Digestive System - Paper <=1998
1998_2nd
semester_7_PartA
The
composition of bile is mainly bile salts and bicarbonate ions. Its secretion and
release are regulated by the acitivity of the parasympathetic fibres which cause
a stimulatory effect and produced increased amounts of bile. Sympathetic effects
have the opposite effect, resulting in less bile produced.
The
secretion of bile is also regulated by entergastrones, which are hormones
produced by enterendocrine cells located within the small intestine. The tips of
these cells are sensitive to the acidity of the chyme released from the stomach,
and also are sensitive to the presence of any partially hydrolysed proteins and
lipids. Thus, when the acidity and state of the chyme is different to what is
expected, this triggers the release of secretin and CCK. These hormones are
released into the underlying connective tissue and into the capillary blood
where they travel to the liver and have an effect on the production of bile.
Secretin
has a stimulatory effect on the production of bicarbonate. If the acidity is
increased, then this initiates more bicarbonate to be produced as part of the
composition of bile and therefore increase the pH of the chyme. This brings
about the homeostatic balance within the chyme of the small intestine.
When
bile is produced by the hepatocytes they are released into the bile canaliculli
which are between the tightly packed hepatocytes. The bile here is then
collected in bile ductules, and these ductules empty into the common hepatic
duct. The cystic duct which is an extension of the gall bladder also connects
with the common hepatic duct and then these converge to empty into the small
intestine as the common bile duct. The bile produced by the liver is normally
taken in by the gall bladder for storage and concentration and when the hormones
are released, bile is then released into the small intestine.
1998_2nd
semester_Q7_PartA
Pancreatic
juice is largely composed of water, electrolytes such as bicarbonate ions and
pancreatic enzymes. The latter two are the most important components of the
pancreatics juice. Most of the enzymes secreted by the pancreas are in inactive
form such that they so not participate in autodigestion of the pancreas itself.
Pancreatic juice secretion is regulated by both neural and hormonal mechanisms.
The neural mechanisms involved are via the excitatory signals received by the
parasympathetic signals (mainly the vagus nerve). Sympathetic activity reduces
the secretion of pancreatic juice. Hormonal mechanisms are also evident.
Enterogastrones are the hormones responsible for the stimulation of secretion of
the pancreatic juice and its components. The enterendocrine cells present within
the mucosa of the small intestine are sensitive to the acidity of the chyme
secreted by the stomach and also are sensitive to the level of hydrolysis of the
proteins and lipid components of the chyme. Thus if the acidity it too high or
there are partially hydrolysed components evident, the these cells secrete
secretin and cck.
The
secretin has a stimulatory effect on the secretion of the pancreatic juice. They
induce more bicarbonate ions to be secreted therefore bringing the pH of the
chyme back to normal. The pancreatic acinar cells also have special receptors to
CCK, and when they hormones bind to these receptors they induce secretion of the
pancretic enzymes.
The
pancreas is a highly ducted organ, and the functional units are the acini.
Histologically, pancreatic acinar cells surround the ducts that form the
pancreatic duct and they are responsible for the secretion of the pancreatic
juice. The pancreatic duct directly empties into the small intestine.
1996_2nd
semester_Q6
Major
Points (Composition): HCL
--> Pepsin
-->
Intrinsic
Factor
Major
Points (Structures): Cells
of the mucous lining -->
parietal
cells -->
chief
cells -->
location
in the gastric glands -->
active
forms of pepsin
Major
Points (Gastric Juice Reg.): Neural
and hormonal mechanisms -->
short
and long reflexes -->
afferent
limb -->
integration
centre -->
efferent
limb -->
synapse
directly between intramural neurons -->
enteric
nervous system -->
Hormonal
mechanism -->
Enterogastrones
-->
Gastrin
secreted by G cells in response to the properties of chyme -->
gastrin
has a influence on gastric secretion -->
occurs
during gastric phase of regulation -->
intestinal
hormones and enterogastric reflex -->
during
intestinal phase -->
production
of inhibitory hormones.
1996_2nd
semester_Q7_PartA
Major
Points (Structure): Lobules
-->
surrounded by connective tissue -->
portal areas contain bile ductules, portal artery and vein -->
drain into central vein -->
sinusoid between hepatic cords -->
kuffer cells -->
presence of bile canaliculli between tightly packed hepatocytes -->
drain into ductules -->
common hepatic duct -->
to
gall bladder/small intestine.
Major
Points (Bile salts): Bile
salts are derived from cholesterol -->
act
similar to detergents in chyme to help emulsify the lipid droplets -->
greater
surface area -->
means
the pancreatic enzymes can faster hydrolyse these droplets in chyme.
