If you’re new to the channel, welcome, my name is Patrick and this channel is all about
anatomy and how we learn about it.
As always, notes for this lesson are linked in the description if you want to check those
out, otherwise let’s get started.
When you’re first learning digestion, I recommend separating it into two parts: mechanical
digestion which is the physical grinding and processing of food into smaller bits, and
chemical digestion, all the enzymes and acids which break foods down further so we can get
them ready to pass into the bloodstream.
But there’s a caveat here: digestion is only the process that breaks down food, while
the products of digestion undergo absorption to enter the blood and lymph.
That’s why we emphasize digestive histology so much.
Chemical digestion is microscopic, so our anatomy of interest is microscopic as well.
That being said, let’s start in the oral cavity.
You already knew that the mouth was a big deal for mechanical digestion because chewing.
But our saliva does a fair amount of chemical digestion too.
You’ve got a handful of major salivary glands — one is sublingual, under your tongue,
one is submandibular, under your jaw, and one is the parotid, it pumps in saliva from
the side of your face.
At the microscopic level, you might notice some familiar cuboidal epithelium and these
things are prime for secreting enzymes.
In particular, salivary amylase and lingual lipase.
And pro tip, any time you see the suffix -ase at the end of a word, it’s an enzyme, a
protein that makes a chemical reaction happen faster.
In this case, they’re helping to break bonds between complex nutrients.
And the prefix will tell you what it breaks down.
Lipase breaks down lipids while Amylase breaks down complex carbs into simpler carbs.
At this point in digestion, the chewed up lump of what used to be food is called a bolus
— cuz unless you’re about to baby bird that thing into someone else’s mouth, it’s
going down the hatch.
Pardon my manners, I’m supposed to be a respectable anatomist.
Down the esophagus.
It’s a thick muscular tube lined with a bunch of stratified squamous cells, which
makes sense.
It doesn’t break anything down, mechanically or chemically.
It just moves the bolus to the stomach with a ripple pattern called peristalsis — peri
for around, stalsis for squeeze
Now, the tissue level anatomy of the stomach is important because it does both mechanical
and chemical digestion.
The stomach is coated in smooth muscle, the type of muscle organized into big sheets which
lets it squeeze together.
In the stomach, this squeezes the bolus towards the pylorus, the tiny hole that leads from
the stomach into the small intestine.
Then this piece of the stomach, the antrum keeps those ripply contractions coming, grinding
the bolus against the pylorus which is just clamped shut.
This grinding process breaks down food until it’s no larger than two millimeters.
Only then can it pass through the pylorus into the duodenum, the first part of the small
intestine.
But the entire bolus isn’t immediately small enough to pass through.
So your stomach will move the bolus away from the pylorus, and keep grinding and chemically
digesting it until it’s small enough to pass.
And your stomach mucosa, or mucous membrane, can throw a lot of chemical digestion at that
bolus.
Most of the epithelial cells are simple columnar cells, but there are also these little gastric
pits that have a few secreting glands — oxyntic glands and pyloric glands and they both work
together.
Oxyntic glands are made of two cell types: parietal cells which pump out super concentrated
hydrochloric acid and chief cells which make a precursor to an important enzyme.
Not the enzyme itself, but the thing that will become an enzyme.
What I love about these two cells is how they actually use each other to work.
Like, hydrochloric acid is powerful.
It denatures or unfolds some of the protein we eat, but not all the way.
Fortunately, that acid also helps turn pepsinogen, the precursor secreted by the chief cells,
into the active enzyme pepsin.
Then pepsin, in this acidic environment, can further break down the peptide bonds between
proteins to more completely digest them.
And as great as this acid is for breaking down proteins, we’ve still got this delicate
stomach mucosa that we don’t want to burn holes into.
So the other type of glands, the pyloric glands, have cells that secrete a mucous rich with
bicarbonate, a more basic chemical that coats the gastric mucosa to protect it.
Those glands also have G-cells that secrete gastrin, which is a hormone that stimulates
the parietal cells to make more hydrochloric acid when needed.
So by the time the bolus is done in the stomach, it’s more of a soupy acidic, partially digested
mess, and at this point we’d call it chyme.
And it’s about to see some serious chemical digestion in the small intestine.
After chyme passes through the pylorus it hits the first section of the small intestine
called the duodenum.
I imagine this thing like a little witch’s cauldron — it’s basically a mixing pot
for stomach contents and enzymes, including more amylase and lipase, but also enzyme precursors
like trypsinogen and proelastase sent from the pancreas.
