– [Professor] Okay, so we’re gonna talk about large intestines now. So, the first topic that’s
covered in this part is water balance, so
it’s important to realize that once again, there’s a lot of water going into the GI tract. I mean, not as much as the kidneys, but we have to re-absorb most
of it or we’ll be dehydrated. Okay, so the digestive tract receives nine liters of water a day. So they say about 0.7 is in food, about two liters is in drink, and then 6.7 liters is in secretions. Now, it’s important to realize
that eight liters of it is gonna be absorbed by
the small intestines, and about a liter is gonna be absorbed by the large intestines. I probably won’t ask you those numbers, but I want you to realize
that the small intestines are really important for absorption. Okay, now the water’s gonna
be absorbed by osmosis. That makes sense. Okay, and it’s gonna follow salt. So when the salt goes in, or the organic compounds
like amino acids go in, the water will follow. And I know you all know this, so diarrhea occurs when the
water doesn’t get absorbed. So the feces will pass
through too quickly, and absorption won’t occur
if the small intestines or the large intestines
are irritated or inflamed. So that could be caused
by an autoimmune disease, it could be caused by an
infection, like a virus or a bacterial infection. Diarrhea will also occur, or
the feces will be more liquid, if they have a high concentration
of solute, like lactose. So if you can’t digest milk
sugar, it can cause diarrhea. You can also think about… Like, a baby when they’re
just taking a bottle. If you increase the amount of sugar, like if you give them
apple juice or something, what you’re doing is you’re
increasing the amount of sugar. And that’ll pool the water,
so that’ll increase the water that goes into the small intestines and the large intestines. And that’ll make the feces
go through more easily. Same thing with prunes and grownups, or some of the laxatives like MiraLAX. What we’re doing is we’re
increasing the amount of solutes. Okay, and then the water will follow that. So here’s large intestines. So if we, I dunno why I have
two pictures, I guess just… Alright, I don’t know what I did here. But if we look here at this small picture, you can see here is where
the small intestines will hook into the large intestines. Okay, the cecum is this blind pouch where they’re gonna line up. Okay, so the small intestines,
lemme go over here. Small intestines comes in,
okay, so the ilium comes in, but where it lines up
it’s not that we line up exactly like this. We line up more like this. So this part is the cecum. Okay, and it’s considered a blind pouch. So sometimes food gets stuck in there, and that’s where appendicitis can occur. So here’s the appendix, which
is just hanging off the edge. It’s just like the size of a pinky finger. Okay, and then the cecum. Okay, the ileocecal valve,
that’s the connection. Okay, and then we’re gonna go… We have the ascending colon, is going up. Okay, and then the right
colic flexure is the bend, and then transverse colon. Okay, so there’s transverse colon. And then we have the
left colic flexure, okay, which is just the bend. And then the descending colon. Then we’re gonna go
into the sigmoid colon, which is kinda shaped like an S. Which, sigmoid. And then the rectum, and
the anal canal or the anus. There’s a lot of extra stuff on here. So these pouches are called haustra. Each one is a haustrum. Okay, and they’re pouches. And then, this band right here,
they don’t have it labeled. This band is kind of a
remnant of a layer of muscle, it’s the taenia coli. Okay, and you can think of it,
it kind of creates ruffles, so you can think of a, of, oh shoot… Like a valance, a curtain. Or valance, however you say it. And how you put the
curtain on the curtain rod, and there’s too much fabric
and you push it all together. That’s kind of what the haustra
are, they’re these pouches, and the taenia coli would be like the curtain
rod going through. Except it’s muscle,
it’s not a curtain rod. Sometimes, at these bends,
like when you have a baby, this is where gas gets stuck. Okay, because it’s gotta make that bend. Now if we look at this
little picture of the anus. Okay, so here’s the rectum coming in, and there’s the anus. The muscles, the levator
ani, this right here, okay? The anal canal. There’s gonna be veins in there. Okay, two sphincters. So the internal anal
sphincter is involuntary, and the external anal
sphincter which is voluntary. So it’s gonna work like
the urethra in urination. Alright, so large intestines,
it’s gonna receive about a liter of material
from the small intestines. Mainly what it’s gonna do
is it’s gonna absorb water and sodium and chloride. About 100 milliliters of water
is lost every day as feces. So the large intestines, even though they call
it the large intestines, it’s shorter than the small intestines but the diameter is bigger. Okay, so it’s about five feet long, and two-and-a-half inches in diameter. Okay, and we talked about the cecum. And the appendix hangs off
the cecum and then the valve, the ileocecal valve is gonna control… So it’s gonna control
movement from small intestines to large intestines. And we already talked about
this, we talked about that, and there’s the rectum. Okay, so the histology is way more simple. So the mucosa is jump
simple columnar epithelium. There aren’t any circular folds or villi to increase the surface area, but there are intestinal crypts
