Download The Human Digestive Tract

S E C T I O N
10.2
The Human Digestive Tract
pharynx
oral cavity
E X P E C TAT I O N S
List, in order, the structures
through which food passes in
the human digestive tract.
salivary
glands
Distinguish clearly between
the structure of the stomach
and the small intestine.
Describe how the digestive
system helps maintain our
internal environment.
Figure 10.10 A view of the
human digestive tract and its
associated organs.
parotid
sublingual
tongue
submandibular
esophagus
diaphragm
liver
gallbladder
duodenum
transverse colon
ascending colon
caecum
appendix
anus
Within mammals, there is considerable variation
in digestive system details. There is even more
variation among invertebrate digestive systems.
The human digestive tract will serve, however, to
point out the essential features characteristic of an
open tube arrangement (see Figure 10.9). On average,
it takes about 24–33 h for each meal you eat to
complete its passage through your digestive tract.
stomach
pancreas
pancreatic duct
common bile duct
small intestine
descending colon
sigmoid colon
rectum
anal canal
moistens or lubricates food so it will pass into the
next part of the digestive system more readily.
hard palate
soft palate
Parts of the Human Digestive Tract
uvula
The Mouth
In mammals, the mouth is equipped with a number
of teeth arranged along the upper and lower jaws.
The teeth vary in number and structure, depending
on species. The structure, number, and arrangement
of teeth in the human mouth is shown in Figure
10.11, as are details of the structure of the mouth
itself. Not evident from the diagram is that the
upper surface of the tongue is covered with tiny
pimple-like structures called papillae. The papillae
house most of the taste buds that allow us to tell
whether our food is sweet, sour, bitter, salty, or
some combination thereof. The uvula hanging from
the middle of the back edge of the soft palate
prevents food from entering the pharynx when we
swallow.
Upon entering the mouth, food quickly comes in
contact with saliva, which is secreted by three
pairs of salivary glands that assist in the chemical
process of digestion (Figure 10.10). Saliva also
tonsil
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MHR • Internal Systems and Regulation
molars (3)
premolars (2)
canine (1)
incisors (2)
Figure 10.11 This illustration of the human mouth shows
the number, type, and arrangement of the teeth, plus other
details.
The two parotid glands, which are located
slightly below and in front of the two ears, are the
largest of the salivary glands. The smallest of the
salivary glands, called the sublingual glands, are in
the floor of the mouth just inside of the incisor
teeth. Slightly below and behind the sublinguals
are the third and final pair of salivary glands,
known as the submaxillary glands. In all cases, the
glands open up into the mouth cavity by means of
ducts — tubular canals for carrying glandular
secretions from one part of the body to another.
The Esophagus or Gullet
After leaving the mouth, the food passes into a tube
called the esophagus, passing the covered opening
of the trachea or windpipe on the way. If you place
your fingers over your “Adam’s apple” and
swallow, you will notice that both it and your
trachea move up. This movement closes the trachea
against the covering called the epiglottis. This
action seals off the glottis in order to prevent food
from entering the trachea (Figure 10.12).
The esophagus is lined with circular and
longitudinal muscles along its length, which is
about 24 cm. These muscles work together to push
the food along. Mucin, a lubricant, is secreted by a
number of small, tubular glands located in the back
of the throat and in the walls of the esophagus. The
circular muscle ring at the lower end of the
esophagus (before the entrance to the stomach) is
thickened considerably to give its owner (whether
a person or another mammal) some involuntary
control over the flow of food into or out of the
stomach. The movement of food out of the
stomach, up the esophagus, and out of the mouth is
called regurgitation. Most of us have experienced
regurgitation when we are sick.
The Stomach
After passing through the esophagus, the food
enters the next organ of the digestive tract, the
stomach (Figure 10.13). The stomach is a muscular,
J-shaped, sac-like organ whose interior lining is
packed with millions of gastric glands. These
glands secrete the gastric juice so important in
digestion. The stomach differs structurally from the
esophagus by having a third layer of muscle fibres
called the oblique layer. Muscles lining the
stomach work to break food physically into smaller
pieces and mix it with the gastric juices, rendering
it into a thick liquid called chyme.
ower
esophageal
sphincter
esophagus
fundic region
of stomach
cardiac region
of stomach
duodenum
pyloric
sphincter
body of
stomach
rugae
pyloric
canal
pyloric region
of stomach
Figure 10.13 A cross sectional view of the stomach. Note
the multitude of folds called rugae on the inner walls, and
the esophageal and pyloric sphincters.
nasopharynx
soft palate
tonsil
uvula
hard palate
tonsil
food bolus
epiglottis
covering
glottis
trachea
esophagus
Figure 10.12 During the act of swallowing, the trachea
moves up against the epiglottis to seal off the glottis and
prevent food from entering the trachea.
