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>> If we are what we eat,
then somehow food has to become nutrients.
So how does food become nutrients?
Well that is the story of digestion.
Digestion is the process of breaking
down foods to nutrients.
Now remember though we eat carbohydrates, fats, proteins,
vitamins, minerals and water.
Which of those six need to be broken down?
Well obviously the big ones
and we call those the macronutrients.
The macronutrients are carbohydrates,
fats, and protein.
Vitamins, minerals and water do not have to be digested
so this whole system is all about breaking
down carbohydrates, fats, and proteins.
So here is the gastrointestinal tract or the digestive tract.
It is a flexible, muscular tube that begins
at the mouth and ends at the ***.
It has a space, an inner-space in this tube
which is called the lumen; and you think of it
as a disassembly line-- it is all about disassembly
and there are parts in this disassembly factory that train
to break down certain nutrients and so at the mouth we put
in food, we masticate, we chew it, we have taste buds
on our tongue that helps us decide to put
that food in our mouth.
Some of us put food in and swallow it
in pretty much the same breath,
but ideally if we could use our teeth, chew, rip, tear,
that is mastication of that food, we then move the material
to the back of the mouth, the tongue helps us, it's a muscle,
it pushes it back and then we swallow.
We don't think anymore about that food but when we swallow
that food it now becomes a bolus;
and the bolus is in the pharynx.
So to move the bolus from the pharynx
into the next disassembly point we have
to have a bolus that is liquid.
It is moist and so in the mouth we have salivary glands
and salivary glands are considered accessory organs
for digestion.
Salivary glands will secrete saliva.
It sounds fair enough and that saliva has in it an enzyme
that is going to match and work on a particular nutrient.
The fancy name for that nutrient is amylose.
Anything ending in o-s-e is a sugar
so amylose is what you would call starch.
So if we put starch in the mouth there will be an enzyme
in the saliva that will start
to chemically break down the starch.
We are trying to physically, mechanically,
break down whether it be a starch, a fat or a protein
but nothing chemically for fats or protein.
It's all about starch.
So our bolus makes it past the epiglottis which is covering
up the trachea, that's where air goes and that goes to the lungs.
We don't want food down there; if we do we start coughing
and we're going to choke and we don't want that.
So the bolus is going to move down the esophagus
and the esophagus is basically just the tube or the conduit
that joins the mouth to the stomach.
At the end of the esophagus our bolus will be moved
into the stomach.
The stomach is this J-shaped sack that will be another place
of physical, mechanical digestion.
The walls, the ceiling,
the floor of this little sack are going to come in
and anything in the loom and it's going to be squashed.
But also in the stomach we have the beginning
of protein digestion because the stomach has to be acidic;
and so the acids will really go to town on the protein.
What happens to the starch
and fats well they get physically mechanically broken
down but not chemically really.
So a few hours later what leaves the stomach will be a
semi-liquid paste which is called chyme.
That chyme is going to now move into the bulk
of this digestive tract which is the small intestine.
The small intestine is made up of the duodenum,
the jejunum, and the ileum.
But at the beginning of the small intestine
which is the duodenum we have attachments
to other accessory organs.
We have the common bile duct and the bile duct connects
to the gallbladder, it sits right under the liver,
and that bile duct also has a tube going to the pancreas;
so that pancreatic duct joins up with the bile duct
which then empties into the duodenum.
So we have contributions from the pancreas and the gallbladder
and the gallbladder is sitting right under the liver.
The liver is also considered an accessory organ for digestion
because it makes bile, sends it to the gallbladder,
the gallbladder holds that bile
until chyme goes into the duodenum.
So the duodenum, the jejunum,
that's where digestion really happens.
All of those big macronutrients--
we had a little teaser going on for starch in the mouth
and we had a little teaser going on in the stomach
but now we've got our fats and carbohydrates
and protein really broken down.
And we need a lot of space so that's what we have
with this twisted, curved small intestine.
At the end of the small intestine will be the ileum
and it joins to the large intestine.
The large intestine is large because of its diameter.
The lumen is larger than the small intestine.
From the entrance into the large intestine,
we have the large intestine ascending then it goes
across that's a transverse, then it goes down, descending.
We many times call the large intestine the colon
but the colon at the end
of the descending colon becomes the sigmoid colon and that joins
to the ***, and the last exit point
where we eliminate any material that is of no use
to us whatsoever is the ***.
So we have a digestive tract that starts at the mouth,
goes esophagus, stomach, small intestine made up of duodenum,
jejunum, ileum, to the large intestine, up across, down,
finishing at the *** and then the ***.
What we have to have is some kind of control
of the material going through each part;
and so what we're provided with are sphincters.
Did you notice that there was a lower esophageal sphincter,
there was a pyloric sphincter, there was an ileocecal valve?
Those are control points between esophagus to stomach;
between stomach to small intestine;
between small intestine to large intestine.
So there is control removed
from one disassembly point to the next.