Visible Body | 3D Tour of the Digestive System

- [Laura] Laura here.

Today we're going to take a
tour of the digestive system

using Human Anatomy Atlas.

The digestion process begins

as soon as food enters the oral cavity.

The secretions of the
parotid, submandibular,

and sublingual salivary glands

start breaking down food
at a chemical level.

Saliva contains an enzyme called amylase

which kicks off the process

of breaking starches into simpler sugars.

The teeth and tongue work together

with the muscles of the jaw

to break the food down
mechanically into tiny pieces.

Once it's ready to be swallowed,

ingested food is referred to as a bolus.

The tongue begins the swallowing process

by pushing the bolus along the hard palate

and towards the oropharynx,
that is the back of the throat.

The involuntary pharyngeal
phase of swallowing begins

when the oropharynx,
soft palate, and uvula

close off the nasopharynx.

The transition of the bolus

from the pharynx to the
esophagus only takes a second.

The epiglottis closes off the trachea

to prevent food from
falling into the wrong tube.

The bolus is pushed down the esophagus

by the involuntary muscle
contractions of peristalsis.

It enters the stomach through
the cardiac sphincter.

The stomach consists of
several layers of smooth muscle

in addition to its
outer layer, the serosa,

and its innermost epithelial
layer, the mucosa.

Its interior is extremely
acidic and contains pepsin

which helps break down proteins.

After being processed in the stomach,

the bolus becomes a
substance known as chyme.

Chyme passes through the pyloric sphincter

and into the duodenum,

the first portion of the small intestine.

This is when more of
the digestive system's

accessory organs join the party.

The liver produces bile

which aids in the breakdown of lipids.

The gallbladder stores
and concentrates the bile

created by the liver.

Bile from both the liver and
gallbladder enters the duodenum

through the common bile
duct and duodenal papilla.

Endocrine cells in the
pancreas produce insulin

which helps regulate blood glucose levels,

and glucagon, which
helps the liver convert

glycogen to glucose.

Exocrine cells in the pancreas
produce pancreatic juice

which is full of enzymes
that help break down

proteins, carbohydrates, and fats.

Pancreatic juice enters
the common bile duct

through the pancreatic duct.

Chyme spends about 12 hours
in the small intestine,

moving from the duodenum to
the jejunum to the ileum.

The bulk of nutrient absorption
occurs during this time

aided by finger-like
structures called villi

that line the small intestines.

Lymphatic vessels called
lacteals absorb dietary fats.

Blood vessels in the walls
of the small intestines

carry absorbed nutrients

such as amino acids, sugars, and vitamins,

up to the liver through
the hepatic portal vein.

Once the chyme's time in
the small intestine is done,

it moves into the cecum,

the first part of the large intestine.

Just off of the cecum, is a
structure called the appendix.

Though humans can survive
without an appendix

and it usually is removed
if it becomes infected,

it does serve as a
reservoir for gut bacteria.

The remainder of the absorption

of water, electrolytes,
and vitamins occurs

as food moves through the
large intestine/colon.

The large intestine contains gut bacteria

that serve important digestive
and immune functions.

After the cecum, the food moves

through the ascending
colon, transverse colon,

descending colon, and sigmoid colon.

The tenia coli are bands of smooth muscle

that run along the
different parts of the colon

and help move material through
the colon to the rectum.

Along the path from one
end of the large intestine

to the other,

unabsorbed material is converted to feces.

When feces enters the rectum,
it stretches the tissue there,

triggering the defecation reflex.

The relaxing of the
internal anal sphincter

allows feces to move into the anal canal.

The voluntary relaxation

of the internal and
external anal sphincters

allows feces to leave the anus.

And that's the path of a piece of food

from ingestion to elimination.

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