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GI Tract Overview
Aim of the lectures
To provide a basic understanding of
the structure and function of the
gastrointestinal tract and its normal
function
Margaret John
School of Nursing & Midwifery Studies UWCM©
Functions of the GI tract
1. Ingestion
2. Digestion
3. Absorption
CONCEPTS to ENGAGE
Motility - mechanical breakdown, propulsion
• Secretion - enzymes, water & ions
Control • of motility and secretion by nervous system and
hormones
• Mastication, salivary secretion and swallowing
Nutrient Transport mechanisms
• Diffusion, facilitated and Active Transport
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Objectives from Fox
• Describe the functions of the digestive system and lists its
structures and regions.
• Describe the layers of the gastrointestinal tract and the
function(s) of each.
• Describe the structure of the gastric mucosa, list the
secretions of the mucosa and their functions, and identify
the cells that produce each of these secretions.
• Explain the roles of HCl and pepsin in digestion and explain
why the stomach does not normally digest itself
• Describe the structure and function of the villi, microvilli,
and crypts of Lieberkuhn in the small intestine.
• Describe the location and functions of the brush border
enzymes of the intestine.
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Digestive System
• GI tract divided
into:
– Alimentary
canal.
– Accessory
digestive
organs.
• GI tract is 30 ft
long and
extends from
mouth to anus.
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Functions of the GI Tract
Motility:
• Movement of of food through the GI tract.
– Ingestion:
• Taking food into the mouth.
– Mastication:
• Chewing the food and mixing it with saliva.
– Deglutition:
• Swallowing the food.
– Peristalsis:
• Rhythmic wave-like contractions that move food
through GI tract.
Functions of the GI Tract
Digestion:
• Breakdown of food particles into subunits
(chemical structure change).
Absorption:
• Process of the passage of digestion (chemical
subunits) into the blood or lymph.
Storage and elimination:
• Temporary storage and elimination of indigestible
food.
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Functions of the GI Tract
Secretion:
Includes both exocrine and endocrine secretions.
Exocrine:
•HCl (Hydrochloric Acid),
•H20 (water),
•HC03- (Bicarbonate),
•Bile-made by the liver, stored in gall bladder & secreted into
duodenum,
•lipase (breaks down fat),
•pepsin (breaks down protein),,
•amylase (breaks down carbohydrates),
•trypsin (breaks down protein),and
•histamine (stimulates production of HCl), etc. are secreted
into the lumen of the GI©UWCM/SONMS/GI
tract.
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Functions of the GI Tract
Endocrine functions:
• Stomach and small intestine secrete hormones to
help regulate the GI system.
Examples
• Gastrin,
• Secretin,
• Cholecystokinin (CCK),
• Gastric Inhibitory Peptide (GIP),
• Somatostatin, etc.
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Salivary Glands FUNCTIONS
Lubrication
•Thins and dilutes food for
swallowing
Anti acid function,
•Buffers and neutralises
acids, particularly
important when there are
acids of bacterial origin
present
Bacteriostatic Bacteriolytic
• Acts on endogenous
bacteria
Digestive: Carbohydrates
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UWCM/SONMS/nutrition/MJohn
(? fat)
Oesophagus
A muscular tube that carries the food from
the back of the throat to the stomach.
• It must contract in a very co-ordinated
fashion so we don't regurgitate our food or
feel that our meals are sticking as we
swallow.
It is inflammation within the oesophagus
that gives the sensation of heartburn.
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©
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Food chunks up
here and amylase
continues to work
until food drops
into the stomach
Layers of GI Tract
• Mucosa
– Lining
• Submucosa
– (beneath the
inner layer)
• Muscularis
– (Muscle
Layer)
• Serosa
– (connective
tissue outer
layer)
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Mucosa
This lines the lumen of GI tract.
It consists of simple columnar epithelium.
• The major absorptive and secretary layer
– Lamina Propria: thin layer of columnar epithelium
with many lymph nodes
– Muscularis mucosae:Smooth muscle just inside
the above layer -responsible for the folds.
– Increase surface area
– Goblet cells: Secrete mucus throughout the tract
.
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Submucosa
Thick, highly vascular layer of connective
tissue.
• Absorbed molecules enter the blood and
lymph vessels in this layer.
• Submucosal plexuses (nerves):
– Provide autonomic nerve supply to the
muscularis mucosae.
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Muscularis
Responsible for segmental contractions and
peristaltic movement through the GI tract.
– Inner circular smooth muscle.
– Outer longitudinal smooth muscle.
• Contractions of these layers move food through
the tract and pulverize the food.
