Hi. It's Mr. Andersen and welcome to my podcast on the digestive system. One
of my favorite animals is the flesh fly. The flesh fly sucks in ooze from carrion. It then
creates a little bubble that evaporates so it can leave the material behind. Because
there's so much fluid inside here. And so the flesh fly is an example of a fluid feeder.
And it's kind of a unique way of feeding. We also have suspension feeders. An example
would be like a blue whale taking in a bunch of ocean water, squirting the water out and
then using baleen to to hold the krill behind it. Or a substrate feeder like a caterpillar
that lives within the substrate of a leaf. But most of us are bulk feeders. That means
that we take our food in in bulk and then we have to digest it and then eventually absorb
it into our body. And so I want to start with a picture of a doughnut. The reason why is
it starts you salivating because you might want to eat a doughnut. But it also has a
hole in it. And if you were to stick your finger through the hole in a doughnut, that's
a lot like food moving through the hole that is your digestive system. In other words it's
going to move all the way from your mouth out your anus, but it's not really inside
you. When you stick your finger through the hole of a doughnut it's not technically inside
the doughnut. Just like the food that goes through your digestive system, is not technically
inside your body. It's just moving through the hole in your body. We can eventually absorb
that material inside our body, but not until it gets very small. And so you are what you
eat. What does that mean? Well there are basically four types of macromolecules that make up
life, from carbohydrates to nucleic acids. And so you're going to take food, like this
pizza. You're going to break those polymers down into tiny little monomers and then you're
going to weave that back into you. So when you look at me right now, what you're really
seeing is protein that was in food I ate weeks or months ago. And so a good way to see if
you understand the digestive system is if when we're done with this podcast, I'm going
to show this pizza again, could you review each of these four major macromolecules from
carbohydrates to proteins, lipids, nucleic acids? Where it's digested? And if you have
an understanding where and how it's digested, then you really understand what's going on.
But the whole thing begins with our Pavlovian response. And this was a Russian scientist
who used dogs. He'd measure the amount of saliva they had. There was a bell that he
would ring. But basically he conditioned them so that when he rang the bell they'd start
to salivate. And the same thing works with us. There's a great episode of "The Office"
where Jim gets Dwight to Pavlovian respond to a little ringing of a bell and a mint.
So you may want to watch that. So basically when we see it, we start to salivate. We have
glands, three big glands, that are going to produce saliva. And those are going to empty
into our mouth. Now the saliva has water in it. But it mostly has mucus. And then an enzyme
called amylase. And amylase breaks down starch, amylose starch. And so it's going to breakdown
or start the breakdown of that carbohydrate. And so basically our teeth are there to chew
up that bulk food. Our tongue is there to move it back to the teeth. But eventually
we're going to form a bolus and that bolus is going to move down our esophagus. Now,
right here we have an epiglottis that will close because we don't want our food to go
into our trachea and into our lungs. And so if you swallow, right now you can actually
feel that epiglottis moving as we slide that food down into our esophagus. We have control
up here, but it eventually moves into smooth muscle. Eventually ends up in the stomach.
So what's going on in the stomach? Well we'll have a sphincter right here which closes it
off. And a sphincter right here. And so the food will sit inside our stomach. We're going
to break it down chemically and mechanically. There are a series of muscles that go this
way. Muscles that go this way. And muscles that go this way. And basically they're going
to churn up your food. And if there's nothing inside there you can hear that, your stomach
growling as it tries to grind up nothing. So basically that's mechanical digestion and
it will sit there for minutes if not hours grinding up the food. But we also have chemical
digestion here. And there are two cells that you should remember. Those are the chief cells
and the parietal cells. Basically they're going to be cells in the lining of the stomach.
We also produce mucus because we don't want to digest the stomach itself. So there's going
to be mucus here. But the two things that are produced are hydrochloric acid. Those
are going to be produced by the parietal cells. So hydrochloric acid is going to move out
here. The function of that isn't to digest the food, it is to create an acidic environment
where the other chemical, which is called pepsin can start to breakdown proteins. And
so protein digestion starts to occur right here in the stomach as we start to unravel
those big proteins into single strands of amino acids that we can eventually breakdown
in the duodenum. Okay. So that's the stomach. Next we move into the duodenum. The duodenum
is going to be this first portion of the small intestine. There are a few important things
that connect here. The first one is going to be the gall bladder. And then we're going
to have this bile duct that will empty down into the duodenum. What does that contain?
