>>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.