and they only produce mucus. So the mucus is there to reduce
friction, it’s a lubricant. Okay, now in the gut,
there’s a whole big thing. And if you take me for micro, we’ll definitely talk a
lot about the microbiome. So the microbiome is a big push to study, so it’s all the microorganisms that actually normally live in your body. So in the large intestines, there are microbes that
are supposed to be there, and they’re beneficial to you. Okay, so they’re gonna
digest complex carbohydrates, they’ll have breakdown proteins, they’ll have breakdown lipids. They do produce carbon
dioxide, hydrogen sulfide, which is like the sulfur
smell, methane gas, and then some other gasses too. One important thing that they do is they’re gonna help
synthesize vitamins B and K. And there’s a whole bunch on this guy, James Craig Venter Institute, jcvi.org, he’s done some research on the microbiome. There’s also a bunch on the NIH website if you’re interested in it. We talk about it in microbiology. Okay, so the rectum is going
to expand to store feces. There are rectal valves to
keep things from leaking out. So that’s to prevent leakage. Now the anal canal, there are… So the anal canal is right in here. There are columns. There’s anal sinuses, the
sinuses actually release mucus, which is gonna be a lubricant. There are veins in here, they’re called the hemorrhoidal veins. They can become hemorrhoids if they… They’re just like varicose
veins of your anus. So the veins become too
weak, the valves weaken, and then the blood pools. And it can be hereditary, or it can be a result of pushing too much. And then the internal anal
sphincter is involuntary, and then the external anal
sphincter is voluntary. The movements in the large intestines, so they do have weak peristalsis, so removing things along
through peristalsis. But we also have this thing
called haustral churning. So what happens, okay so
if these are the haustra, so these are the pouches, okay, what happens is the feces, or
what’s gonna become the feces, it’s going to move from
one haustrum to the next. Okay, so the relaxed haustrum fills, reflex contractions, increases churning, and it just moves from
haustrum to haustrum down the large intestines. Okay, mass movements occur
usually after, or during a meal, and they’re two to three times a day. So it’s more powerful
peristaltic contractions, and it’s the taenia
coli is what’s doing it. Okay, so they begin in the
middle of the transverse colon, and they’re gonna hopefully
stimulate a bowel movement. So it forces the feces to move
across the transverse colon and down the descending colon. Okay, so two major reflexes
from the large intestine. So one is called the gastrocolic reflex. So when the stomach actually distends, okay so when you eat, the
stomach distends, it stretches, and that’s gonna cause movement, okay? And that causes mass movement. So that’s kinda like you eat,
and then you have to poop. I know, so sophisticated. Okay now, the defecation
reflex, here’s what happens. Okay, the rectum fills and it stretches. So that’s gonna start
the urge to defecate. Sensory input goes to the spinal chord, then the parasympathetic nervous system is gonna increase its stimulus to the sigmoid colon and the rectum. The parasympathetic
nervous is going to tell the internal anal sphincter to relax. Okay, so that relaxes. Okay, and then your brain
says is it the time, is it the place, and the cerebral cortex will tell the external
anal sphincter to relax. And then you push down, you bear down by holding your breath and
pushing down on your abdomen, and that’s gonna force the feces out. Okay, so defecation reflex. And I think picture of that… No, I don’t. There’s pictures of that,
it’s not that tricky. Of everything that you’ve learned. Okay, so disorders. So I have colorectal cancer on here. So it’s the second-most common
type of cancer in the US. Most of the time it occurs in
the distal descending colon, the sigmoid colon or the rectum. And mainly because that
has the longest contact with your feces. So the feces has waste in it that could be stressing out the cells, and the cells could mutate. Okay, so polyps. This is a polyp. Okay, a polyp, those are
outgrowths of the mucosa. So if you do a colonoscopy, they’ll go in and they
can remove these polyps, and then they’ll biopsy them. Okay, so risk factors. So maybe not enough fiber in the diet. Definitely family history. Ulcerative colitis. And with anything, the older we get, the more likely that our
cells mutate and are damaged. If the cancer is limited to the mucosa, the five-year survival
rate is really good. However if it’s deeper,
the prognosis is worse. So the key is early detection, so they recommend at age 50 that you start to do fecal occult blood tests, so you’re looking for blood in the feces. They do a sigmoidoscopy and a colonoscopy. So, sigmoid colon, and then the colon. And you guys know this, but I guess we… So the colon and the large
intestines are the same. Okay, they equal the same thing, and they just use different terms. So no big deal. Some other disorders, so diverticulitis, those are just bulges. Diverticulosis, these are
bulges in the intestines, and then when they get
inflamed is diverticulitis. Yeah, so these are like the pouches, and then things can get trapped in there and cause inflammation. And then two other
disorders that are on here is constipation, okay so you can’t poop, and lots of medications cause that. And then diarrhea.

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