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Gastrointestinal tract, Gastrointestinal secretion, Small intestines, The…
Gastrointestinal tract
Composed of the
buccal (oral)
cavity, **
oesophagus
,
stomach,
small intestine
(
duodenum, jejunum, ileum),
liver
,
gall bladder, pancreas, colon, rectum, and anus **
Food
enters the
buccal cavity
and is
mechanically
broken down
by the teeth through the process of mastication (chewing).
This is a
harsh
process on the
epithelium
of the
buccal cavity,
which is why the
epithelial
layer
of the
mouth
and
oesophagus
are
composed
of
squamous epithelial cells
cells are
scraped
off and
swallowed
by and with the
bolus
of food produced by mastication and the action of the tongue. The
squamous
epithelial cells
are
rapidly replaced.
This process is
lubricated
by the
secretion
of
saliva
from the
salivary glands
, otherwise each time you ate you would
damage
the inside of your
mouth
which could
limit
your
chances of survival.
The
generalised structure
of the
GI
tract at the level of the
oesophagus
,
small intestine
, and
colon
includes
three muscle layers
.
longitudinal
,
circular,
Muscularis mucosa
Two nerve plexi,
Myenteric plexus
(between the muscles)
Sub-mucosal plexus
(beneath the mucosa)
Key
functions
of the
GI track
.
Secretion
, fluids,
bicarbonate,
enzymes, acid, hormones, bile
Digestion
,
Chemical
and
mechanical breakdown
of
carbohydrates
,
lipids
, and
protein
Absorption.
water,
sugars
,
amino acids
,
peptides,
lipids,
vitamins
,
minerals,
medical therapeutic agents
Motility
Uni-directional
movement
and
mixing
of
food/contents
of
buccal cavity
through the
entire GI tract,
ending
with the
excretion
of faecal matter from the
anus
.
Gastrointestinal secretion
Cephalic phase
, secretion is caused by: The thought of food. The sight of food. The smell of food. The taste of food. The presence of food.
(Senses)
Gastric phase,
acid
response
to
stimuli
within the
stomach
. effective stimuli:
The pyloric gland mucosa of the
antrum
secretes the hormone
gastrin
from
G-cells
.
the
oxyntic gland mucosa
secretes
acid
from the
parietal cells
, and p
epsin
from
chief cells **
enables the relative contribution of endocrine control vs neural control to be investigated
The
efferent pathway
of the
cephalic phase
involves the
vagus,
directly
innervating G cells
in the antrum to secrete gastrin and also directly innervates the parietal cells to secrete acid
Sham feeding is achieved by simulating the presence of food in the stomach.
Heidenhain.
(
by inserting fake food into the non-pouch area of the stomach
stimulates acid secretion from the area of the denervated pouch
. As the pouch is
denervated
, the mechanism must be the
stimulation of gastrin release by the vagus
.
Sham feeding causes an increase in both gastrin and acid secretion. When the
antrum
is denervated the
gastrin response is lost
but a
cid is still secreted.
The vagus must therefore also innervate
parietal cells
directly.
Distension
, food products ( the post potent is beef extract)
Caffeine
and
alcohol.
the mechanism of action includes
gastrin releas
e both
long and short reflexes
Both long
vago-vagal reflexes
and
intramural reflexes
will
stimulate acid secretion
and gastrin secretion.
.
Gastrin
secretion is also stimulated by
secretagogues
(any substance which provokes a response) such as
alcohol,
which in turn
stimulates
an increase in
*acid production
The
neural innervation
of the
parietal cells,
whether from i
ntra-mural reflexes or from vago-vagal reflexes
, utilises
acetylcholine
as the
neurotransmitter.
Small intestines
composed of the
duodenum,
the
jejunum a
nd the
ileum
duodenum
as this is the
first
section
of the
small intestine
immediately
after
the
pyloric sphincter
.
to increase
surface area
the
small intestine
possesses structural features such as
villi,
and the
columnar epithelial cells
of the
duodenum
possess
microvilli.
Goblet cells
secrete mucous
which
reduces friction
as
chyme
and
solids
are move through the
lumen
, and various
hormones
are
secreted
by
endocrine cells
at the
bottom
of the
crypts
found along the
intestinal wall
The
sub-mucosal layer
of the duodenum also contains
Brunner's glands,
which
secrete bicarbonate
and contribute to the
neutralisation
of
gastric acid
The colon
( large intestines)
Primary function
is to
absorb water
and
electrolytes
, and the
formation of faeces
the
colon
possesses a
muscular wall
and many
goblet cells
which
secrete mucous
to
facilitate
the
movement
of
faeces
through the
lumen
lack of villi
and
microvilli,**
many more
goblet
cells.
Makes the
colon
wall
distinctively
different** to the small intestines
The stomach
3 functional areas
The fundic region
(or fundus)
Secretes acid,
Site
of the
lower oesophageal
sphincter
which allows food to
enter t
he
stomach
and
prevents
it from
leaving
the
stomach
back through the
oesophagus.
