Lecture 4.1: The Stomach Flashcards by Purva Shegunshi

Stomach Functions (4)

Storage
Haematopoietic
Protection
Digestion

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Stomach Functions: Storage

Ingestion»digestion – holds it for a few hours
Mixing – production of chyme

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Stomach Functions: Haematopoietic

Production of intrinsic factor needed for vitamin B12 absorption

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Stomach Functions: Protection

Mucus rich lining (self care) bacteriolytic

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Stomach Functions: Digestion

Secretion of protease (pepsin) aided by HCl

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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)

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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)

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Stomach mucosa is simple columnar epithelia which
produces….?

A protective alkaline secretion

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Where do gastric glands empty their secretions?

Gastric pits which are dotted within the epithelia in the millions

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What do Mucous Neck Cells do?

Secrete Mucous

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Where are Chief Cells (Peptic Cells) found? What do they do?

Basal Region
Secrete Pepsinogens

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What do Parietal Cells (Oxyntic Cells) secrete?

HCl and intrinsic factor

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What do G-Cells (Enteroendocrine Cells) secrete?

Gastrin

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What cells secrete Grehlin?

Gr Cells

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What cells secrete Somatostatin?

D Cells

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What types of glands and cells are mostly found in the cardia of the stomach? (1)

Mucous Secreting Glands

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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

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What types of glands and cells are mostly found in the pyloric region of the stomach?

Mucus Secreting Cells
G Cells (releases Gastrin)

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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

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What stimulates production of HCl by parietal cells?

Arrival of food

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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?

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