So let’s move on
now to the stomach. The stomach is
specialized for this mechanical
and chemical digestion. So it's a
muscular sack that churns,
contracts and squashes,
like I say churns
the ingested bolus of food.
It also carries out significant chemical
digestion that release pepsinogen
which is an enzyme,
which is converted into pepsin helps
the break down of our
gastric juices. And it converts this bolus
of food into a semi-liquid
which is known as chyme. If we look
at the diagram we can see here
we have got the oesophagus
joining the stomach
and this region of the stomach we know
is the cardiac region.
So this region here is known as the
cardiac region and it opens up
via the cardiac orifice. That's the
junction between the oesophagus and the
stomach. We can then look at some
other regions of the stomach.
Superior to this junction,
this region here we have the Fundus
so we have the Fundus in this region.
We have the cardiac region here.
The fundus is usually filled with
gas that has been produced via this
chemical digestion. And then we
have this large space in here
which is the body of the stomach.
And we can see on the internal structure
of the stomach, it's flowing into this
series of folds called rugae.
And that helps to increase the
ability of the stomach to distend
outer in size we got
these elevation. We can
also see in this region here we have
what's known as the pylorus and this is a
narrowing of the body of the stomach
that leads it towards the duodenum
and importantly, in this region we
have what's known as the
pyloric sphincter. Really important
sphincter that regulates
the passage of chyme through
this space into
the duodenum and that’s the pyloric sphincter.
When the pressure within
the stomach becomes so high, then
food passes through this chyme,
passes through this sphincter
and enabling it to carry on
through the gastrointestinal tract
and fit to be digested. We can also
recognize a few other landmarks
or boundaries; external
features of this stomach.
We have this nice long curvature around here
as called a greater curvature. I am going to
appreciate the series of blood vessels that
run along here later on. And we can also see
that here we have a lesser curvature
Another important landmark is that it has
a peritoneal attachment with the
liver. We can see we have the other
structure of cardiac notch this junction
between the Fundus and the oesophagus.
And we can see where the lesser
curvature take us down towards
the pylorus, where we have the Angular
incisure so we can see that down here.
On this side of screen we can actually see
this muscular bag and all its smooth
muscle fibers arranged
either longitudinally or
align or arranged kind of circular that
surround the muscles. And these muscles
contracting enable that mechanical
digestion to occur.
So we can see the various layers
of muscles that are surrounding
the stomach. If we look at
the internal surface of the
stomach so here we can see the section
through the stomach. We have got the
outer surface of the stomach here and
we have got the internal surface here. We can see
in this thickness of the stomach,
we can see the various muscle layers
we can see the circular and longitudinal
muscle layers which we can see here.
And we can also see the mucosa,
this most inner lining of the
stomach and that’s really important,
this inner surface, this mucosa.
It’s important because it contains
what are known as the gastric
pits. The gastric pits that
communicate with some
gastric glands. And deep within these
gastric glands we can just may count
some gastric glands down at here, right
bottom of these pits. We can see these individual
pits that open up onto the surface of the
stomach. This two types of cells.
This is not the Physiology
course so we are not going into too much detail here.
But briefly these two different
types of cells - these parietal and
these chief cells are important
in that chemical digestion.
The parietal cells secrete
So secretes hydrochloric acid as
protons, as Hydrogen ions
and as Chlorine ions
into the lumen of the stomach.
This reduces the pH to around
about 1 or 2. The importance of this
is it kills micro-organisms
it denatures proteins, and it helps to
break down the plant cell walls.
It also importantly, this
activates the pepsin.
Pepsin is released by this
second group of cells called chief cells
and these secrete pepsinogen.
Pepsinogen is inactive
Pepsinogen is inactive and it requies the acidic pH
created by the parietal cells
to become active. So this
acidic lumen converts pepsinogen
to pepsin. And it
is the pepsin that breaks down the proteins.
So without this acidic
lumen, created by the parietal
cells, proteins in the
ingested bolus wouldn't be
able to be digested.