在本课程中，学生将学习认识和应用说明人体九个器官系统中整体人体机能（作为完整有机体）的基本概念。
一般结构和功能
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來自 Duke University 的課程
人体生理学导论（中文版）
90 個評分
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從本節課中
消化系统
在此模块中，我们将探讨消化道的内部运作。我们重点讨论胃肠道特定区域的功能，复杂食物在这些区域中被处理为身体可吸收以用作能源物质的溶质和养分。 此“加工厂”以从嘴到肛门的单向方式工作，需要胃酸、酶、碱和液体协调分泌来正常运作。对胃肠 (GI) 道及其附属器官（唾液腺、肝脏和胰腺）来说不平常的是，它们无需来自大脑的意识输入即可及时协调运作。 取而代之，消化道通过局部产生化学物质（激素和旁分泌）以及肠道神经系统（自主神经系统的组成部分）的协调运作来整合其多样化的动作。在此模块的最后一课中，我们将介绍消化道的正常能动性，以及导致胃肠不适（如呕吐和腹泻）的扰动。
與講師見面
Jennifer CarbreyAssistant Research Professor
Department of Cell Biology
Emma JakoiAssociate Research Professor
Department of Cell Biology
Welcome.
discussion of the gastrointestinal tract. This is the or I call it the GI tract,
this is the tract that enables us to bring nutrients, electrolytes and water
into the body. So we're moving it from our external
environment to the inside of the body and then into the bloodstream for delivery to
all the cells of the, of the. Of the body.
So how does this thing work. So the GI track is a tube, which is about
five meters in length. And this tube has an opening at the top
which is your mouth, and that's what's shown here.
So here's the mouth. And at the, and at the end of the tube,
we have the anus. And the tube is, is going to allow then
food materials to be coming into the mouth from your diet and these are going
to be complex foods such as steak and potatoes.
And we need to, to convert this steak and potatoes into simple molecules which can
be used by the cells of the body. for fuel.
So we want to be able to convert then the steak and the potatoes into amino acids
and into simple sugars. This is done by the gastrointestinal
tract and it sort of acts like a processing plant.
So the first thing that happens is that these complex food materials enter
through the mouth and as they enter into the mouth then we chew on them and break
them down into smaller. And smaller particles this increases the
surface area of the particle and allows then for attack by enzymes.
So what we're increasing the surface area and this material then is very rapidly
going to be transmitted from the oral cavity through the esophagus which is
essentially a tube down the to the stomach.
Once we reach the stomach. Once we're in the stomach then we're
going to start the process of digestion. And this is going to be converting the
complex, the complex materials such as a steak into polypeptides and then from
polypeptides into amino acids. Digestion is going to occur within the
stomach, but it's also going to occur within the next region which is called
the small intestine. And in the small intestine we will
further break down these particles to very simple molecules such as glucose and
amino acids. And then, we're going to absorb these
things these small molecules across the ephithelial cells, which are lining this
tube and enter into the body. Material that's left over is going to be
released from the anus as fecal material. So this is going to be the waste products
that cannot be used by the body. In a typical day you're taking in
something like a litre to two litres of solids as well as fluid in, in a given
meal and as it passes through the GI tract you will add about seven litres of
secretions. Which can be acids, it can be buffers and
it can be enzymes and as it moves through the small intestine then.
We could have as much as nine litres of fluid.
But this, most of this fluid is going to be reabsorbed as it's moving through the
small intestine, and so that we end up with fecal matter which is only about 200
to 500 millilitres. And the rest of the material then will be
reabsorbed as we're moving through the small intestine, and into the large
intestine. So that means that along this track,
we're going to have regions that are have specific functions.
And we're going to want to be moving our material in a unidirectional manner, so
we start at the mouth, and we're going to be moving towards the anus.
And this unidirectional manner is, is going to be due to smooth muscle
contraction which is, which is propelling the boldest of food.
And food stuffs through the tract. In addition to having the, the GI track
itself. The GI track proper we have an other
organs which are going to help in this process.
In this processing of food, of food. The first of this is the salivary gland.
The salivary gland secrets saliva which moistens the food particle as it coming
in through the mouths for chewing, and that allows the food particle to easily
slides down the esophagus to the stomach. The second is that we have the liver and
we have the pancreas, and these two these two organs are going to be secreting,
materials which are going to aid in the digestion of the foodstuffs within the GI
tract itself. And that's going to be predominantly,
within the small intestine. As we're moving down this track, we have
specific sphincters, which are gating the entry and exit from the track.
And these, the, at the top, we have a voluntary sphincter, which is for
swallowing. And at the bottom, we have a voluntary
sphincter. Which is, for avoiding the feaces, under
our control. So we have skeletal muscle that,
sphincters at the top and at the bottom of the tube.
There are also sphincters within different regions of this tube, which are
smooth muscle sphincters, there are under involuntary control.
And the smooth muscle sphincter, for instance, resides here.
