652
UNIT FIVE
17.1
INTRODUCTION
Digestion
(di-jest
yun) is the mechanical and chemical
breakdown of foods into forms that cell membranes can
absorb.
Mechanical digestion
breaks large pieces into smaller
ones without altering their chemical composition.
Chemical
digestion
breaks food into simpler chemicals. The organs of
the
digestive system
carry out these processes, as well as
ingestion, propulsion, absorption, and defecation.
The digestive system consists of the
alimentary canal
(al
i-men
tar-e kah-nal
), extending from the mouth to the
anus, and several accessory organs, which release secretions
into the canal. The alimentary canal includes the mouth,
pharynx, esophagus, stomach, small intestine, large intes-
tine, and anal canal. The accessory organs include the sali-
vary glands, liver, gallbladder, and pancreas.
Figure 17.1
and reference plates 4, 5, and 6 show the major organs of the
digestive system.
The digestive system originates from the inner layer
(endoderm) of the embryo, which folds to form the tube of
THE GUT MICROBIOME
get our microbial residents. Also, we can add bacteria to foods to prevent cer-
tain infections, an approach called probiotics. For example, certain
Lactobacillus
strains added to yogurt can protect against
Salmonella
foodborne infection.
The numbers and types of microorganisms that live in our intestines vary
somewhat from person to person, and these di±
erences may be one reason why
some people can eat a great deal and not gain weight, yet others gain weight
easily. Studies show that an item of food may yield di±
erent numbers of calo-
ries when eaten by di±
erent people. One investigation of the energy in a cookie
found that even though the package listed 110 calories, it yielded anywhere
from 90 to 110 calories, depending upon who ate it.
N
inety percent of the cells in an adult body, if the digestive
system is included, are not human—they are microorgan-
isms collectively called micro²
ora. Terms from ecology are
used to describe the microbes within us, including commu-
nity, ecosystem, and biome. The Human Oral Microbiome
Database (www.homd.org), for example, lists more than 600 species that live
in the mouth. Each person has about 200 of the oral bacterial types.
To assess the microbial biome at the other end of the digestive tract,
researchers analyzed DNA fragments in stool samples from a twenty-eight-
year-old woman and a thirty-seven-year-old man, both of whom had not
taken medications that could have a±
ected the micro²
ora. By comparing the
DNA pieces to those of known microorganisms, the researchers discovered
that the “distal gut microbial community” includes more than 6,800 species.
Researchers also tracked the formation and changing nature of the
human gut micro²
ora by classifying bacterial DNA in a year’s worth of stool col-
lected daily from soiled diapers. Bacteria in the stool varied greatly from baby
to baby at the outset, but by their ³
rst birthdays, the gut communities were
more alike and more closely resembled the microbial communities in adults.
Each person’s large intestine comes to support 500 to 1,000 bacterial species.
The microorganisms that live in our large intestines are crucial to our
health. They produce more than eighty types of enzymes that digest plant
polysaccharides that our bodies cannot break down, as well as easing pro-
cessing of certain sugars. Our “gut” residents also synthesize essential vita-
mins and amino acids and break down certain toxins and drugs.
We can use knowledge of our gut microbiome to improve health. For
example, unusual bacterial communities can re²
ect disease. Speci³
c micro-
²
oral pro³
les are associated with colorectal cancer, diarrhea, in²
ammatory
bowel disease, and peptic ulcers. A new focus of drug development is to tar-
Several million microorganisms are normal residents of our digestive tracts.
Escherichia coli,
pictured here (6,800×), produce vitamin K and if present in low
numbers, will not cause diarrhea.
the alimentary canal. The accessory organs develop as buds
from the tube.
PRACTICE
1
What are the general functions of the digestive system?
2
Which organs constitute the digestive system?
17.2
GENERAL CHARACTERISTICS
OF THE ALIMENTARY CANAL
The alimentary canal is a muscular tube about 8 meters long
that passes through the body’s thoracic and abdominopelvic
cavities
(f
g. 17.2)
. The structure of its wall, how it moves
food, and its innervation are similar throughout its length.
Structure of the Wall
The wall of the alimentary canal consists of four distinct
layers developed to different degrees from region to region.
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