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Acid Reflux causes and Treatment

Digestive Functions Overview

Digestive Functions Overview


– In this video, we’ll
start to introduce what the digestive system does
and a little bit about how it does it. The digestive system basically
is all about getting nutrients out of food and drink. But of course, it’s a little
bit more complicated than that. There are really six
activities involved in the process of getting
nutrients out of your food or whatever you drink. Ingestion is probably
the most basic one. Ingestion is basically
taking food into the body and that is what
happens at the mouth. Ingestion is pretty
straightforward. You bite off a piece
of food and take it in. Propulsion is the movement
of food through the GI tract. So you have to get that
food from the mouth all the way through the
esophagus, and stomach, and small intestine, and
large intestine to the anus. And propulsion
involves two processes. First, you swallow, so your
tongue moves that bolus of food to the back of your mouth. And you activate
a reflex that has you squeezing and contracting
some skeletal muscle and swallowing
that piece of food. But about a third of the
way down the esophagus, you have an involuntary means
of propulsion taking over. And that is peristalsis. Peristalsis refers to waves
of contraction and relaxation that squeeze food
through the GI tract. Imagine squeezing a tennis
ball through a stocking. That squeezing motion that
you make with your hands to move the tennis
ball through, that’s what your GI tract does
to move food through. And you see
peristalsis happening in the lower esophagus,
in the stomach, and in the small
and large intestine. In the large
intestine, you actually have big waves of
peristalsis that occur a few times a day,
moving big boluses of food through the large intestine. Or, by that point, it’s
mostly waste products. Let me show you what a
peristaltic wave looks like in the stomach. [MUSIC PLAYING] Do you see it? This is that wave
of contraction. [MUSIC PLAYING] You may have heard the
term, gastric motility. Digestive motility,
or gastric motility, is the propulsion of food
through the digestive tract. With age, this function
of the digestive system actually declines, in part due
to a decrease in smooth muscle tone. Peristalsis gets weaker. And the gastroesophageal
sphincter gets weaker. So with the decreased
peristalsis, you are more likely to develop
constipation as you get older. And with a weakened
gastroesophageal sphincter, you’re more likely to develop
acid reflux as you get older. The next couple of activities
that the digestive system does are all related to breaking
food down into nutrients that your body can actually use. So if you think about
eating an apple, taking a bite of an apple, can
one of the cells in your body take up that apple? Can one of the
cells in your body even take up a single
bite from that apple? No. So you have to basically break
that bite down into molecules, into individual molecules. And your digestive
system does this through both
mechanical means, where you’re just physically chopping
up or mixing that food, and through chemical
means, where you actually have digestive
enzymes breaking apart large molecules in that
food into smaller molecules. Mechanical processing
includes chewing. When you take a bite of food and
you chew it up with your teeth, you’re basically chopping it and
mushing it into smaller pieces. It also occurs– what happens
to your food in the stomach. The stomach is a
very muscular organ. It actually has three
layers of muscle around it. And when food gets
into the stomach, the stomach just
goes to town on it. It’s beating it up. It’s pummeling it. It’s pinching it. It’s pulling it apart. So this churning in the stomach
helps break a big food mass apart into smaller
masses, as well. Segmentation is also a form
of mechanical processing. And segmentation is just
basically mixing food with enzymes. So in a way, this is
preparatory to starting the chemical digestion process. So it’s rhythmic constrictions
of non-adjacent segments. So first, this segment squeezes. And then, this segment squeezes. And in the process,
you get the food and the enzymes mixed together. And you see segmentation
in both the small intestine and the large intestine. It also takes that
partially-digested food product and moves it along
the intestinal wall, so that absorption is easier. And the large
intestine segmentation is referred to as haustral
churning, the haustral segments of the large intestine. LI for large intestine. So once that big food
bite has been broken down into smaller segments,
the digestive enzymes can access the
large molecules in those little partially-digested
food segments. And enzymes are proteins
that act as catalysts. So they basically make chemical
reactions happen more quickly. And the digestive enzymes, the
chemical reaction that they do, is called hydrolysis, which
is where you use water to break apart a bigger molecule
into two smaller molecules. Just say you have molecule A