There’s an important reason why the pancreas sends precursors and not fully formed enzymes.
If the pancreas secreted active enzymes, it would digest itself, which is what happens
during pancreatitis.
Instead, it makes those precursors, and ships them to the duodenum where they mix with another
enzyme called enterokinase — entero- for intestine, -kine- for movement, -ase for enzyme.
It’s an intestinal enzyme that kickstarts other enzymes.
First it converts trypsinogen into trypsin.
Then trypsin can convert chymotrypsinogen into chymotrypsin, procarboxypeptidase into
carboxypeptidase, proelastase into elastase.
For all of you noticing the pattern, yes, just drop the -ogen or pro- and you can figure
out the name for the active enzyme.
And a lot of these enzymes further break down the peptide bonds in proteins.
But there are more than just pancreatic enzymes in there.
The epithelial cells of the duodenum also secrete a dipeptidase, an enzyme that breaks
down peptide bonds in proteins, and multiple disaccharidases, enzymes that break down compound
sugars into simple sugars.
Like maltase is the enzyme that breaks down maltose into two glucoses, but we’ve also
got lactase for breaking down lactose, and sucrase for sucrose.
I’ve said it before but I’ll say it again.
Physiologists are the worst at naming things.
But you’ve still got a problem.
You know how water and oil don’t mix?
Well, the lipids, or fats from your food aren’t mixing with the rest of that soupy, squishy
chyme.
So your liver and gallbladder secrete a substance called bile into the duodenum.
It’s a mixture of cholesterol, fatty acids, electrolytes, bile salts, and bilirubin that
all act together to emulsify certain lipid particles into the liquid.
We need to do that because some of our enzymes like pancreatic lipase, work better if they
can access more lipid surface area.
At this point, you’ve digested food about as far as you can.
Those big proteins have been broken into amino acids and oligopeptides, and complex carbs
into simple sugars.
Now we can focus on absorption and excretion.
The point here is to allow diffusion of nutrients through a really thin cellular layer made
mostly of enterocytes, literally intestine cells.
But there are also the awesomely named crypt cells that supply the small intestine with
stem cells and secrete chloride.
To help with that diffusion, the small intestine is covered in all these wiggly projections
called villi that increase surface area available to absorb nutrients.
Each of them are about half to one and half millimeters long and covered in columnar epithelial
cells.
Then those things have even tinier /micro/villi on them that increase the surface area by
a factor of 600 and more surface area allows more diffusion of nutrients from the small
intestine.
I’m going to keep this video focused on histology, but I’ll dedicate a video to
the absorption of different nutrients in the future.
For now, suffice to say that all of the nutrients depend on electrolytes and diffusing through
that columnar epithelium to enter the bloodstream.
Ohh, and 80% of your water is absorbed in this step too.
By the time we’ve absorbed nutrients through the small intestine, we exit through a section
called the ileum.
For those familiar with skeletal anatomy, I’m sorry.
It’s pronounced the same as your pelvic bone but spelled differently.
Anatomists suck.
From the ileum, we enter the large intestine where we absorb the rest of the water and
get this thing ready to ship out.
I said ship.
With a P. Like for poop.
Overall, it’s shorter intestine and has a much larger lumen.
It has this segmented look because of these pouch-like structures called haustra, which
on the inside, are separated by semilunar folds.
But we need some quick nomenclature since the large intestine has so many distinct sections.
The first chamber is the cecum — which I remember because, when it comes to contents
from the small intestine, this is the first section to see it coming.
Look, I know that’s a borderline dirty joke, but we’re almost at the anus so anything
goes.
Hanging off the cecum is the appendix, which houses a bunch of good gut bacteria.
Then there’s the main section of the colon which gets divided into the ascending, transverse,
and descending colons, as well as the sigmoid colon which connects to the rectum subtext
I hardly knew him, and finally the anus.
That’s the big picture level, but what about the tissue level?
Well, it’s still lined with simple columnar epithelium with microvilli, and has a similar
lamina propria as the small intestine, but with a few unique features of its own.
The colon has crypts, but no villi, it doesn’t secrete any digestive enzymes but does have
a bunch of goblet cells which secrete a protective mucus.
Which it uses to protect itself from all the gut bacteria hanging out in the lumen.
If you move out from the lumen, you’ll see a much thicker muscular layer too, because
that’s the muscle that moves feces along the colon.
Eventually this transitions into the thick sphincters around the anus.
One of which is involuntary and almost always clamped shut, while one is under voluntary
control, holding back our poops.
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Have fun, be good, thanks for watching.