Download called peristalsis. Although

LEssoN
2 MovrNc THRoucH THE DIGESTTvB TRect
The human digestive
tract (see Frgure 2.1) is
a sin$e, convoluted tube
that is about B to 10
meters (m) long. Its walls
are composed of an inner
layer of circular muscle
that is surrounded by a
Food
enters mouth
Pharynx
(back
9
Esophagus. The journey from
of mouth)
layer of longitudinal muscle.
the mouth to the stomach
takes approximately 6 seconds.
(The stomach has a third
layer composed of diagonal
muscle.) These muscular
Within 2 hours, the pyloric
layers are held together
sphincter opens to release
by eonneetive tissue and
chyme, a little at a time,
The stomach fills as food is
are covered and protected
into the small intestine.
swallowed. Food may remain in
the
stomach up to 4% hours.
by epithelial tissue.
The circular and longiWithin 4 to 12 hours, digested
tudinal layers of muscle
nutrients are being absorbed
work together to produce
into the bloodstream.
wavelike motions that
push food slowly through
Within 18 hours,
the digestive ttact and
excess water passes
back into the blooG
help break it down into
stream, leaving solid
smaller and smaller partiwaste (feces).
cles. This wavelike motion is
called peristalsis. Although
the force of glavity has a
minor role in the passa$e of
food throu$r the digestive tract,
peristalsis provides the major push.
The peristaltic contractions are so
strong that food would even continue
Appendix
to move forward through your diEiestive
tractrd you were standing on your head!
The solid waste usually
Another muscular activity that is essential
takes between t2 and 24
provided
to digestion is
by the sphincters.
hours to move through the
These ringB of thickened muscles facilitate the
large intestine and to be
eliminated through the anus.
passage of food from one area of. the digestive
tract to another. They usually ensure that food
continues to move in one direction. Sphincter
Frgure 2.1 Simplified illustration of the human digestive
muscles are found at the openings of the drgestract and the digestive processes
tive system, that is, at the lips and anus. Other
sphincter muscles are located at points where
two digestive orf,ans meet (for example,
between the esophagus and the stomach).
U2
STCA,lSru HuMAN Boov Svsrprras