tyer消化系统专业英语

2021年1月19日发(作者：modernism)
Chapter 4

The Digestive System

In this passage you will learn:
●

The digestive system as a whole
●

Anatomy and physiology of the major organs in the digestive system
●

Associated medical terms
Introduction.

The
digestive
system,
also
called
the
gastrointestinal
or
alimentary
canal,
contains the organs involved in the ingestion and processing of food. The primary functions of
the digestive system are: ingestion
—
the entry of food into the body ; digestion
—
the physical and
chemical breakdown of food into nutrients that can be used by the body's cells; absorption
—
the
passage
of
these
nutrients
from
the
gastrointestinal
tract
into
the
bloodstream;
and
elimination
—
the excretion of solid waste materials that cannot be absorbed into the blood.

Anatomy
and
Physiology.
Anatomically,
the
digestive
system
consists
of
a
30-foot
long,
mucous membrane-lined tube beginning with the mouth, where food enters the body, and ending
with the anus, where solid waste is excreted. The digestive system is composed of 9 main organs:
mouth,
pharynx,
esophagus,
stomach,
small
intestine,
large
intestine,
liver,
gallbladder
and
pancreas. The liver, gallbladder, and pancreas are also called accessory organs of the digestive
system. Although food does not pass through these organs, they aid in the processing of food and
nutrients. The organs of the digestive system are illustrated in
Figure 5-1 and described in the
paragraphs below.

Figure 5-1
The oral cavity.
The process of digestion begins in the mouth, also called the buccal cavity,
which
is
the
opening
through
which
food
enters
the
body.
The
mouth
has
the
ability
to
mechanically
reduce
the
size
of
the
food
that
is
placed
in
it.
The
lips
form
the
opening
to
the
mouth, while the cheeks or bucca form the borders of the oral cavity. The structures within the
oral cavity including the teeth, tongue, and palate, are involved in the chewing (mastication) and
swallowing (deglutition)of food. They also play a role in speech.
The
teeth
are
used
to
cut,
tear,
and
crush
food
into
smaller
pieces.
They
are
classified
mainly by their shapes and locations. Incisors and cuspids are in front of the mouth; molars are in
the back. Each tooth consists of a mass of nerves and blood vessels, called pulp, surrounded by a
hard
substance
called
cementum
and
a
white
smooth
substance
called
enamel.
The
teeth
are
embedded in fleshy tissue called gums or gingivae. Together, the gums and other structures that
support the teeth are known as the periodontium. Humans have two sets of teeth, the deciduous
dentition of child-hood , which fall out and are replaced by the permanent dentition of adulthood.
The
tongue

extends
across
the
floor
of
the
oral
cavity
and
is
attached
by
muscles
to
the
lower jaw bone. It manipulates food in the mouth during mastication and deglutition. The tongue
is covered with a series of cone-shaped small projections called papillae in which there are taste
buds that can sense flavors, such as sweet, bitter, salty, and sour.
The palate
forms the roof of the mouth. It is divided into two parts; the hard palate and the
soft
palate.
The
hard
palate
forms
the
anterior
portion
of
the
mouth,
while
the
soft
palate
lies
posterior to it. Hanging from the soft palate is a small tissue called the uvula. The word uvula
means
little
grape.
Around
the
oral
cavity
are
three
pairs
of
salivary
glands.
These
exocrine
glands
produce
a
fluid
called
saliva,
which
is
released,
from
the
parotid
gland,
submandibular
gland and sublingual gland on each side of the mouth.
During swallowing, the soft palate and
uvula move upward to prevent food from entering the nasal cavity, the uvula also helps to guide
the food into the pharynx.
The pharynx,
or throat, is a long muscular tube that serves as a passageway for food from
the
mouth
to
the
esophagus
and
as
a
passageway
for
air
from
nose
to
the
windpipe
(trachea).
When
swallowing
occurs,
a
flap
of
tissue,
the
epiglottis,
covers
the
trachea
so
that
food
can
't
enter and becomes lodged there.
The esophagus
is a long muscular tube extending from the pharynx to the stomach. Food is
propelled by rhythmic contractions of muscles in the wall of the esophagus. This process, called
peristalsis,
is
also
how
food
is
moved
through
the
stomach
and
intestines.
It
is
something
like
squeezing a marble (bolus of food) through a rubber tube.
The stomach,
a pouch-like organ located in the upper part of the abdominal cavity, connects
the esophagus with the small intestine. It is composed of an upper portion called fundus, a middle
section
known
as
the
body,
and
a
lower
portion,
called
the
antrum.
Entry
of
food
from
the
esophagus into the stomach is controlled by a
ring of
muscles known as the cardiac sphincter.
The cardiac sphincter relaxes and contracts to move food from the esophagus into the stomach,
whereas the pyloric sphincter allows food to leave the stomach when it has sufficiently digested.
When
the
stomach
is
empty,
the
mucous
membranes
lining
its
walls
are
highly
folded;
buried
within
these
folds,
or
rugae,
are
numerous
digestive
glands.
As
the
stomach
fills,
the
nigae
unfolded, exposing the digestive glands and stimulating them to secrete digestive enzymes and
hydrochloric acid. These substances help transform food present in the stomach into a semifluid
substance called chyme. The pyloric sphincter allows food to pass into the small intestine only
after it has been transformed into chyme.
The small intestine
(small bowel) is the region of the gut where nearly all of the chemical
digestion of the nutritional components of food takes place. It is a coiled long tube that winds
from the pyloric sphincter of the stomach to the beginning of the large intestine, filling much of
the
abdominal
cavity.
By
convention,
the
small
intestine
is
divided
into
three
sections.
(l)The
duodenum,
which
is
only
a
25cm
section,
receives
chyme
from
the
stomach,
helps
regulate
gastric emptying. (2) The jejunum, the central section, is the primary absorptive region. (3) The
last section, the ileum is attached to the large intestine. In the wall of the entire small intestine are
millions of tiny, microscopic projections called villi. It is through the capillaries in the villi that
completely digested nutrients pass into the bloodstream and lymph vessels. Materials that cannot
be absorbed pass from the small intestine to the large intestine.
The
large
intestine.

