Lecture 4.1: The Stomach Flashcards
Stomach Functions (4)
Storage
Haematopoietic
Protection
Digestion
Stomach Functions: Storage
Ingestion»digestion – holds it for a few hours
Mixing – production of chyme
Stomach Functions: Haematopoietic
Production of intrinsic factor needed for vitamin B12 absorption
Stomach Functions: Protection
Mucus rich lining (self care) bacteriolytic
Stomach Functions: Digestion
Secretion of protease (pepsin) aided by HCl
Structure of the Stomach (5)
- Lower Oesophageal Sphinchter
- Cardia (surrounds opening of oesophagus into the
stomach) - Fundus (region above the cardiac orifice)
- Body (Corpus)
- Pylorus (consists of pyloric antrum and pyloric canal
which terminates at pyloric orifice)
Generalised structure of the wall of the GI tract (out to in)
- Serosa, continuous with parietal peritoneum which
lines the abdominal cavity - Muscularis externa = smooth muscle: outer layer is
longitudinal, inner layer is circular (in stomach there is
a 3rd oblique layer) - Submucosa – loose connective tissue
- Muscularis Mucosa (2 thin layers of smooth muscle)
- Lamina propria
- Layer of epithelial cells (with basement membrane)
Stomach mucosa is simple columnar epithelia which
produces….?
A protective alkaline secretion
Where do gastric glands empty their secretions?
Gastric pits which are dotted within the epithelia in the millions
What do Mucous Neck Cells do?
Secrete Mucous
Where are Chief Cells (Peptic Cells) found? What do they do?
Basal Region
Secrete Pepsinogens
What do Parietal Cells (Oxyntic Cells) secrete?
HCl and intrinsic factor
What do G-Cells (Enteroendocrine Cells) secrete?
Gastrin
What cells secrete Grehlin?
Gr Cells
What cells secrete Somatostatin?
D Cells
What types of glands and cells are mostly found in the cardia of the stomach? (1)
Mucous Secreting Glands
What types of glands and cells are mostly found in the fundus and body of the stomach? (4)
- Mucus Secreting Neck Cells
- Acid Secreting Cells (Parietal or Oxyntic)
- Chief Cells secreting Pepsinogen
- Other local hormone producing cells
What types of glands and cells are mostly found in the pyloric region of the stomach?
- Mucus Secreting Cells
- G Cells (releases Gastrin)
Why is it important for pH of stomach acid to be low (1-3)? (4)
- Helps breakdown connective tissue and muscle
- Activates pepsinogens
- Can combine with calcium and iron to facilitate their
absorption - Defence mechanism – killing pathogens
What stimulates production of HCl by parietal cells?
Arrival of food
Structure of Parietal Cells
- Cytoplasm of these cells has an elaborate series of
branches - These fuse to form deep canaliculi
- Increased surface area available for release of HCl
What proteolytic enzymes are released from chief (peptic) cells?
Pepsin
Pepsin is released as inactive precursors known as?
Pepsinogen
What are pepsinogens contained in?
Membrane bound zymogen granules
What is Intrinsic Factor?
- Glycoprotein that binds to VitB12 (cobalamin) in the
upper small intestine - Protecting it from digestion
- Intrinsic factor/Vit B12 complex absorbed in the ileum
What is the use of Vit B12?
Needed for erythropoiesis
What happens if there is a lack of Vit.B12?
Pernicious Anaemia
It is an autoimmune condition that affects your stomach
How does an ulcer form?
Breaching of the mucosal barrier –> Gastritis –> Persistent Erosion –> Ulcer
Control of Gastric Secretions (3 phases)
- Cephalic
- Gastric (major)
- Intestinal (minor)
(Cephalic Phase) Anticipation of food – sight, smell – accounts for …% acid secretion
~40%
Cephalic control is mediated by what nerves?
Vagus Nerves
How does vagal activity increase acid secretion?
- Postganglionic neurons in myenteric plexus release
ACh – stimulate parietal cells - Vagal stimulation promotes gastrin release from G
cells (via gastrin releasing peptide from enteric
neurons) – in turn promotes acid release - ACh and gastrin stimulate release of histamine from
mast cells and enterochromaffin like cells - Histamine can trigger acid release
What hormones can prevent acid release?
- Histamine H2 antagonists (cimetidine)
How long does the Gastric Phase of acid release last?
2-3 hours
Gastric Phase accounts for …% of acid secretion
~50%
What is Gastric Phase of acid secretion triggered by?
- Triggered by food in stomach
- Distension and the presence of amino acids and.
peptides (EtOH and caffeine also very potent) - Distention promotes local myenteric reflexes and
longer vasovagal reflexes which result in ACh release
and therefore increased secretions - ACh also promotes the release of gastrin – stimulus for
acid secretion
Intestinal Phase accounts for …% of acid secretion
~10%
As the duodenum expands with chyme a low pH, fat and hypertonic chyme results in…? (4)
- Somatostatin release (from D cells) inhibit parietal cells
and this acid secretion - Acid promotes secretin release which in turn inhibits G
cells - Fatty acids cause the release of GIP, CCK which both
inhibit acid secretion - Secretin, GIP, CCK – collectively called enterogastrones
When empty, what is the volume of the stomach?
~50ml
When full, what is the volume of the stomach?
~4000ml
In the absence of eating, gastric contractility occurs episodically about every…mins?
90
Bursts of contraction (of the stomach) are called…?
Migrating Motor Complexes (MMCs)
What hormone is gastric motility stimulated by?
- Motilin
- Small polypetide (22aa) from enterochromaffin cells
- Acts via GPCR on enteric neurons to promote smooth
muscle contraction - Erythromycin binds to motilin receptors
Gastric Motility when we are eating: Mixing (4)
- Waves of contraction originate in the corpus and
move towards the antrum - Pushing stomach contents ahead of them to the
pylorius - Pyloric sphincter closes in advance of the wave –
which stops just short of the sphincter - The antrum now contracts and contents move back to
wards the corpus –retropulsion - This is sometimes called the gastric mill
How many hours to reduce contents to a appropriate size to pass through the pyloric sphincter to the duodenum?
3hrs
What is a significant predictor of the rate of gastric
emptying?
The composition of chyme
What in the duodenum increases contraction of pyloric sphincter?
Fatty Acids
What receptors detect hyperosmolar chyme and delay
emptying of the stomach?
Duodenal osmoreceptors
Presence of what in the duodenum delays gastric emptying?
Appearance of acid in the duodenum delays gastric emptying
Why was ranitidine (used for acid reflux) stopped being prescribed?
Ongoing investigations uncovered levels of N-Nitrosodimethylamine (NDMA), a probable human carcinogen, increase over time
What is the Triangle of Death in Medicine?
A medical term describing the fatal combination of hypothermia, acidosis and coagulopathy