The
corpus body
Secretes
acid
(H+) &
pepsin
The antrum
(pyloric region
Secretes
gastrin
Site of the
pyloric sphincter
which
controls
the
movement
of
chyme
from the
stomach
to the
duodenum)
Main
functions
of the
stomach
Receives food
To
mechanically
breakdown
food
To
chemically
breakdown
food (digestion of
protein at low pH
)
to
mix
and
store
food
To
deliver
food
(chyme)
to the
duodenum
Destroy bacteria
Motility of the stomachs 3 functions
Storage
during swallowing the stomach relaxes to accommodate food.
(receptive relaxation)
caused by 2 reflex pathways.
Vago-vagal
Pathway, the
inhibitory
neurotransmitter
-
vasoactive intestinal polypeptide (VIP)
Vago-sympathetic
Pathway. inhibitory neurotransmitter-
noradrenaline
Mixing
.
Peristaltic waves
originating in the
corpus
move toward the
pylorus
(A).
Fluid
elements of the
chyme
are forced through the
pylorus
solid
elements are forced backwards causing
mixing
and
grinding
of the
stomach contents
Emptying
This is stimulated by both
hormonal
and
neural effects
Neural **
Vago-vagal reflex
enhances peristalsis The
duodenum
is activley relaxed to
receive
the
chyme** from the stomach.
Hormonal
The
gastrin relaxes
the
pylorus-
small quantity of
chyme
to enter the
duodenum.
Gastrin
enhances peristalsis
Gastrin constricts
the
LOS
to
prevent leakage
of the stomach content to the oesophagus. Gastric
emptying
needs to be
controlled.
This is
determine
d by the
receptivity
of the
duodenum.
If there is
chyme in
the area of the
duodenum
immediately next to the
pylorus,
then gastric emptying is
slowed.
This control is achieved by both
reflex and hormonal action
Gastrointestinal Motility
Food must be moved through the gastrointestinal tract starting at the mouth and ending with the excretion of faeces from the anus. gut utilises smooth muscle. This
smooth muscle
displays
contractions
which can be
tonic
(that is to say that they display tone) and
phasic
(that is to say that the contractions happen in waves).
Peristalsis
is a
coordinated
wave
of
muscle contraction
. This
coordination
is
achieved
and
controlled
by the
intramural plexi
of the
GI tract
which
stimulate
the
circular
and l
ongitudonal muscle
in such a way as to
promote unidirectional movement
of a
bolus
of food
Tonic contractions
, feature of the
sphincter muscles
which are found between functionally different sections of the GI tract
Phasic contractions,
are responsible for the
mixing
( known as
segmental contractions)
Contraction of the
smooth
muscle is
influenced
by
intrinsic electrical rhythm
that is
generated
by
smooth muscle cells
which behave like a
pacemakers
Known as
basic electrical rhythm
pace maker cells are
responsible
for the
slow wave
s are called the
myenteric interstitial cells
of Cajal (ICC)
Highly specialised cells
The chief cell
. These cells
secrete
the
proenzyme pepsinogen
Parietal cells
These cells
secrete protons
(H+) to keep the
acidity
of the
stomach
around
pH 2
Gastrin Secreting Cells
(a.k.a. G-cells) These cells
secrete
the
hormone
gastrin
into the
circulatory system
Mucous Cells
Secrete
mucous
and
bicarbonate
(HCO3-) as a
protective lining
of the
stomach
to
prevent
the
acid
and
enzymes
secreted here from
digesting
the
stomach tissues.
creating a
mucous barrier
that is approximately
0.18mm thick
. the side of the mucous barrier that contacts the
surface cells
has a
pH7
. the
gastric juices
at the other side of the
barrier
is kept at
pH2
Intestinal motility
movement
in the
small**
intestine
is
segmentation
. This is a
rhythmic contraction
and
relaxation
of
circular muscle
in
alternating
small
segments
and its purpose is to
facilitate
the
mixing
of the contents of the
small intestine **
The rate of
segmentation
is at its
highest
in the
upper small intestin
e. Therefore there is a
tendency
to very
slowly
move
material
through the
intestine.
segmentation
also occurs in the
colon
. However, here it is known as
haustration.
to facilitate the
reabsorption of water
and
electrolytes
Colonic propulsion
also
increases
following a
meal
, This is stimulated by the
gastro-colic reflex
and circulating
gastrin.
Swallowing
Swallowing
is a complex process
coordinated
by the
swallowing entre
in the
medulla oblongata
. consisting of 3 processes: the
Voluntary phase
( initiates the response) The
pharyngeal phase
involuntary and initiates the
peristatic
wave, that moves food from the
pharynx
down the
oesophagus
. The
oesophageal phase
, this is
involuntary
and
promotes
the
movement
of
food
into the
stomach.
The reflex
of the
lower oesophageal sphincter
onset of
swallowing
is caused by
inhibitory VOP-ergic nerves
which are supplied by the
vagus
from the swallowing centre in the
medulla oblongata
.
failure
to
close
the LOS after swallowing is the causes of
gastro-oesophageal reflux disease.