Gastric Anatomy

1
Gastric Anatomy Physiology
2
Anatomy

• In adult life, stomach located T10 and L3
  vertebral segment
• Can be divided into anatomic regions based on
  external landmarks
• 4 regions
• Cardia
• Fundus
• Corpus (body)
• Antrum

3
Anatomy

• Cardia- region just distal to the GE junction
• Fundus- portion above and to the left of the GE
  junction

4
Anatomy

• Corpus- region between fundus and antrum
• Margin not distinctly external, has arbitrary
  borders
• Antrum- bounded distally by the pylorus
• Which can be appreciated by palpation of a
  thickened ring of smooth muscle

5
Anatomy

• Position of the stomach varies with body habitus
• In general- it is fixed at two points
• Proximally at the GE juction
• Distally by the retroperitoneal duodenum

6
Anatomy

• Anterior- in contact with Left hemi-diaphragm,
  left lobe and anterior segment of right lobe of
  the liver and the anterior parietal surface of
  the abdominal wall
• Posterior- Left diaphragm, Left kidney, Left
  adrenal gland, and neck, tail and body of
  pancreas
• The greater curvature is near the transverse
  colon and transverse colon mesentery
• The concavity of the spleen contacts the left
  lateral portion of the stomach

7
Vasculature
8
Vasculature

• Well vascularized organ
• Arterial flow mainly derived from Celiac Artery
• 3 Branches
• Left Gastric Artery
• Supplies the cardia of the stomach and distal
  esophagus
• Splenic Artery
• Gives rise to 2 branches which help supply the
  greater curvature of the stomach
• Left Gastroepiploic
• Short Gastric Arteries
• Common Hepatic or Proper Hepatic Artery
• 2 major branches
• Right Gastric- supples a portion of the lesser
  curvature
• Gastroduodenal artery
• -Gives rise to Right Gastroepiploic artery
• -helps supply greater curvature in
  conjunction with Left Gastroepiploic
  Artery

9
Anatomy

• Venous Drainage
• Parallels arterial supply
• Lymphatic drainage
• Lymph from the proximal portion of the stomach
  drains along the lesser curvature first drains
  into superior gastric lymph nodes surrounding the
  Left Gastric Artery
• Distal portion of lesser curvature drains through
  the suprapyloric nodes
• Proximal portion of the greater curvature is
  supplied by the lymphatic vessels that traverse
  the pancreaticosplenic nodes
• Antral portion of the greater curvature drains
  into the subpyloric and omental nodal groups
• In general- The lymphatic drainage of the human
  stomach, like its blood supply, exhibits
  extensive intramural ramifications and a number
  of extramural communications. Therefore spread
  beyond is often beyond region of origin at a
  distance from the primary lymphatic zone

10
Anatomy

• Nerve Supply
• Left and Right Vagus Nerves descend parallel to
  the esophagus within the thorax before forming a
  peri-esophageal plexus between the tracheal
  bifurcation and the diaphragm
• From this plexus, two vagal trunks coalesce
  before passing through the esophageal hiatus of
  the diaphragm

11
Anatomy

• Left (anterior) Vagus Nerve
• Left of the esophagus
• Branches
• Hepatic Branch
• Supplies liver and Biliary Tract
• Anterior gastric or Ant. Nerve of Latarget

12
Anatomy

• Right (posterior) Vagus Nerve
• Right of the esophagus
• Branches
• Celiac
• Posterior Latarget
• Innervates posterior gastric wall

13
Anatomy

• Parasympathetic innervation of Stomach- Vagus
  Nerve
• 90 of fiber in vagal trunk is afferent (info
  transmitting from stomach to CNS)
• Sympathetic innervation of Stomach- Splanchnic
  Nerve
• Derived from spinal segement T5-T10

14
Anatomy

• Microscopic Anatomy
• Glandular portions of stomach
• Lined by simple columnar epithelium
• This luminal surface is interrupted at intervals
  by gastric pits
• Opening into these gastric pits are one or more
  gastric glands that have functional significance
• Mucosa has three types of gastric glands
• -Cardiac
• -Oxyntic
• -Antral

