Digestion is the process by which
the gastrointestinal system
retrieves important nutrients for
the body, and chemically changes
the unused food into waste.
Mastication, or the chewing of food
in the mouth, is the first step
of digestion.
Saliva initiates digestion, and changes
the chewed food into a soft mass
or bolus.
Saliva makes the bolus slippery
making it easier to be swallowed and slide
down the back of the throat and esophagus.
The bolus passes through the
esophageal sphincter before it enters the
stomach.
Inside the stomach, hydrochloric acid is
released.
Breaking down large food molecules into
smaller ones, and liquifying the bolus.
The liquified bolus, now called chyme
then passes through the pyloric sphincter
and enters the duodenum,
the first section of the small intestine.
It is here that enzymes released from the
pancrease, liver, and gall bladder
further break down chyme into elements
that can be easily absorbed and used
by the body.
The small intestine is lined with a
heavily folded inter mucosa, and small
finger-like projections called villi.
The villi enable digested food to enter
the blood stream.
It is here in the small intestine
where all nutrients and vitamins are absorbed.
Chyme can travel through up to
20 feet of small intestine
before it passes through the ileocecal valve
to enter the large intestine.
Very little digestion occurs in the large
intestine.
Undigested chyme that enters the large
intestine is considered waste.
The waste becomes more and more solid
as it passes through the large intestine
because water is continuously being
re-absorbed from the waste.
Waste collects in the rectum, or
end of the large intestine, until the brain
signals for it to be expelled from the body.