Under the liver is the stomach. Locate the point where
the esophagus enters the cardiac region of the stomach.
Gastric glands in the wall of the stomach secrete pepsinogen, hydrochloric acid, and rennin. Pepsinogen is activated
by hydrochloric acid to become pepsin, which digests proteins. Rennin is an enzyme that hydrolyzes milk protein.
Food leaves the stomach as a fluid suspension, chyme. It
enters the duodenum, the first part of the small intestine.
Find the pancreas, a glandular mass lying in the angle
between the curve of the stomach and duodenum. It secretes several enzymes into the duodenum that digest proteins, lipids, carbohydrates, and nucleic acids. Certain cells
in the pancreas act as endocrine cells and secrete the hormones insulin and glucagon. In fact, insulin used in human
diabetes therapy can be extracted from the pancreases of
pigs collected at slaughterhouses.
Although it is not part of the digestive system, identify
the spleen attached by mesenteries to the outer curvature of
the stomach. It is made of lymphatic tissue and is important
in development of immunity and the scavenging of iron
from red blood cells when they break down.
Slit the stomach lengthwise, cutting through the cardiac and pyloric sphincters, muscles that regulate passage
of material into and out of the stomach. The internal surface of the stomach is covered by gastric mucosal cells,
which secrete mucus that prevents the stomach from digesting itself. When this protection fails, a peptic ulcer
develops.
The small intestine is made up of three sequentially
arranged regions: duodenum, jejunum, and ileum. These
areas are difficult to differentiate from each other. Cut out a
2-cm section of the small intestine about 5 cm posterior
from the stomach, slit it open, and place it under water in a
dish. Use your dissecting microscope to observe the velvety
internal lining made up of numerous fingerlike projections
called villi. The villi are highly vascularized, containing
capillaries and lymphatics that transport the products of digestion to other parts of the body, especially the liver.
The ileum opens into the large intestine, or colon.
They join at an angle, forming a blind pouch, the cecum,
which in primates and some other mammals often ends in a
slender appendage, the appendix. In many herbivores, the
cecum is very large and contains microorganisms that aid
digestion by breaking down cellulose.
The rectum is the caudal part of the large intestine,
where compacted, undigested food material is temporarily
stored before being released through the anus. The colon of
vertebrates contains large numbers of symbiotic bacteria, especially Escherichia coli. These bacteria produce vitamin K,
which is absorbed and plays a vital role in blood clotting.
Histology of Small Intestine
your fetal pig aside for the moment and obtain a prepared slide of a cross section of a mammalian small intestine. Examine it under scanning power with the compound
microscope. Compare what you see to figure 28.5.
384
Investigating Digestive and Gas Exchange Systems
The central opening is called the lumen and is the space
through which food passes as chyme during digestion.
Switch to low power and observe the small fingerlike projections of the intestine's inner surface. These are villi and are
covered by a layer of cells called the mucosa. You should be
able to distinguish two cell types in the intestinal mucosa:
goblet cells and columnar epithelial cells. Examine them
with the high-power objective. The goblet cells secrete
mucus into the small intestine, serving as a lubricant for the
passage of chyme. Epithelial cells are involved in absorption.
Return to the low-power objective and observe the submucosa, a layer of connective tissue that underlies the mucosa. Look for the blood vessels and lymphatic vessels that
ramify through this layer. Sugars, amino acids, glycerides,
and other components of digested food must move through
the mucosal cells into the submucosa before they can enter
the circulatory system and be distributed throughout the body.
To the outside of the submucosa are two smooth muscle layers: an inner circular layer and outer longitudinal
layer. The inner circular muscles change the diameter of the
intestine, and the outer muscles alter its length. These muscles contract in a wavelike motion called peristalsis, which
pushes chyme through the digestive tract. The small intestine is covered by a layer of peritoneal cells, that together
with underlying connective tissue, is called the serosa.
Figure 28.5 shows scanning electron micrographs of
the three-dimensional arrangement of the small intestine.
Note how the villi and microvilli increase the surface area.
What important process following digestion is facilitated
by this increased surface area?
Invertebrate Respiratory Systems
Insect Trachea/ System
The mammalian respiratory system represents only one
method of gas exchange in terrestrial animals. Insects and
other terrestrial arthropods make use of another, a rather remarkable tracheal system, which allows the exchange of
gases to occur independent of the circulatory system.
In a tracheal system, air enters through several small,
lateral openings called spiracles and passes into a branching system of tubules called tracheae and tracheoles. The
tracheoles continue branching into a system of fluid-filled
tubes where gas exchange occurs (fig. 28.6). The tracheoles
branch so extensively that they are never very far from any
cell in the body. Gas exchange occurs by diffusion from the
cell to the tracheole with little participation of the circulatory system. There are apparently no special muscles that
circulate air in the tracheal system, but contraction of the
abdominal and flight muscles aid the exchange of air. Large
insects often have air sacs. When compressed by movement of surrounding organs, they force large volumes of air
through the system.
If your instructor has not prepared a demonstration dissection of a tracheal system, obtain a live, large insect
(cricket, grasshopper, or roach) and a petri dish filled with
wax to be used as a dissecting pan. Anesthetize the insect
28-6
Lumen
Mucosa
layer
Submucosal
layer
Columnar
epithelium
Goblet
cell
Muscle
layer
~
Figure 28.5
Microstructure of intestine, (o) Photo taken
through a light microscope of cross section of small intestine,
(b) Scanning electron micrograph of cross section of small
intestine showing villi (Vi), lumen (Lu), submucosa layer (Su),
and muscle layers (Mu). The epithelial cells on the surface of
the villi have highly folded membranes, microvilli (Mv), which
greatly increase the absorptive surface area of the cell layer, as
seen in (c) a transmission electron micrograph showing highly
folded cell membrane, [b] From R. G. Kessel and R. H. Kardon.
