are tall columnar mucin-secreting cells which line the entire
gastric mucosa as well as the gastric pits.
 Mucosa neck cells are present deeper in the gastric pits, these cells are thought to
be the progenitors of both the surface epithelium and the cells of gastric glands.
 Glandular cells form the gastric glands. There are three types of gastric glands –
Main gastric glands, Cardiac tubular glands and Pyloric gland.

28. 1. Main gastric glands, found in the body and fundus of stomach. These are simple
tubular glands. The alveoli of main gastric glands contain two types of cells:
 Chief cells, also known as peptic or zymogen cells, are basophilic and secrete
proteolytic proenzymes, pepsinogen I and II.
 Parietal cells (oxyntic cells) are acidophilic. These secrete hydrochloric acid
(HCL) and the intrinsic factor.
2. Cardiac tubular glands are found in the mucosa of cardia (a small conical part of
the stomach) just around the distal end of oesophagus. These secrete soluble
mucus.
3. Pyloric glands are found in the antrum and pylorus region of the stomach. These
glands contain two types of cells:
 Mucus cells, which secrete soluble mucus, and
 G-cells, responsible for release of the hormone gastrin.

29. SUBMUCOUS COAT
 The submucous coat consists of loose areolar connective tissue connecting the
muscular and mucous coats of the stomach.
MUSCULATURE OF STOMACH
 The muscle coat of stomach has three layers, an outer longitudinal, middle
circular, and an inner oblique.
 The stomach and duodenum are divided by a thickened circular smooth muscle
layer called pyloric sphincter.
SEROUS COAT
 The serous coat of stomach is part of the peritoneum which covers the organ.

31.  The arteries supplying the stomach are derived from the coeliac artery and
include:
1. Left gastric artery: It courses along the lesser curvature of the stomach from left
to right, disturbing branches to both surfaces.
2. Right and left gastroepiplotic arteries: They course respectively, from the right
and left side along the greater curvature of the stomach, anaestomose with
branches of splenic artery and supply blood to the stomach.
 The veins draining blood from the stomach include right and left gastroepiploic
veins, and several short gastric veins. All of these eventually join the portal vein. A
small quantity of the blood is returned to azygos veins instead of portal vein.

32. GASTRIC JUICE
COMPOSITION
 Gastric glands secrete about 2-2.5 L of gastric juice in the lumen of stomach per
day.
 It is acidic with a pH varying from 1 to 2.
 Important constituents of gastric juice are:
 Which includes,
 Electrolytes such as Na, K, Mg, Cl etc,
 The electrolyte content of gastric juice varies with the rate of secretions.
 At low secretory rates, Na concentration is high and H concentration is low, but as
acid secretion increases Na concentration falls.
WATER 99.45%
SOLIDS 0.55%

33.  Enzymes present in the gastric juice are: Pepsin, Gastrin lipase, Gastric gelatinase,
Gastric amylase, Lysozyme and Carbonic anhydrase.
 Mucin or mucus is of two types: Soluble and insoluble mucus.
 Soluble mucus secreted by mucous cells of pyloric and cardiac glands.
 Insoluble mucus secreted by surface foveolar cells (tall columnar mucin-secreting
cells) lining the entire gastric mucosa.
 Intrinsic factor is secreted by parietal cells of gastric glands.

34. SECRETION OF HCL
 Hydrochloric acid (HCL) is secreted by the parietal cells (also called oxyntic cells).
 Gastric glands secrete about 2.5 L of HCL in a day having a pH of approximately
1.0
Mechanism of HCL secretion
 Various theories have been put forward to explain the origin of H+ of HCL.
 Hydrochloric acid is made up of hydrogen and chloride ions.
 The hydrogen ions are believed to be generated inside the parietal cell from
metabolic CO2 and H2O present in the cell.
 The enzyme carbonic anhydrase present in abundance in the parietal cells is
essential for the secretion.
Carbonic anhydrase
CO2 + H20 H2CO3 H+ + HCO3
-
 Because of the high intercellular negativity, the Cl - present in the parietal cell is
forced out into the lumen of gland through the Cl - channels located on the apical
membrane of the cell.