Compared
with
the
small
intestine,
the
large
intestine
is
relatively
inactive
and
has
much
less
functions.
Extending
from
the
end
of
the
ileum
to
the
anus,
It
is
divided into four parts: cecum, colon, sigmoid colon and rectum.
The cecum,
a pouch on the right side, is connected to the small intestine by the ileocecal
valve, which controls the passage of fluid waste from the small intestine into the large intestine.
Hanging off from the cecum is the appendix, a small organ with no clear function.
The colon
, which comprised the main length of the large intestine, has three divisions. The
ascending colon, extending from the cecum to the upper abdominal area. The transverse colon
passes
horizontally
to
the
left
toward
the
spleen
and
turns
downward
(splenic
flexure)
to
the
descending
colon.
The
sigmoid
colon,
shaped
like
an
S,
is
at
the
distal
end
of
the
descending
colon
and
leads
into
the
rectum.
As
fluid
waste
from
the
small
intestine
passes
through
the
various sections of the colon, water is reabsorbed into body. As a result, the fluid waste turns into
a solid material known as stool or feces.
The
rectum

serves
as
a
reservoir
for
feces.
It
terminates
in
the
lower
opening
of
the
gastrointestinal
tract,
the
anus,
which
is
the
external
opening
through
which
feces are
released
from the body.
Accessory digestive organs.
Three important accessory organs of the digestive system are
the liver, gallbladder, and pancreas. Although food does not pass through these organs, each plays
an indispensable role in the proper digestion and absorption of nutrients.
The liver,
a large glandular organ located in the upper right quadrant of abdomen, produces
agreenish fluid called bile. Bile contains cholesterol, bile acids and bile pigments. It has a deter-
gent-like effect on fats. It breaks apart large fat globules so that enzymes from the pancreas
can digest the fats. This action is called emulsification. Bile produced in the liver passes through
the hepatic duct to the cystic duct and into the gallbladder. Besides producing bile, the liver has
several
other
vital
functions.
It
manufactures
blood
proteins,
destroys
old
erythrocytes
and
releases bilirubin, removes poisons ( detoxification) from the blood, stores and releases glycogen
needed by the body.
The gallbladder,
a pear-shaped sac behind the lower portion of the liver, stores bile, which
is continuously secreted by the hepatic cells between meals. When the stomach and duodenum
are full, the gallbladder contracts, forcing bile to pass through the cystic duct to the common bile
duct and into the duodenum where it helps in digestion.
The
pancreas,
an
elongated
organ
just
behind
the
stomach,
manufactures
digestive
juice
containing enzymes (amylase and lipase)that aid in the digestion of proteins, starches and fats.
These digestive juices pass into the duodenum via the pancreatic duct. The pancreas also secretes
insulin and glucagon. Those hormones are needed to help release sugar from the blood to be used
for energy by the cells of the body.
Conclusion.
Our
digestive
system
kicks
in
the
minute
food
enters
our
mouths.
Moving
through the digestive tract
———
down the esophagus, into the stomach, ending up in the small
intestine
——
food is chemically broken down into smaller molecules, and nutrients are absorbed
into the body. From here, solid wastes move into the colon, where they remain for a day or two
until they are passed out of the body.
New Words and Phrases