As we're moving from the esophagus into the stomach.
And then from the stomach as we're moving from the stomach into the beginning of
the small intestine which is the duodenum.
And then again we have a involuntary sphincter as we're moving then from the
from one region of the colon to the, to the anus.
So the track, then, is going to be this movement of food materials along the
track. And as it's moving along the track, we're
going to be changing the actual structure of these materials and then, absorbing
the materials and so that, at the end, we have waste product which is not useful to
the body. And that will be what's, what's expelled
from the body. So let's look a little bit more at the
anatomy of the GI tract itself. So, I told you that the tube is a
muscular tube. On the outermost regions of the tube, we
have two layers of muscle, and that's what's diagrammed here.
The inner layer which is here. The inner most layer of this muscle is
called the muscularis externa is this, this muscle, this smooth muscle is
oriented circumferentially around the lumen.
So when it contracts then it's going to, to make the diameter of the tube smaller.
Conversely, if it relaxes it will make the diameter of the tube larger.
So its governing then the diameter of the tube.
The muscle which is present on the outermost layer, the muscularis externis,
then this muscle is oriented along the long axis of the tube.
When it shortens, we are going to shorten the tube.
And this is going to allow for peristaltic movement, of the bolus of
food as it moves down the track. Sort of a milking kind of an action, as
it moves down the track. On the innermost aspect of the tube this
is a lumen. And the lumen is obviously where the food
particles are are first delivered. And surrounding that lumen we have an
epithelium. There's a simple layer of cells.
At the top of the tract there's multiple layers of cells.
Which are very flat and very much like what you see on your skin and this is a
ware and tear surface because we're, it's for abrasion.
We're having food materials coming in which can be a bit rough and it's simply
a conduit from the mouth down to the esophagus, down to the stomach.
And so within the esophagus we have this ware and tear epithelium no absorption
and no secretion. We have a similar type of epithelium on
So if
and ware and tear epithelium at the bottom of the tube.
In the stomach, the epithelium is a single layer of cells and these cells are
going to be secretory. And we'll talk about what they're
secreting in just a second. And then as we move into the small
intestine, in the small intestine, then, the cells are also a single layer of
cells, but here we have an absorptive function.
So we're moving the smaller particles, which we have generated in the lumen of
the track, across these cells and into the, and into the body.
So the first part of the. the tract that I want to talk about today
is going to be the stomach. and we're going to talk about how the,
what the, what secretions the stomach makes and how it starts the process of
digestion. And then, the next time we come in, we'll
talk about how this process is actually regulated.
So the stomach, has essentially three types of cells in the major portion of
the stomach. And the major portion of the stomach is
called the fundic. The fundic region, of the stomach.
Here, the cells, which are on the very surface.
Which are facing the lumen. are called surface cells and, and then
going down into the glands, into the invaginations, so it looks like this.
So these would be our surface cells. And then going into the invagination
which is the gland, we will have the neck cells.
So these epithelial cells are lining these surfaces.
Looks like this. The surface cells and the neck cells are
secreting mucus. This is a complex carbohydrate containing
material, which is a protective coating, which is going to prevent the acid, which
is generated by by the stomach, from eroding through, through this epithelial
layer. The second type of cell is called the
parietal cell and the parietal cells are located also within the glands
themselves. The parietal cells have two major
functions. One is they make HCL.
They make this acid that is going to be starting the digestion of protein.
And secondly, they make a factor which is called intrinsic factor.
This is a carrier and this carrier binds vitamin B12 which, which in the human,
must come from the diet. So, you cannot make this vitamin.
And the vitamin is absolutely necessary for making red blood cells.
Vitamin B12 comes in from the diet, it binds to this intrinsic factor, it's
carrier. And then it passes through the small
intestine, to the distal region of the small intestine, which is called the
elium. And in the elium, that carrier then, has
a receptor, and so the vitamin B12, can move across those cells, across the
elio-epithelial cells, and the vitamin B12 is now entering into the body.
In the absence of intrinsic factor. The individual will have profound anemia.
So they have pernicious anemia. And this anemia is simply means you can't
make the heat, you can't make these red blood cells.
And so you will have lower oxygen carrying capacity within the body.
The third type of cell which is present within this region of the stomach is
called the chief cells and the chief cells sit down here in the very base of
the glands. The chief cells secrete what's called
pepsinogen. Pepsinogen is an inactive enzyme which is
called pepsin. Anything that has an OGN on the end it is
an inactive enzyme. These are, these are classified as as
zymogens. So they're inactive enzymes.
And we'll talk about inactive enzymes some more when we talk about the
pancreas. So this is an inactive enzyme pepsin,
pepsinogen. Which when, it is, it is secreted and
into the lumen of the stomach, the pepsinogen in the presence of acid is
converted to pepsin. And that requires HCL.
So we have to have a very acidic condition for this enzyme to become
activated. The pepcin starts to break down protein
into polypeptides. So we're starting out digestion of
protein with this enzyme. Now because the secretion of acid is so
important I want to spend some time talking about it.
