Now, we have these salivary glands, we know
what the different salivary glands are,
we know how the ions move.
But what is the control factors
for salivary gland secretion?
The cephalic phase is going
to be very important phase
in the digestive process
especially for salivary glands.
In fact, this is the classic Pavlovian
response in response to a stimulus.
So Pavlov trained dogs to
salivate in response to a bell.
And he did that by having the dogs eat
at a certain time when he rang a bell.
Then after a while, he removed the
food and would just ring the bell,
and sure enough, the dogs
would still salivate.
Same kind of process
that is involved here.
It’s simply the brain acting
via the sympathetic and
parasympathetic nervous system
to engage salivary secretions.
So what are the
The first one is acetylcholine.
Acetylcholine causes an increase in serous
secretions from all of the salivary glands.
Vasoactive intestinal peptide is also released
from the parasympathetic nervous system.
It acts primarily to increase blood flow,
which increases salivary secretions.
Interestingly, both the sympathetic
and parasympathetic nervous systems
increase the amount of
So norepinephrine acting through
can also increase the amount
of salivary secretions,
but not in this great
of quantity or rate
as the parasympathetic nervous
system and acetylcholine.
So let’s review now gastric secretions
and what their primary functions are.
First and foremost is
gastric secretions are
very important in
protecting the GI system.
The amount of acid that
is released will actually
kill most bacteria
in most other form.
Substances help denature proteins,
and that will help protect the
lower aspect of the GI system.
It does act as kind of a
reservoir or a storage unit,
which means it kind of holds
onto food stuff for a while
and then gradually releases
it into the small intestine.
And that slow release is very important
because it acts to kind of regulate
how much food enters
the small intestine.
You wouldn’t want to have this
process to not be regulated
because then as soon as you
ate a very large meal,
all of that substance would hit the
small intestine at the same time.
And there wouldn’t be enough enzymes
and enough surface area
in the small intestine
for there to undergo
So the stomach holds it for a little
bit and then gradually releases it.
The nice thing about the stomach is there
is a little bit of absorption function,
but that has to be through
And finally, you’re going to
prepare most of the food stuff
for digestion of
And you do that by releasing both acid,
as well as an enzyme called pepsinogen,
which is then converted to pepsin,
and that helps to
break down protein.
So some of that digestion can
start to occur in the stomach
before it gets to the small intestine,
and therefore, improve
Now that we have the
digestive functions overall,
let’s talk about where these
digestive juices are secreted from.
These are gastric pits in the stomach that
are lined with surface epithelial tissue,
mucous neck cells, and then finally, our
parietal cells towards the bottom.
The mucous neck cells are
very important because
what they do is provide a little bit of
protection from the endothelial layer
so that the lining of the
stomach is not eaten up
by the acid that’s produced
from the parietal cells.
What are the other constituents
that are released?
Well, hydrogen ions being
the most important,
this is something that we
probably should discuss first.
Hydrogen ion secretion is actually
very, very difficult to do
because of the concentration gradients
between what is in the plasma
and what you need
in the stomach.
So in the plasma, you might
have a pH of around 7.4.
Usually, the norm is in
between 7.35 and 7,
but you need to move that all the
way down to a pH of about 1 to 1.5
That takes a lot of energy.
So you should think of,
if this takes energy,
you’re probably needing to
pump, and you’re right.
You need a hydrogen ion pump.
So, let’s talk a little bit more
about how that process works.
Now, to get the hydrogen ions
out because it’s an ion,
we need to talk about
three other ions
One is chloride, which is going to
be secreted, sodium and potassium.
So those are the ions involved
besides hydrogen ions.
They are secreted in various flow
rates that we’ll discuss in a moment.
The other interesting thing
about parietal cells
is they also secrete something
called intrinsic factor.
And in intrinsic factor, we
won’t deal with too much today,
but it’s very important in
vitamin B12 absorption,
and we’ll discuss that in the lecture
on digestion and absorption.
Pepsinogen is also
secreted from chief cells,
and that is converted from pepsinogen
to pepsin, which is its active form.
The gastric secretions of ions are
done in this secretion format,
the same that you saw
in the salivary glands
where we have secretion
on the X-axis
and you have the ion
concentration on the Y-axis.
If you look at a substance like sodium,
if you have a higher value in the
plasma than in the gastric secretions,
you know that it’s being reabsorbed.
If the value is higher in the
secretions versus the plasma,
you know it is being secreted.
So this should be their first
indication that sodium is reabsorbed,
and it’s reabsorbed in the
flow rate dependent manner.
Meaning that as you get faster flow rates,
what you’re going to get is
increases in its reabsorption.
Potassium is being secreted.
Why do I know it’s secreted?
Because the values in the gastric
secretions are greater than the plasma.
Chloride is also being secreted,
and this is done in the flow
rate dependent manner as well.
The faster the flow rate, the
more chloride is secreted.
And finally, hydrogen ions are
secreted in a great number.
In fact, the amount that is secreted in
hydrogen ions is so many folds higher
in the gastric secretions than
it is located within the plasma.
So now that we have these
let’s talk about the mechanisms
or transporters available
to get those ions
to the right place.