36.  HCL participates in the breakdown of proteins.
 It provides an optimal pH for the action of pepsin.
 It hinders the growth of pathogenic bacteria.

37. PEPSINOGEN SECRETION
 Pepsinogen is an inactive precursor (proenzyme) of pepsin. It is mainly secreted by
chief cells of the main gastric glands. A small amount of pepsinogen is also
secreted by pyloric glands.
 Pepsinogen is synthesized and stored as zymogen granules in the apical region of
chief cells.
 Pepsinogen secretion is stimulated by vagal stimulation, gastrin and histamine.
 Pepsinogen is converted to pepsin (the active form) by the action of HCL or
preformed pepsin.
Pepsinogen HCL Pepsin
Pepsinogen PEPSIN Pepsin

39. FUNCTION OF PEPSINOGEN
 Pepsin, the active form of pepsinogen, is a proteolytic enzyme that begins the
process of protein digestion.

40.  Mucus is of two types – Insoluble and Soluble.
INSOLUBLE MUCUS
 Insoluble mucus is secreted by the mucus – secreting cells lining the entire gastric
mucosa. The insoluble mucus is so viscid in nature that it forms a gel-like coat over
the mucosa. These cells also secrete bicarbonate ion which make the mucus
alkaline with pH of 7, that forms an extremely important protective layer saving the
stomach from the destruction by HCL.
SOLUBLE MUCUS
 Soluble mucus is secreted by mucous cells of pylorus and cardiac glands.

41.  Intrinsic factor (IF), a glycoprotein, is secreted by the parietal cells of gastric
mucosa, chiefly by those in the fundus.
FUNCTIONS
 The intrinsic factor is essential for the absorption of vitamin B12. It forms a
complex with B12 which is carried to the terminal ileum where the vitamin is
absorbed.
 Deficiency of intrinsic factor in some patients with idiopathic atrophy of gastric
mucosa may cause a serious disorder called pernicious anaemia.

42.  Mechanisms regulating the gastric secretion include neural control and chemical
control.
NEURAL CONTROL
 Neural control over the gastric glands is exerted by local enteric plexus involving
cholinergic neurons and impulses from the CNS via vagal (extrinsic) innervation.
 Vagal stimulation increases the secretion of HCL by parietal cells and pepsin by
chief cells. Vagal stimulation increases H+ secretion by a direct path and an indirect
path.
CHEMICAL CONTROL
Chemical control on gastric glands is exerted mainly through,
1. Role of gastrin.
 Gastrin, a harmone, is secreted by the G-cells into the blood circulation (and not
into gastric juice). It reaches the stomach through the arterial circulation and
stimulates secretory activity of the parietal cells and chief cells.

43. 2. Role of histamine.
 Histamine is released from the enterochromaffin-like (ECL) cells found in the
base of the gastric gland.
 ECL cells bear both gastrin receptors and ACh receptors.
 They release histamine in response to both circulating gastrin as well as ACh
released by vagal fibres.
 The histamine released stimulates HCL secretion from parietal cells by acting on
H2 receptors.
 H2 receptor-blocking drugs, such as cimetidine and ranitidine, inhibit H+ secretion
by blocking the stimulatory effect of histamine.
3. Role of somatostatin.
 Somatostatin is secreted by D-cells located adjacent to G-cells or the parietal cells
in the gastric glands. Somatostatin inhibits HCL secretion.
4. Role of low pH (<3) in stomach.
 Low pH (<3) in the stomach inhibits the secretion of H+ by parietal cells by a
negative feedback mechanism.
5. Intestinal influences.
 Chyme containing acid, fats and products of protein digestion when reaches the
duodenum causes the release of several intestinal hormones like secretin,
cholecystokinin (CCK), and gastric inhibitory peptide (GIP).