• Myenteric plexus located between 2 muscle
layers.
– It is the major nerve supply to GI tract.
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Serosa
Binding,
protective
outer layer.
connective
tissue.
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Regulation of the GI Tract
Nerve Supply
– Parasympathetic Nervous System:
• Stimulate motility.
– Sympathetic Nervous System:
• Reduce peristalsis and secretory activity.
The GI system Nervous Supply
– Submucosal plexus and myenteric plexus:
• Local regulation of the GI tract.
Endocrine secretion:
– Molecules acting locally.
Hormonal secretion:
– Secreted by the mucosa.
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Oesophagus
Concept: Deglutition (swallowing)
• Oesophagus Connects pharynx to the
stomach.
• Upper third contain skeletal muscle.
• Middle third contains a mixture of skeletal
and smooth muscle.
• Terminal portion contains only smooth
muscle.
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Oesophagus
Peristalsis:
•Local reflexes in response
to distention of the wall of
the Oesophagus by bolus.
(food ball)
•Wave-like contractions:
– Circular smooth muscle
contract behind, relaxes
in front of the bolus.
– Followed by
longitudinal contraction
(shortening) of smooth
muscle.
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Stomach
Most distensible part of GI tract.
• Empties into the duodenum.
• Functions of the stomach:
– Store food.
– Initiate digestion of proteins.
– Kills bacteria.
– Moves food (chyme) into intestine.
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Stomach
• Contractions of
the stomach churn
chyme.
• Mix chyme with
gastric secretions.
• Push food into
intestine.
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Stomach
• Gastric mucosa
has gastric pits in
the folds.
• Cells that line the
folds deeper in
the mucosa are
gastric glands.
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Gastric Glands
Gastric Juice:
– Goblet cells: mucous.
– Parietal cells: HCl and intrinsic factor.
– Chief cells: pepsinogen.
– Enterochramaffin-like cells (ECL): histamine
and serotonin.
– G cells: gastrin.
– D cells: somatostatin.
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Digestion and Absorption in the
Stomach
• Proteins partially digested by pepsin.
– Hydrochloric acid changes the structure of
proteins to kick start the digestive rocess
• Carbohydrate digestion by salivary amylase
is soon inactivated by acidity.
• Alcohol and aspirin are the only commonly
ingested substances absorbed.
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Protective Mechanisms of Stomach
• Alkaline mucus contains HC03- (bicarbonate
which bufers HCl and protects mucosa).
• Tight junctions between adjacent cells.
• Rapid rate of cell division (3 days).
• Prostaglandins which inhibit gastric
secretions.
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Small Intestine
In folds - villi.
Covered with columnar
epithelial cells and goblet
cells.
•Epithelial cells are rubbed
off by passing food and
replaced by replaced
•Lamina propria contain
lymphocytes, capillaries,
and a central lacteal
(Lymph duct).
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Absorption in Small Intestine
• Duodenum and
jejunum:
– Carbohydrates,
Amino acids, Lipids,
Ca++ (Calcium), and
Fe++ (Iron),
• Ileum:
– Bile salts, vitamin B12,
electrolytes, and H20.
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Intestinal Enzymes
Microvilli contain ‘brush border’ enzymes
– Brush border enzymes remain attached
to the cell membrane & react with chyme.
Absorption of nutrients requires both
brush border enzymes and pancreatic
enzymes.
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Intestinal Contractions and Motility
Contraction in the small
intestine:
Peristalsis:
– Slow movement.
Segmentation:
– Major contractile
activity of the small
intestine.
– Contraction of
circular smooth
muscle
– This will also mix
chyme
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Large Intestine
• Absorption of H20, electrolytes,
vitamin B complex vitamins, vitamin K, and folic
acid.
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Circulation between the liver &
the intestine
• compounds are
secreted by the liver
into the bile ducts.
• these compounds
flow into the intestine
with the bile.
• Eliminated in the
faeces.Or
reabsorbed back into
the liver
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Gall bladder
Sac-like organ attached to the inferior
surface of the liver.
•Stores and concentrates bile.
•Contraction of the muscle layer of the
gallbladder, ejects bile into the common bile
duct.
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Pancreas
Exocrine:
– Secretes
pancreatic
juice.
Endocrine:
– Secrete
insulin and
glucagon.
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Regulation of Gastric Function
Gastric motility and secretion are automatic.
• Waves of contraction are initiated
spontaneously by pacesetter cells.
• Extrinsic control of gastric function is divided
into 3 phases:
– Cephalic phase.
– Gastric phase.
– Intestinal phase.
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