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ALIMENTARY SYSTEM 1 (page 1 of 5)
THE BURDEN OF GI DISEASE AND INTRO TO GI
PHYSIOLOGY.
Label a diagram of the alimentary system identifying the following: mouth,
oesophagus, salivary glands, stomach, pancreas, liver, gall bladder, duodenum,
jejunum, ileum, colon, rectum, and anus; mark on the diagram the position of the
sphincters – gastro-oesophageal, pyloric, ileo-caecal, anal and sphincter of Oddi.
⇒ The blue labels indicate the salivary glands
⇒ The green shows the position of the sphincters.
⇒ The sphincter of Oddi is the one guarding the
common bile duct)
Gastro-oesophageal / Cardiac
Pyloric
Ileo-caecal
Anal
⇒ GI tract is hollow tube from mouth to anus made of smooth muscle and lined by epithelium +
associated structures (salivary glands, liver and pancreas):
Mouth – oesophagus – stomach – small intestine (duodenum, ileum and jejunum) – colon
(ascending, transverse and descending) – rectum - anus
⇒ Flow through tube regulated by activity smooth muscle and 4 sphincters (specialised regions of
smooth muscle) – gastro-oesophageal, pyloric, ileo-caecal and anal
⇒ Bile (liver) and pancreatic secretions enter the duodenum via common bile duct and regulated
by sphincter of Oddi
Label a simple diagram of a typical gut wall section identifying: epithelial lining of
gut lumen, mucosa, circular and longitudinal muscle, myenteric and submucosal
plexuses and extrinsic nerves
GUT EPITHELIUM
Typically the gut wall has
the epithelial lining
(mucosa), 2 layers of
smooth muscle (circular
and longitudinal) and 2
nerve plexi (sub-mucosal
and myenteric)
MUCOSA - Lamina Propria (connective
tissue) and Muscularis Mucosa
SUBMUCOSAL PLEXUS
CIRCULAR MUSCLE - Muscularis Mucosa
(Smooth Muscle) - part of the Mucosa.
MYENTERIC PLEXUS
EXTRINSIC NERVES
LONGITUDINAL
MUSCLE MUSCLE
(Muscularis Externa)
State that the epithelium may be specialised for secretion or absorption, and
invaginations may form glands of specialised cell types (gastric glands, Brunner’s
glands), e.g. HCl parietal cells, pepsin chief cells
Briefly explain the terms “intrinsic” and “extrinsic” nerve supply to the gut
⇒ Intrinsic the gut has an enteric nervous system comprising a submucosal plexus
(containing sensory and motor neurons, and autonomic postganglionic fibres; it regulates
movements of the mucosa and the vasculature tone, as well as controlling secretions)
and the:
myenteric plexus (which contains the same types of nerve fibers but controls mostly the
motility of the GI tract, particularly the strength/frequency of contraction); the plexuses
communicate with each other, and also within each other.
⇒ Extrinsic nerve supply is the nerve supply to the enteric nervous system; the autonomic
innervation comes from the vagus and pelvic nerves, and also the sympathetic ganglia
Gut neural plexi have multiple interconnections (“gut brain”) both within/between plexi as well as
connections from external nerves of ANS. Both local and “long” (involving brain) reflexes are present.
State that these nerves may modify secretion, absorption, motility and blood flow of
the gut
State that the extrinsic nerve supply has both a parasympathetic (mainly excitatory)
and a sympathetic (mainly inhibitory) component
State that mechano- and chemoreceptors in the gut wall may invoke both local
reflexes involving the intrinsic nerve system and “long” reflexes involving the brain
via afferent nerve fibres in the vagus and splanchnic nerves
State that blood flow to the gut is about 30% of resting cardiac output and that
venous drainage is via the hepatic portal vein which passes through the liver before
returning to the heart
State that activity of the gut muscles mixes contents with the digestive secretions
and also aids efficient absorption of nutrients
Describe the blood flow to the gut
At rest, about 30% of the cardiac output goes to the gut – which can reduce to 5% in
exercise; most blood returns to the vena cava via the liver (the hepatic portal vein)
Distinguish between muscle activity involved in “mixing” movements and that
involved in “propulsive” (peristalsis) activity
Mixing (segmentation activity) is governed by the myenteric plexus, occurring in areas
of the intestine with large volumes of chyme; a segmentation is initiated with the
contraction of circular muscle which segments the intestine; then the muscle fibers within
each segment contract; then the original contraction relaxes, and new contractions form;
this creates a “sloshing” effect, occurring most rapidly in the duodenum (12/min); it mixes
the food with digestive juices and also aids absorption by presenting “fresh” chyme to
the mucosa
Propulsive (peristalsis – migrating motility complex) is also governed by the
myenteric plexus, and is characterised by a series of progressive contractions beginning
in the lower part of the stomach and down to the terminal ileum within 2hrs; it is an
organised movement and so requires an intact nervous system
The stomach has a third layer of muscle (oblique) to grind the food before entry into the
small intestine
Describe how the structure of the mucosal lining of the gut leads to a large surface
area for absorption
There are mucosal folds (x3); the mucosa itself is made of a villus structure (x20); the
luminal membrane of the epithelial cells has microvilli (x10) – a brush border; cumulative
absorption area ~ tennis court (x600).