44. Meal-related gastric secretion can be divided in to three phases
PHASES OF
GASTRIC
SECRETION
GASTRIC
PHASE

45.  Cephalic phase of gastric secretion occurs before the entry of food into the
stomach.
 The secretion is initiated by the thought, sight, smell or taste of food. Neurogenic
signals originate in the cerebral cortex and appetite centres of amygdala or
hypothalamus. The impulses are transmitted to dorsal vagal nuclei and from there
through vagii to the stomach.
 Emotions also influence this vagally-mediated gastric secretion. Anger and hostility
are associated with increased gastric secretion and motility. Fear and depression
decrease the gastric secretion and motility.

46.  Gastric phase of gastric secretion occurs when food enters the stomach.
 The presence of food in the stomach induces gastric secretion by following
mechanisms:
 Distension of the body of stomach.
 Distension of the antrum initiates vagally, mediated and local reflexes that result in
gastrin release from the antral G-cells. Gastrin release is inhibited when pH
becomes low [<3].
 Products of partial protein digestion also stimulate gastrin secretion and this
increases secretion of gastric acid mainly.
 Low pH causes increased pepsinogen secretion through local reflexes.

47.  Intestinal phase of gastric secretion begins as the chyme begins to empty from the
stomach into the duodenum.
 In contrast to the excitatory cephalic and gastric influences the intestinal influence
on the gastric secretion is chiefly inhibitory in nature. Intestinal factor inhibits
gastric secretion by enterogastric reflex and through several hormones such as
secretin, cholecystokinin (CCK), gastric inhibitory peptide (GIP), vasoactive
intestinal polypeptide (VIP) and somatostatin which inhibit gastric secretion. The
inhibitory influences discussed above help to terminate the gastric secretion when
all the food has left stomach

49.  The peristaltic activity of the gastric musculature has been given various names
depending upon its features and motor function subserved by it. Gastric motility
can be described as:
Motility of the empty stomach, which includes:
 Migrating motor complex, and
 Hunger contractions.
Gastric motility related to meal, includes:
 Receptive, relaxation,
 Mixing peristaltic waves and
 Gastric emptying.

50. MOTILITY OF EMPTY STOMACH
1. Migrating motor complex (MMC) is the name given to the peristaltic wave that
begins in the oesophagus and travels through the entire gastrointestinal tract
(migratory motor activity) during interdigestive period.
 The MMCs remove any food remaining in the stomach and intestines during
interdigestive period in preparation for the next meal; because of this they have
been called the interdigestive housekeepers.
 The MMC are abolished immediately after the entry of food in the stomach.
2. Hunger contractions. Mild peristaltic contractions occur in the empty stomach,
which over a period of hours increase in intensity and are called hunger
contractions. MMCs are probably responsible for hunger contractions.

51. GASTRIC MOTILITY RELATED TO MEALS
1. Receptive relaxation and accommodation. Storage function of stomach is
accomplished by receptive relaxation and accommodation.
 The passage of each bolus of food stimulates the stretch receptors of oral region
and produces relaxation. By the end of meal about 1to 2 L of food can be
accommodated.
 Receptive relaxation is a vagovagal reflex initiated by distension of stomach and is
synchronized with the primary peristaltic waves in the oesophagus.
2. Mixing peristaltic waves. The presence of food in the caudal region (distal body
and antral part) of stomach increases the contractile activity of this part of stomach.
This enhanced contractile activity (a combination of peristalsis and retropulsion) is
called mixing waves which mix the food with stomach acid and enzymes and break
it into smaller and smaller pieces. When the food is mixed into a pasty consistency
it is called chyme.
3. Gastric emptying. Gastric emptying occurs when the chyme is decomposed into
enough small pieces (typically less than 1 mm3) to fit through the pyloric sphincter.
 Gastric emptying results from a progressive wave of forceful contraction which
sequentially involves antrum, pylorus (pyloric sphincter) and proximal duodenum;
thus all the three function as a unit.