The circular muscle layer at the junction of the
stomach and the next part of the digestive tract is
also thickened, much like the ring at the junction
of the esophagus and stomach. Here, however, the
muscle layer forms a valve called the pyloric
sphincter, which contracts and relaxes to control
the flow of food leaving the stomach.
The Small Intestine
After exiting the stomach, the food enters the small
intestine, which is subdivided into three regions.
The duodenum, which is generally U-shaped, is
the shortest and widest of these regions. Like the
esophagus, it lacks a layer of oblique muscle. The
pancreatic and bile ducts open into the duodenum,
making it an important site for the further chemical
breakdown of the partially digested materials
received from the stomach.
Nutrients, Digestion, and Nutrition • MHR
339
100 µm
villus
lacteal
blood
capillaries
duodenum
goblet
cell
intestinal
gland
arteriole
lymph
nodule
venule
lymphatic
vessel
section of wall
photomicrograph of villi
villi
Figure 10.14 The permanent circular folds in the mucous membrane of the
duodenum bear tiny projections called villi, which in turn bear microvilli. The
presence of all three vastly increases the absorptive surface of the intestine.
Wo rd
LINK
The duodenum takes its name from the Medieval Latin
duodenum digitorum, meaning “12 fingers,” because its length
was measured as 12 finger breadths (about 25–30 cm) when it
was first studied.
Like the rest of the small intestine, the
duodenum has permanent, circular folds in its
mucous membrane. These folds greatly increase the
surface area of the intestine. This larger surface, in
turn, increases the amount of digested food that
can be absorbed (Figure 10.14).
Along these folds, and in particular along the
folds in the duodenum, are minute, visible, fingerlike projections called villi (singular villus). The
villi, in turn, have a fine brush-like border of
microvilli. Both serve to further increase the
absorptive surface of the intestinal tract. Minute,
tube-shaped, intestinal glands are in the spaces
between the villi. Their role is to secrete
intestinal juices.
There are also lacteal or lymph vessels in the
villi. The role of these lacteal vessels is to accept
and carry the larger fat particles that are absorbed
from the intestine. These vessels flow into vessels
of the lymphatic circulatory system, as described in
Chapter 9.
Following the duodenum are the jejunum and
the ileum. These last two regions differ only
slightly in structure from the duodenum. The
jejunum (which is about 2.5 m long) contains more
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MHR • Internal Systems and Regulation
folds and intestinal glands than the duodenum. Its
function is to break down remaining proteins and
carbohydrates so the end products can be absorbed.
The ileum, which is about 3 m long, contains fewer
and smaller villi. Its function is also to absorb
nutrients, as well as to push remaining undigested
material into the large intestine.
Math
LINK
The folds in the lining (mucosa) of the small intestine increase
its surface area three times. The villi increase the surface area
another 30 times, and the microvilli increase it by 600 times.
What is the surface area of a small section of tubing that is
280 cm long and 4 cm in diameter? What is the surface area
of this same section of tubing if it were a section of small
intestine, with its folded lining, villi, and microvilli?
BIO
FACT
The overall length of the three regions of the small intestine,
coupled with the tremendous absorptive surface provided
by the folds, villi, and microvilli, has been compared to the
area of a tennis court. It is across this vast surface that the
majority of nutrients necessary for life are absorbed.
The Large Intestine
The large intestine consists of the caecum, colon,
rectum, and anal canal (see Figure 10.10). At about
1.5 m long, it is much shorter than the small
intestine. Its diameter, however, from which it
takes its name, is much greater. A valve separates it
from the small intestine. The sac-like caecum is the
blind end of the large intestine. The appendix, an
organ that plays no role in digestion but which
may play some role in fighting infection, hangs
suspended from the caecum.
Undigested food entering the large intestine
passes up, along, and down the colon, the main
portion of the large intestine. In the colon, water
and dissolved minerals are absorbed from the
undigested food, while intestinal bacteria help to
break it down further to provide more nutrients.
These bacteria also produce vitamins B-12 and K
and some amino acids. The damp mass of
indigestible material that remains at the end of this
process is called feces. It passes into the rectum
and anal canal, which comprise the last 20 cm of
the large intestine. From here, the feces passes out
of the body through the anus, which has rings of
circular muscle called the anal sphincters. These
sphincters allow the body to control the timing of
elimination to some extent.
