Human Physiology – Functional Anatomy of the Small Intestine

Human Physiology – Functional Anatomy of the Small Intestine


>>Dr. Ketchum: So that takes us into the small
intestine. The small intestine is a coiled, hollowed tube, and it’s very long—about
8 to 10 feet. It’s located between your stomach and your large intestine. This is
the primary site of digestion and absorption. And so in terms of digestion and absorption,
this is the primary site for lipid digestion and absorption, carbohydrate digestion and
absorption, protein digestion and absorption, and nucleic acid digestion and absorption.
We’ll come back to the specifics on that when we talk about the absorption and digestion
in the small intestine. So in the small intestine we have three divisions,
and they occur in this order. The first part of your small intestine, and that’s the
duodenum. Some people will refer to this as the “duo-D-num,” but I call it the duodenum.
So that’s the first passage or the first tube when chyme leaves the stomach and passes
into the duodenum. Then that chyme will pass through the jejunum and lastly through the
ileum. So those are the three regions of the small intestine. Again, chyme has to go through
the duodenum, jejunum, and then the ileum. The duodenum is the most important when it
comes to digestion and absorption of nutrients. There are various secretions into the duodenum,
and these secretions come from accessory organs. Remember those are the organs that are outside
of the GI tract. So one of these is pancreatic juice. Pancreatic juice consists of a lot
of stuff, and we’ll come back to that later. So there is some digestive enzymes that make
up this pancreatic juice, and there are some bicarbonate ions that make up the juice as
well. If you think about the contents coming from the stomach into the duodenum…the stomach,
we have said, is very acidic. The duodenum, however, is not acidic because of the bicarbonate.
So we have these bicarbonate ions that get secreted into the duodenum. So in reality,
the duodenum is neutral to slightly basic, and cannot be acidic. Otherwise you will not
be able to digest your food. Bile enters the duodenum as well. Bile is
secreted from the liver; the liver is another accessory organ. The bile can be stored in
the gallbladder, and the bile contains bile salts, which aid in fat digestion. So for
example, what I mean by this is let’s take a large fat droplet. So this is a lipid or
triglyceride, and then we apply bile salts which contain bile. And so what the
bile salts are going to do is going to take that big old fat droplet and it’s
going to break it down. I don’t like using the term “break down,” because bile salts
are not enzymes. But what it does do is it emulsifies fat—that’s a much better term
to use than break down: It emulsifies fat. So we have these smaller fat droplets. So
the whole purpose to do this is so that we can increase surface area for an enzyme called
lipase. Lipase will breakdown fat. So now we have more surface area for that enzyme
to act upon. Okay, so here we are once again. We have absorption in the small intestines.
It occurs within the first 20% of the intestine, and it is arranged to have a large surface
area for absorption. So let’s take a closer look at how it has a large surface area for
absorption. So here what we’re looking at is the small intestine; the lumen would be
here of the small intestine. And then what you have are these finger-like projections,
and these project into the lumen of the small intestine. And what these things are called?
They’re called villi. One of these is called a villus, and multiple are called villi. So
villi increase the surface area of the epithelium. So let’s take a look at the top part, okay,
or even the side components of this villus. So when we zoom in on that, what we would
see is that we have these epithelial cells that have microvilli. So these are even smaller
projections that sit on top of the villi. And what these microvilli form is the brush
border. The brush border will actually secrete what are called brush border enzymes, and
it will also contain transmembrane proteins. Remember that we have a lot of absorption
taking place in the small intestine, so you’re going to be needing those transmembrane proteins
to do that. You also need brush border enzymes to help break down the larger biomolecules.
Because we’re doing absorption here, these villi have to have association with blood
vessels. So if you look real closely at this one, it contains your blood vessels. Here’s
your artery in red, some veins are in blue, and here are the lacteals. We’ll talk about
lacteals later. Okay, so here then, because we have villi and microvilli, there’s a
lot more surface area for absorption and digestion. Then associated with these villi are the crypts.
Think about a crypt being a partially underground chamber. So here’s this one villi, and if
you look down into the wall of the small intestines, there’s the crypt. So again, these little
cells lining the crypt here are epithelial cells and they secrete the bicarbonate. You
have bicarbonate coming from the pancreas as well, but the small intestine also secretes
bicarbonate and when it secretes its bicarbonate, it gets secreted into the lumen of your small
intestine. So secretion takes place in the proximal small intestine. Proximal meaning
it’s closer; it’s the part of the small intestine that’s closer to the stomach.
Later the bicarbonate gets absorbed. It’s absorbed into the distal part of the small intestine.
So the further away it gets from the stomach, it will get absorbed. And we’re going to
come back to that idea later.

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