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Overview of the Digestive System






Aims:
1) Functions of the layers of the GI Tract: Serosa, Muscularis
Externa, Submucosa and Mucosa
...




3) Mechanisms regulating digestion and absorption of nutrients
...


What are the 4
layers of the GI
Tract?

5) Mechanisms which regulate the secretion of bile and role of
the enterohepatic circulation
...

Ø Epithelial part provides selective barrier between
lumen and the body
...

Ø Loose connective tissue has abundant blood supply +
lymphatic nodules with lymphocytes + plasma cells +
macrophages
...

2) Submucosa:

Consists of the dense connective tissue +

Meissner’s Neve Plexus
...

3) Muscularis Externa: Consists of 2 smooth muscle cell layers
with Myenteric Nerve Plexus in between the layers
...

Ø Myenteric Nerve Plexus generates these peristaltic
contractions
...










Ø Serosa is present in the parts of the GI Tract in the
peritoneal cavity
...



What is the saliva a
mixture of?

What does amylase
do?

What secretes the
glycoproteins?
Why do we need
glycoproteins?

What electrolytes
are present?
What cells produce
the secretions?

Functions of the Salivary, Gastric and Pancreatic Secretions
1) Salivary Secretions:
Ø Secreted

from

Ø Secretions are a fluid mixture
of

electrolytes

carbohydrates in the oral cavity
...

Ø Saliva

has

electrolytes:
Na+, HCO3-, Cl-

Secretions



K+

in

the
are

produced
acinar

by

cells

in

the glands
...
Na+ is reabsorbed, HCO3- is
secreted

and K+

secreted
...


secretions
...


and



salivary

gland
...

Ø Acid is secreted by

parietal

the



Potassium
pump
...

Ø Parietal cell is activated by:



v Ach (from postganglionic neurones at surface of
cells– neurocrine)
...




endocrine)
...


2)

G-Cell produces the hormone GASTRIN
...

4)

ECL-Cell is activated + releases HISTAMINE which

works as a paracrine messenger
...

6)

Acid produced by the parietal cell is released into the

lumen of stomach
...


How is reflux
treated + why only
in short term?

What drug would
need to be used in
addition in long
term?




Ø Too much aid production may result in acid reflux
...

Ø Omemprazole is an inhibitor of the protein pump used by the
parietal cell to produce acid
...

However normally the acid produced inhibits the D-Cell
...

Ø Need to inhibit histamine production = H2 antagonist
(Famotidine)
...

Ø Aqueous part of secretion = rich in HC03
...

Ø Also an enzyme rich component = proteolytic enzymes
such

as

trypsinogen,

procarboxypolypeptidase,
deoxyribonuclease
...




What is the main
action of enzymes
in the:
Salivary glands?
Stomach?
Pancreas?
Luminal
membranes of cells
in small intestine?




How are substances
absorbed?

What is the
concentration of
Na+ in and out of
cell and how does
this influence
absorption?


Digestion + Absorption
Digestion:
-

Enzymes in salivary glands = attack carbohydrates mainly
...

-

Enzymes which complete digestion are found on the cytoplasm

+ luminal membranes of the cells lining the small intestine like
maltase, lactase and sucrase
...

-

Enzyme action is aided by either acid produced by the

stomach or bile secreted by the liver
...

-

Movement of substances across the luminal membrane is often

dependent on Na+ or a similar carrier system
...

-

Hence a high concentration of Na+ on mucosal surface

facilitates glucose uptake whereas a low Na+ concentration will
inhibit it
...

-

Proteins + oligopeptides have 6/7 different transport systems

What other type of
transport system is
there + for what?

to transport them from small intestine lumen into the enterocytes
...
There is also a system which is independent of Na+ and instead





5 involve a Na+ co-transporter pump and 2 of these 5 also require
uses H+ (for di- or tri- peptides)
...


-

Bile contains HCO3- ions and bile salts to solubilise fats
...

-

muscle contraction + enters the duodenum portion of small
intestine via bile duct
...


the liver
...


What stimulates
contraction of the
gall bladder?

What is a micelle?

of

activity = increased bile





production

Ø Parasympathetic VAGAL

Production of bile is
stimulated by
what?

What is
emulsification?

The

stimulated by:



What is the role of
bile salts?

During a meal, bile is secreted from the gland via smooth

acids
in

+

the

amino
chyme

stimulate CCK (cholecystokinin) to be secreted which
causes contraction
...
This uses contractile activity of GI Tract to
disrupt + emulsifying agents like bile salts
...

Ø Digested product + bile salt = micelles formed which
keeps insoluble products in soluble aggregates – small
amounts released = absorbed via diffusion into cells
...




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