joined covalently to molecule B, you add some water– remember, water is H2O, but
it can be written as HOH– and that turns these
into A-OH, so you get a hydroxy group added to
A and the hydrogen added to B. That’s what hydrolysis is. And that’s what
digestive enzymes do. So this way, they can take
apart a whole protein chain into individual– break it down
into individual amino acids by hydrolysis. And for a quick visual
reference to what enzymes do, remember that enzymes are very
specific for their reactions. So if you want to break
apart the sugar maltose, you need an enzyme
called maltase, which can break maltose apart
into two glucose molecules. But if you want to take a
different sugar, fructose– sorry, a different
sugar, sucrose, and break that down into
glucose and fructose, you need to use a different
enzyme, called sucrase. So enzymes are very specific. And the digestive
enzymes, ones that are active in your
digestive tract, they’re produced by
the salivary glands, they are produced by
the small intestine, they’re produced in the stomach. The stomach produces
a protease, an enzyme that chops up proteins. And they’re also
produced by the pancreas. So in the small intestine, you
actually have digestive enzymes from both the pancreas and
the small intestine, itself, acting on your food product. Once food has been broken down
through both mechanical and chemical means
into nutrients, you have absorption taking place. Absorption is the
uptake of nutrients. So those nutrients in
your digestive tract are a good start,
but they really don’t do your body
any good until they get into circulation. So they have to get across the
cells of the digestive tract into either the
blood or the lymph, and then, eventually,
back into the blood. This is kind of a cartoon
schematic of what’s going on in absorption. You have the lumen of
the small intestine here, because the
small intestine is where the vast majority
of absorption takes place. You have digestion
going on in the lumen. Large molecules getting broken
down into smaller nutrients. Those nutrients broken
up into smaller nutrients, those nutrients getting taken
up by the epithelial cells that line the lumen of
the digestive tract, across those epithelial
cells, and then down here into the blood. Most nutrients are taken
up directly by the blood. Large fat molecules,
because they’re hydrophobic and they’re packaged in a
way that they don’t actually fit in the blood, so they’re
taken up into the lymph. The last activity of
the digestive system, the last activity on
the disassembly line, is defecation. Defecation is just
getting rid of the stuff that we couldn’t
use, elimination of solid, indigestible waste. And this includes– a
small part of the feces that you excrete are fiber from
food that your body couldn’t break down and that the
bacteria in your large intestine couldn’t break down. But you also actually shed
a number of epithelial cells in the digestive system and
a number of bacterial cells, actually. It’s been estimated
that the majority of the weight of your feces is
actually bacterial cells shed from the large intestine. But it’s all gotten
rid of at the anus. This diagram summarizes
where all of these activities actually occur. So you’ve got ingestion
happening up at the mouth. You have propulsion with
swallowing in the oropharynx and upper esophagus. Peristalsis taking
over in the esophagus, stomach, small intestine,
ans large intestine. Mechanical digestion,
chewing in the mouth, churning in the
stomach, segmentation in the small intestine,
haustral churning in the large intestine. Absorption happening mostly
in the small intestine. Nutrients and water
taken up, some water is absorbed in the
large intestine. The majority of the
water is absorbed in the small
intestine, but you get most of the rest of it taken
up in the large intestine. Chemical digestion
is wherever you see these little green drops,
but I’m going to add something, because this diagram, it shows
chemical digestion happening in the stomach, in
the small intestine, and a little bit in the
large intestine, where you have bacteria doing
some chemical digestion. So it’s not your
own enzymes, it’s the bacterial enzymes
breaking food down. But I’m going to add a
drop up in the mouth, because saliva is where chemical
digestion actually begins. Lastly, of course, you have
the final activity, defecation. OK, so after
studying this video, it’s just a beginning to
talking about the physiology of the digestive system. But you should be able
to talk a little bit about the different steps, the
different activities involved in going from ingested food
to nutrients circulating in the blood. You should be able to talk about
peristalsis and segmentation, talk about mechanical processing
and chemical digestion. Sometimes mechanical
processing is referred to as mechanical digestion. You should be able
to talk about where mechanical digestion
and chemical digestion and where absorption– where these all take place.

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