1995_2nd
semester_Q6
The
stomach’s main function is in mechanical and chemical fragmentation of the
food that it stores following deglutition, and it contains cells part of its
gastric glands that help it perform this specialised function.
The
gastric wall of the stomach contains three layers of smooth muscle, and this
helps in its specialised function of peristaltis. Peristaltis is the contraction
of this smooth muscle which helps it mix, churn the food in the presence of
gastric juice and turn it into chyme. This chyme is then released into the small
intesting. The smooth muscle cells of the stomach contain specialised pacemaker
cells. These have an unstable membrane potential and as result depolarise
spontaneously. The spontaneous depolarisation causes the contraction of the
muscle, producing peristaltic movements.
The
gastric mucosa is made up of a simple columnar epithelium with lots of goblet
cells. The goblet cells produce mucous which has a functional purpose. The
mucous acts as a protective barrier against the acidity of the gastric juice
secreted by the cells that occupy the glands of the stomach. The gastric glands
contain many cells which secrete constituents of gastric juice. The parietal
cells are located dispersed between the chief cells, and its main function is to
secrete HCL which gives gastric juice its acidic properties. Functionally, HCL
is useful for converting pepsinogen into pepsin. The chief cells are located at
the bottom of the glands and their main function is to produce pepsinogenn which
is the inactive form of pepsinogen. The presence of HCL above, converts
pepsinogen into pepsin and then positive feedback results. The parietal cells
also secrete intrinsic factor which is vital in absorption of B12 in the small
intestine. There also enteroendocrine cells located basally in the gastric
glands and these are responsible for secreting enterogastrone hormones. Some of
these hormones include: secretin, CCK and gastrin and these are important in
regulation of gastric secretion and the activity of bile production by the
liver, pancreatic enzyme production by the acinus and also the gall bladder.
The
gastric wall contains specialised glandular structures which contains
specialised cells, that help it perform specialised functions such as production
of gastric juice. It also contains specialised smooth muscle, which help it act
as a mechanical fragmenter of food into chyme.
1997_2nd
semester_Q6
Major
Points (Oesophagus):
Non-Keratinized
Stratified Squamous Epithelium -->
abrasion
caused by food movement -->
tunica
adventitia instead of serosa -->
attachment
to outside tissues -->
structural
integrity -->
thrown into
folds -->
prevent
entry of food -->
submucosal
glands secrete mucus -->
easy passage
of food along the tube -->
thickening
of the smooth muscle near entry into stomach -->
acts as a
sphincter -->
upper third,
skeletal, middle mixed muscle, lower third, smooth muscle -->
voluntary
control to involuntary control of swallowing.
Major
Points (Stomach): simple
columnar epithelium -->
lots of
goblet cells -->
gastric pits
-->
act as
glands -->
cells and
functions -->
enteroendocrine
cells and functions to gastric motility and secretion -->
muscle layer
-->
two layers -->
functional
implication -->
peristaltis
during second phase of gastric motility -->
pacemaker
cells -->
basic
electrical rhythm -->
muscle
layer is three instead of two.
Major
Points (Small Intestine): Simple
columnar epithelium -->
lots of goblet cells -->
function is to protect against acidic nature of chyme -->
three parts -->
function of each part -->
surface area modifications -->
muscle layers assist in segmentation and peristaltis -->
pacemaker cells depolarise at different rates in neighbouring sections of the
small intestine -->
brush border enzymes -->
function in breakdown of proteins, carbohydrates, nucleic acids -->
intestinal crypts.
Major
Points (Large Intestine): Simple
columnar with lots of goblet cells to stratified sqamous in anal canal
(function?) -->
function of goblet cells in protection against irritants secreted by bacteria
present -->
intestinal crypts -->
taenae coli -->
three bands -->
longitudinal layer -->
segmentation and peristaltis -->
structures of the small intestine.
1995_2nd
semester_Q6
Major
Points (Stomach): simple
columnar epithelium with lots of goblet cells -->
function of mucous secretion to protect against acidic nature of gastric
secretions -->
gastric pits -->
cells present -->
function -->
presence of two layers of muscle -->
longitudinal layer has three bands -->
main functions of the stomach
Major
Points (ileum): major
function -->
to absorb nutrients broken down in jejunem -->
major processes of absorption -->
through luminal surface -->
through basal cell surface -->
simple diffusion, osmosis, active, facilated, secondary active transport -->
mucosal surface area increase -->
assist in more efficient absorption -->
specialised absorption pathway of lipids