15
Microscopic Anatomy

• Cardiac Glands
• Location- Cardia
• Contain mucous
• Function- secrete mucous (provides a protective
  coat for lining of stomach)
• Oxyntic Glands
• Most distinctive feature of the stomach
• Location- Fundus and Corpus
• Contains many cell types

16
Microscopic Anatomy

• Parietal cells
• Location- neck of gastric pit
• Stimulated by Ach, Histamine and Gastrin
• Secretes HCl Intrinsic Factor
• Chief Cells
• Location- base of gastric pit
• Stimulus- Vagal
• Secretes Pepsinogen (eventually leads to pepsin-
  digestive enzyme)

17
Microscopic Anatomy

• Antral Glands
• Gastrin cells
• Location- mucosa of distal stomach
• Stimulus- amino acids
• Secretion- Gastrin (stimulates HCl production by
  way of parietal cells)
• Somatostatin
• Location- mucosa of distal stomach Duodenum
• Stimulus- HCl or low pH in duodenum
• Actions- Inhibits gastric emptying, Pancreatic
  secretions, and gallbladder contraction

18
Physiology

• The stomach contains a number of biologically
  active peptides in nerves and endocrine cells
• Ex. Gastrin, somatostatin, vasoactive intestinal
  peptide (VIP), substance P, and glucagon, etc
• The two peptides of greatest importance to human
  disease and clinical surgery are
• Gastrin
• Somatostatin

19
Physiology

• Gastrin
• Most important stimulus is a meal
• amino acids that results from proteolysis
• Fat and carbohydrates are not stimuli for gastrin
  secretion
• Gastric distention that occurs from a meal will
  stimulate cholinergic neurons thereby releasing
  gastrin
• Gastrin will then prompt Parietal cell to secrete
  HCl
• Once Gastric distention diminishes,
  VIP-containing neurons are activated causing
  stimulation of somatostatin, thus attenuating
  Gastrin secretion
• Overall, a lumen pH gt3.0 will potentiate gastrin
  release, whereas a pH lt3.0 will inhibit its
  release

20
Physiology

• Somatostain
• Like Gastrin, plays an integral role in gastric
  physiology
• Also, used for important therapeutic applications
  in treatment of digestive diseases
• Main stimulus is a low or acidic (lt3.0)luminal pH
• Many peptides have shown to release somatostatin
• Ex. Secretin, Cholecystokinin and gastrin
• In contrast, stimulation of Vagal nerves along
  with cholinergic neurons inhibit somatostatin
• Overall, the most important gastric function of
  somatostatin is to regulate acid secretion and
  gastrin release

21
Gastric Acid Secretion
22
Gastric Acid Secretion

• Basolateral membrane of the parietal cell
  contains specific receptors for the three major
  stimulants of acid production
• Histamine
• Gastrin
• Acetylcholine
• Each stimulant has its own 2nd messenger system
  which allows for stimulation of the parietal cell

23
Gastric Acid Secretion

• Humans normally secrete 2 to 5 mEq/h of HCl in
  the fasting state, constituting basal acid
  secretion
• Both Vagal tone and ambient Histamine secretion
  are presumed to regulate basal acid secretion
• Gastrin is not thought to play a role in basal
  acid secretion
• Therefore, a Vagotomy or use of H2 blockers (ex.
  Cimetidine) will decrease basal acid production

24
Gastric Acid Secretion

• Stimulated acid secretion begins with
• Cephalic phase
• Thought, sight or smell of food stimulates acid
  secretion
• Mediated by Vagal stimulation
• Vagal discharge
• Directs the cholinergic mechanism for stimulation
• Can be inhibited by Atropine (anticholinergic)
• Inhibits release of somatostatin
• Vagal effects inhibit tonic inhibition that is
  provided by somatostatin