So the secretion of, of gastric acid, is done by the parietal cells, so these are
our parietal cells. And we're still in the stomach in that
fundic region of the stomach. I'm draw-, I've drawn here a parietal
cell, where the lumen of the stomach is on this side.
And the blood portion of the cells are facing the basal portion of the cell is
facing the blood on this side. So the apical portion faces the lumen.
And that's basal side is facing the blood.
So we have a capillary that's running right here.
Now these cells are going to use a reaction which dependent on carbonic and
hydrase. To generate protons.
And the, the and this, these protons are what's going to be then moved into the
lumen of the stomach, and that's going to give us our acid.
So the enzyme is cape, is able to take carbon dioxide, which comes from the
blood. It passes across the plasma membrane of
these cells, and it reacts with water in the presence of carbonic anhydrase.
And we get bicarbonate and we get a proton.
The proton is going to be extruded from the cells into the lumen by a pump.
So this is going to be a ATPase, and this particular ATPase is a hydrogen-potassium
ATPase. And it's so important that it has another
name, and we call that the proton pump. The bicarbonate, which came from our, our
carbonic and hydrose reaction. And the bicarbonate is shown here.
Is extruded from the cells and enters into the blood in exchange for chlorides
so that we use an anti-porter to move bicarbonate out of the cells and chloride
into the cells. The chloride will diffuse across the
cells and leave the cells to enter in the lumen of the stomach, so we now have
chloride and hydrogen within the lumen of the stomach, and so we have just
generated hydrochloric acid. In order to keep the proton pump to
active, we have to have potassium, and this potassium is is being furnished by
the sodium potassium ATPase just sitting on the basel surface of these cells.
Potassium comes into the cells from the blood, potassium will then be extruded
from the cell through the[UNKNOWN] channel, and then potassium is used by
the pump to move potassium into the cell and the proton out of the cell and into
the lumen. The PH of this region can get as low as a
PH of two, and PH, as you recall, is the logarithmic scale for hydrogen ions, free
hydrogen ions. So this is extremely concentrated, a very
strong acid. So this is a very very acidic condition.
Acid is made by the stomach under two conditions.
One is in the fed state, we increase the amount of acid that's going to be made.
So our acid secretion, so if we have a food coming into the body at this point,
acid secretion will increase. rapidly.
So the cells are sensing the ma-, the food is present and they're going to be
making a lot of HCL. And the, by 90 minutes after eating, we
have a peak of secretion of acid within the, within the stomach, within this
region of the stomach. And then the peak acid output drops, so
that by four hours after a meal, then the peak acids eh, secretion drops.
And we are making a basal amount of acid within, within this region.
If you notice, the PH of the area is changing, as well.
So PH is our free proton. And, and, and initially we have a.
A movement from a very acidic condition, that is a PH of two, up towards a PH of
five. So we're going toward a basic condition.
And that's due to the protons binding to the food materials which are coming into
the stomach lumen. So the food is.
Buffering the free, the free proton. And eventually the amount of food that's
within the stomach, is leaving the stomach slowly to enter into the small
intestine and as it leaves the stomach, then the amount of free proton increases
and our PH then falls. Such that by 100, by two hours, after
feeding to four hours after feeding, we have again a very low PH within the
lumen. So the next time we come in here, we're
going to talk about how we're going to control turning on the production of acid
when we have food coming into the body. And how we turn off the production of
acid, when we don't need very high amounts of acid, because the, the stomach
is empty. So one of the general concepts.
So the general concepts are that the gastrointestinal tract is a muscular tube
in which nutrients are going to move in a unidirectional manner from mouth to anus.
Secondly, we have regional specializations along this track, which
enable fragmentation of the food particles.
So we make them smaller, we have larger surface areas.
Digestion, we break the particles from complex proteins into polypeptides and
eventually into amino acids. And then the absorbtion of nutrients so
we can absorb the amino acids across the epithelium that's lining the track and
allow them to enter the body. Third, acids will be secreted by the
parietal cells of the stomach and this is in a specific region of the stomach
called the fundic stomach. And that this process requires carbonic
anhydrase activity to generate the proton and the bicarbonate, and it's generated
from water and CO2, which is being which is being delivered to these cells by the
blood. The protons are secreted into the stomach
lumen by a sod- a hydrogen potassium ATPase, and this is our proton pump.
Bicarbonate is secreted into the blood in exchange for chloride ions.
And as the chloride enters, it crosses the cells and then, is is.
It's transferred by diffusion across the membrane, facilitated diffusion into the
lumin. And we then generate hydrochloric acid.
And forth, the acid secretion can be increased in our fed states, and is
decreased in the fasted states. And the regulation of this, of moving
from a high acid secretion to a lower. acid secretion, that regulation requires
several factors. And we'll talk about that the next time
you come in here. So, see you at that time.
Bye-bye.