It contains bile salts. Bile salts are important because if you've ever tried to mix fat with
water or oil with water, since it's hydrophobic you can't break it down. And so the nice thing
about these bile salts is that they will actually surround the lipids. Make them much smaller
so they emulsify the fat. And so we can start breaking that down. Okay. So that's going
to be important. Next we have a bunch of enzymes that come, oops, let me go back, a bunch of
enzymes that come out of the pancreas. So the pancreas is important because remember
it can regulate blood sugar. But it's also producing, it's a gland, it's producing all
of these enzymes. And so lipases are going to come into the duodenum and those are going
to breakdown lipids. Pancreatic amylase is going to be important because it's going to
breakdown carbohydrates. We also have trypsin and chymotripsin. Those are going to breakdown
different types of amino acids. Or breaks between different amino acids so we can breakdown
those proteins into the building blocks. We also have nucleases that are going to be produced
in the pancreas as well. And so we have tons of enzymes that are produced by the pancreas
and they're going to breakdown the different bulk food as it comes into our body. Next
we move and continue down the small intestine to the jejunum and the ileum. And so if we
kind of follow, this would be the duodenum. And now if we follow food it's going to kind
of move almost like a maze all the way through our small intestine. This first section is
called the jejunum. Basically what happens in here is we're going to finish digesting
the food and then we're going to start absorbing the food. How does that work? Well if these
are the lining of the small intestine, we have these villi and microvilli. There are
going to be capillaries that move in here and basically move out and they're going to
take the nutrients into our body. How do we get those nutrients into our body? Well some
of it's diffusion but a lot of it is active transport actively moving that material in.
And so by the time we get down to the end of that ileum we've pretty much digested and
absorbed most of the food that we need to take in. In fact we're not going to do much
more digestion of those monomers as we move into the large intestine. So what's it do?
Well it's called the colon. It's got three different types to it. It's got the ascending,
transverse and descending colon. But basically as our waste moves through the colon or the
large intestine, we're reclaiming water from our waste. And also there are a bunch of bacteria
that live in here and they can release vitamins from our food so we can get that as well.
And it's important that we have that there. This is one thing that I didn't mention. This
is the appendix. The appendix is actually a vestigial structure. It has a bunch of bacteria
in it. Sometimes it becomes inflamed and that's a big deal because if we release those bacteria
into our body we're in trouble. It doesn't really do anything. But if we look at certain
animals like a koala bear, for example, their appendix or their secum is really large kind
of a thing that looks like this. And so basically what happens is leaves will get pushed down
in here. And bacteria are going to help them break that down. But since we don't eat a
lot of that it's not required. And so let's see how well you did. Could you do each of
these. Where were carbohydrates broken down? Do you remember? What about proteins? What
about lipids? What about nucleic acids? Well let's start with carbohydrates. Carbohydrate
digestion begins in the mouth because we've got amylase in the mouth. But it also is in
the duodenum. Because we have pancreatic amylase. What about proteins? Proteins start in the
stomach, not in the mouth. But in here we're going to have pepsin remember? Hydrochloric
acid to lower the pH. And as we move out here it's going to be trypsin and chimotrypsin
that are going to help break that down. What about lipases or excuse me lipids? Lipids,
there are two things that you should remember. We've got the gall bladder. The gall bladder
is going to give off bile salts to emulsify the lipids. And then we're going to have lipases
that come out of the pancreas. And then finally we have nucleases that are going to breakdown
those nucleic acid. So basically we then reabsorb all these nutrients in through the small intestine.
All of those vessels lead to the liver where we figure out what we want to do with all
those building blocks of life. But eventually we weave it back into our cells. In other
words, where did you get your DNA? Where did you get your sugars? Where did you get your
proteins? It's in your food. And now you've taken all that broken down in the small monomers
and then we can weave it back into the polymers that make us the way we are. So that's your
digestive system and I hope that's helpful.