A
from
mouth
B
to stomach
food mass
longitudinal
muscle
contraction
circular
muscle
Contraction of circular muscles behind food mass
C
The Movement of Food
So far, we have considered what happens to food
in the digestive tract, but not how the food actually
moves through it. This movement is accomplished
by a series of wavelike muscular contractions and
relaxations known as peristalsis. Peristalsis involves
the circular and longitudinal muscles that surround
the various parts of the digestive tract. To move
food, the circular muscles over a food mass relax
while the longitudinal muscles immediately in front
of it contract. The circular muscles immediately
behind the food mass then contract while the
longitudinal muscles over the food mass relax. As
succeeding muscular regions relax and contract, the
food is pushed along (Figure 10.15). If you ever have
the chance to observe a snake after it has swallowed
a mouse, you will see peristalsis in action.
Another action related to peristalsis is used by
the body to mix partially digested food in the
intestines. During this action, known as rhythmical
segmentation, the food is held in approximately the
same part of the intestine while rhythmical
contractions of the circular muscles squeeze it back
and forth (Figure 10.16).
Contraction of longitudinal muscles ahead of food mass
D
Contraction in circular muscle layer forces food
mass forward
Figure 10.15 Waves of peristalsis like the one shown here
move food along the digestive tract.
partially
digested
food mass
PLAY
Your Electronic Learning Partner has video and animation
clips that will enhance your understanding of the human
digestive system.
Figure 10.16 Rhythmical segmentation is a form of
peristalsis that allows partially digested food to be
thoroughly mixed in the intestines.
Nutrients, Digestion, and Nutrition • MHR
341
MINI
LAB
Modelling Peristalsis
You can gain a good idea of how peristalsis works in the
esophagus by conducting a simple experiment using a few
readily available materials. You will need a tennis ball (or
rubber ball of similar size), liquid soap or detergent, a kneehigh nylon stocking, scissors, and a hand lens. To begin,
stretch the stocking gently in all directions with your hands.
Observe what happens to the width and length of the
stocking as you do this. Now cut off the toe end of the
stocking with the scissors. Next, soak the stocking and ball
in water for a few seconds before applying a squeeze or
two of soap or detergent to each. Use your hands to
spread the soap or detergent throughout the stocking and
around the ball.
Hold the stocking up by its reinforced end in one hand and
push the ball inside the stocking opening until it is well
below the top of the reinforced end. With your free hand,
squeeze the stocking material at the top of the ball in the
web between your thumb and forefinger. What happens to
the ball and the stocking? Repeat this squeezing action
SECTION
1.
2.
over and over, bringing the other fingers of same hand in
play as well, if you like, until the ball exits the stocking at
the toe end.
Rinse the stocking, tennis ball, and your hands free of
the soap or detergent when you are through. Place the
stocking and tennis ball where they can dry.
Analyze
1. Use one hand to stretch the stocking from the inside
while you observe it with the hand lens. How do the
textile fibres you see correspond to the muscle fibres
that surround the esophagus and other parts of the
digestive tract? Explain how your squeezing action
modelled the action of these muscle fibres.
2. What did the water and soap or detergent do?
What two secretions serve the same purpose in the
esophagus?
3. Could someone swallow a mouthful of juice while
upside down? Why or why not?
REVIEW
9.
K/U What would be the effect on the digestive
system if mechanical digestion did not take place?
Name two secretions that help to lubricate
10.
K/U Describe two features of the stomach that
make it an important organ of digestion.
MC What do people mean when they talk about
“food that went down the wrong way”? Could this
prove dangerous? Explain your answer.
11.
I Explain how you could design an experiment
to model rhythmical segmentation, including the
equipment and materials you would use.
12.
MC Doctors, after examining a person with colon
cancer, decide to surgically remove the cancerous
region of the person’s large intestine. Suggest what
dietary adjustments that person might have to make
afterward in order to live a healthy life.
13.
MC A detective is assigned to investigate a suspicious
death. What kinds of information could an autopsy of
the person’s digestive tract reveal?
K/U List, in order, the organs of the digestive tract
through which food passes.
K/U
food.
3.
4.
Explain how chyme is made.
5.
K/U What purposes do the villi and microvilli serve?
Where are they located?
6.
K/U Where is the appendix located and what role
does it play in digestion? Does it have another role?
7.
C Using diagrams, illustrate how peristalsis and
rhythmical segmentation work.
8.
342
K/U
C Draw a diagram that shows where the three
sphincters in the human digestive tract are located.
Label it with the functions that each performs.
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