State that the maximum rates of absorption of fat, protein and carbohydrate are
about 10x greater than the normal daily rates
⇒ State that some 7 litres of water + ions are secreted into the lumen of the gut – this is
equivalent to 50% of the extracellular volume- together with some 1.5l in the diet. Need to
reabsorb most of this – faecal volume normally 100-150ml/day
Explain the importance of reabsorption of this secreted fluid
The total daily input to the GI tract (diet and secretions) is 9.5l, but you only shit around
100ml/day
State that nearly all of the nutrients and most of the fluid secreted by the gut is
reabsorbed in the small intestine
No absorption occurs in the stomach, and only 1.4l occurs in the colon; reabsorption
occurs due to osmosis (water follows the absorption of dietary nutrients)
Describe how gut function is regulated by a combination of neural (both central and
local), humoral and local paracrine activity; illustrate this with a suitable example
Neural input (both extrinsic and intrinsic) controls absorption, blood flow,
motility, secretions; humoral control originates from secretions from either the
GI system (e.g. pancreas secreting secretin) or other glands (e.g. adrenal
cortex secreting aldosterone); an example of local paracrine control is
histamine release in the stomach
An example of this integrated control is gastric acid secretion:
Cephalic phase – neural response to sight, smell and taste of food,
preparing the stomach before food actually gets there; this is mediated by
ACh receptors on the parietal cells and the parasympathetic nervous
system (via the vagus nerve); this stimulation also causes gastrin/histamine
secretion
Gastric phase – G-cells in the gastric antrum release the hormone
gastrin; this is mediated by gastrin receptors on the parietal cells; this also
causes histamine secretion -
•
•
•
Vagus - direct effect, also
stimulates gastrin and
histamine release
Gastrin - direct effect and
release histamine
Histamine - direct effect
Intestinal phase – digestive products in the lumen modulate local secretion
(e.g. histamine producing ECL cells in the gastric mucosa: there are
histamine receptors on parietal cells)
The response to these stimuli cumulatively is greater than the sum of their
action; there are also inhibitory mechanisms, for example somatostatin release
from D-cells in the gastric antrum inhibit gastrin release
SUMMARY:
• Initiation - Vagus reflex starts acid before food enters stomach by both direct and
indirect stimulation
• Maintenance - Stretch of stomach wall by contents causes further release of gastrin
(reflex involving intrinsic nerves)
• Termination -Cessation vagal stimulants (food), emptying stomach reduces wall
stretch- reflexes decrease. Other inhibitory mechanisms from small intestine
State that GI disease is a diverse group of conditions that has a great impact on
health and the quality of life and is associated with great economic burden
List some common GI diseases
Acid-related disease GORD (gastro-oesophageal reflux disease), Barrett’s oesophagus
(pre-malignant lesion), peptic ulcer disease (Helicobacter infection, NSAID use)
Chronic intestinal disease IBS (irritable bowel syndrome – disorder of motility, more
prevalent among women), IBD (inflammatory bowel disease – a group of diseases,
including Crohn’s disease, ulcerative colitis – associated with an ↑ risk of bowel cancer),
diverticular disease (most commonly in females), and the shits
Hepatobiliary disease chronic liver disease (injury to hepatocytes, inflammation for
>6mnths, aetiology including viral, immunological and metabolic abnormalities), cirrhosis
(permanent non-functioning tissue, hepatitis infection or alcohol most commonly),
gallbladder disease (e.g. gallstones)
Pancreatic disease acute pancreatitis (mild to life-threatening – blockage of pancreatic
ducts, inflammation due to build up of enzymes), chronic pancreatitis (permanent
damage usually due to alcohol)
Gastrointestinal cancer colorectal (develops slowly and so great potential for medical
screening and treatment), liver (difficult to detect at early stage, high metastasis risk),
pancreatic (difficult to detect at early stage, with poor survival rate)
Infections foodborne illness (>200 possible) and non-foodborne illness
Ion transport mechanisms in the gut are important both for secretion and absorption processes –
⇒ Cell Junctions, basolateral K+, luminal Cl - (fluid secretion into intestine)
Electro-chem gradient
“Tight” - big gradient -stomach
“Leaky” -small gradient -small intestine
Colon in between
⇒ Ion Channels,
⇒ Ion Exchange Transporters (Na+/H+, Cl-/HCO3 -)
Electro-chem gradient
Electro-chem gradient
Anion or cation - not both
Anion or cation
Ion specificity
Ion specificity
Open/closed
Electroneutral
⇒ Active Transport Mechanisms using ATP (Basolateral sodium pump (Na+/K+), Proton pump (H+/K+) - gastric
acid secretion by parietal cell)
Much of the power driving ion movements is derived from the Na-gradient into the epithelial cells
which is maintained by the ATP-driven activity of the sodium pump (Primary active transport).
This gradient also powers the uptake of amino acids and glucose from the gut lumen into the
absorptive cells of the epithelium
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Movement against electro-chem gradient
Uses ATP
Ion specificity
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May be electrogenic
May establish an electro-chem gradient