25
Gastric Acid Secretion

• Gastric Phase
• Begins when food enters the stomach
• The following are responsible for stimulation of
  acid secretion
• Presence of partially hydrolyzed food
  constituents
• Gastric distention
• Gastrin is the most important mediator of this
  phase
• Ends when Antral muscosa is exposed to acid
• When luminal pH is lt2.0 in the antrum, gastrin
  release stops
• Somatostatin release is increased
• Entry of digestive products into the intestine
  begins the intestinal-phase inhibition of gastric
  acid secretion

26
Gastric Acid Secretion

• Intestinal Phase
• Also, releases HCl by way of Gastrin
• Releases secretin to inhibit Gastrin which
  ultimately decreases Acid production

27
Other Factors

• Pepsin
• Secreted from gastric chief cells
• Contributes to the overall coordination of the
  digestive process
• Main function is to initiate protein digestion,
  usually is incomplete
• Partially hydrolyzed protein by pepsin are
  important signals for release of
• Gastrin
• Cholecystokinin

28
Other Factors

• Intrinsic Factor (IF)
• Located in the parietal cells (oxyntic gland)
• Main function is to absorb cobalamin (Vitamin
  B12) form ileal mucosa and then transported to
  the liver
• Secretion of IF is similar to acid secretion
• stimulated
• Ach
• Histamine
• Gastrin

29
Other Factors

• Bicarbonate
• Secreted from the gastric mucosa
• Theory is that bicarbonate is secreted to
  maintain a neutral pH at the mucosal surface,
  even if acidic in lumen
• Cholinergic agonist, vagal nerve stimulation have
  been shown to increase gastric bicarbonate
  production

30
Gastric Motility

• To understand gastric motility the stomach is
  divided into two functional terms as two
  different regions which have distinctive smooth
  muscle
• Proximal 1/3
• 3 layers of smooth muscle
• Outer longitudinal
• Middle Circular
• Inner Oblique
• Distal 2/3
• Only a distinctive outer longitudinal layer
• Gastric smooth muscle ends at the pylorus, a
  septum of connective tissue marks the change from
  pylorus to the duodenum

31
Gastric Motility

• Proximal 1/3
• Have prolonged and tonic gastric contractions
• No action potentials or pacesetter
• Thus no peristalsis
• Distal 2/3
• In general, gastric smooth muscle exhibit
  myoelectric activity based on a highly regular
  pattern, called slow waves
• Slow waves set a maximum rate at which
  contrations can occur (3 contractions/min) they
  do not cause contractions

32
Gastric Motility

• Contraction occur when action potential are phase
  locked with a crest of a slow wave pattern
• When an action potential is combined with a
  pacesetter potential (partially depolarized
  smooth muscle cells) a ring of smooth muscle cell
  contraction moves with peristalsis

33
(No Transcript)
34
Coordination of Contraction

• Receptive Relaxation
• Vagally mediated relaxation of fundus (proximal
  stomach) when degluttination occurs
• Allows the proximal stomach to act as a storage
  site for ingested food in the immediate
  postprandial period
• Meal is accepted without a significant increase
  in intra-gastric pressure
• Soon proximal contractile activity increases
  eventually leading to compressive movement of
  gastric content form fundus to antrum

35
Coordination of Contraction

• Food enters antrum
• Food peristaltically propelled toward the pylorus
• Pylorus closes before the antral contraction
• This coordinated closing allows for small bolus
  of liquid and food particles to pass, while the
  main bulk of the gastric content undergoes
  retropulsion back into the antrum
• Next, there is a churning action in the antrum
  that mixes the ingested food particle, gastric
  acid and pepsin
• Solid food particles gt1mm will not pass through
  the pylorus

36
Coordination of Contraction

• Overall, Liquids are empty more quickly than
  solid
• Liquids empty exponentially
• Solids endure this lag period or antral
  contraction (empties linearly)
• In general
• Proximal stomach is the dominant force in
  determining liquid emptying based on the
  gastroduodenal pressure gradient generated by
  proximal gastric contractions
• Distal stomach is postulated as controlling
  emptying of solids through its grinding and
  peristaltic actions