Tissues and Organs: A Text-Atlas of Scanning Electron
Microscopy. 1979. W. H. Freeman and Company.
with carbon dioxide or by placing it in a freezer. Remove
the wings and legs and locate the spiracles along the side.
With fine scissors, cut through the dorsal surface close to
the lateral margin but do not cut through the spiracles.
(Keep the points of your scissors against the inside of the
exoskeleton and take small snips so that you do not damage
any interior organs.) Run the cuts along the full length of the
insect and join them together at the anterior and posterior
ends. Remove the dorsal strip of cuticle and pin the insect in
the dish, dorsal side up. Next flood the insect with saline solution to prevent drying and to float tissues for observation.
Place the dish under a dissecting microscope and observe.
Gently remove the glistening yellow fat bodies. Vary
the lighting angle while looking for a silver-colored tube
originating from a spiracle. Remove this trachea with a forceps, mount it in a drop of saline on a microscope slide, and
add a coverslip. Observe the trachea with your compound
microscope and sketch it in the circle to the right. Compare
your observations to figure 28.6c.
28-7
Investigating Digestive and Gas Exchange Systems
385
Figure 28.6
Tracrieal systems, (a) Tracheal system of a flea penetrates all parts of its body, allowing direct gas exchange.
(£>) Large insects pump air through their tracheal systems by contracting muscles that press against air sacs, (c) Photomicrograph of
tracheae shows reinforcing rings of chitin that prevent the tubules from collapsing.
Spiracles
Air sacs
(b)
Gills
In aquatic organisms, gills function in respiratory gas exchange and ion regulation. Gills are found in many different kinds of animals, including larval amphibians, fish,
arthropods, and molluscs. Except in molluscs, gills originate in the embryo as featherlike pocketings of the body
wall that are amply supplied with blood. In most cases, a
special mechanism exists to ventilate the gills, that is, to
move water over them. In crayfish, this involves a specialized mouthpart, the gill bailer, which moves water forward
over the gills in a sculling motion (fig. 28.7).
In fish, gill covers called opercula are raised, to draw
water in through the open mouth. The mouth is then closed
and depression of the opercula forces water over the gills
and out through the external gill openings (fig. 28.7b).
In some fish, the pumping mechanism is not sufficient
to satisfy the metabolic demands for oxygen, so that such
fish must constantly swim with its mouth open to improve
the rate of water flow over the gills. In clams as well as in
many other animals, the gills function in gas exchange and
also in filtering food particles from the water stream that
passes over and through the gills.
9
In your lab, you may have some living crayfish in an
aquarium. Place one in a bowl of water and give it time to
resume normal gill ventilation. Take a Pasteur pipette and
place a drop of india ink next to the side of the crayfish just
386
I
Investigating Digestive and Gas Exchange Systems
Figure 28.7
Direction of water movement over the gills
of two animals: (a) crayfish; (b) fish.
(a) Crayfish
(b) Fish
Opercular valve -
behind the thorax. Watch the direction of water flow using
the ink as a tracer. Record your observations below.
28-8
Mammalian Respiratory System
Respiratory Tree Anatomy
eturn to your fetal pig and examine the external openings
(external nares) on the snout. Cut across the snout with a
scalpel about 1 to 2 cm from the end and remove the tip.
The nasal passages are separated from each other by the
nasal septum. The curved turbinate bones in the sinus area
increase the surface area of the passageways, creating eddy
currents that, along with hairs, cilia, and mucus, help remove dust in the inhaled air and humidify it. The floor of
the nasal passages is made up of the hard palate and the soft
palate (posterior to hard palate).
Look into the pig's mouth. Behind and above the soft
palate is the nasopharynx. It may be necessary to slit the
soft palate to observe this. Air enters the nasopharynx from
the posterior end of the nasal passages, then passes into the
pharynx, through the glottis, and into the larynx and ultimately the trachea. If food accidentally enters the glottis,
choking results. The Heimlich maneuver can often "save" a
person who has food wedged in the glottis. Your instructor
will demonstrate this maneuver for you.
In the nasopharyngeal area, look for the openings of
the eustachian tubes. They are very difficult to find. These
tubes allow air pressure to equilibrate between the middle
ear chamber and the atmosphere. (This is why changes in
altitude cause the ears to "pop.") Throat infections often
spread to the ears through the eustachian tubes.
Run your fingers over the pig's throat and locate the
hard, round larynx. Make a medial incision in the skin of
the throat and extend the end cuts laterally, folding back the
skin flaps. Repeat this procedure for the muscle layers.
Note the large mass of glandular material, the thymus,
in this area. In the young pig, the thymus produces lymphocytes, an important component of the immune system. Later
in life, the thymus atrophies and is of little consequence.
As you approach the larynx and trachea in your dissection, use a blunt probe to separate the muscles and expose these structures. Ventral to the trachea, observe the
brownish-colored thyroid gland. Note how both the trachea and larynx are supported by rings of cartilage. The
hyoid apparatus is anterior to the larynx and is a supporting frame for the tongue extensor muscles.
The larynx, or voice box, contains folds of elastic tissue, which are stretched across the cavity. These vocal
cords vibrate when air passes over them, producing sound,
and attached muscles vary the cord tension, allowing variations in pitch. Slit the larynx longitudinally and observe the
vocal cords. Continue the slit posteriorly into the trachea
and observe its lining. The esophagus is located behind the
trachea. Pass a blunt probe into the esophagus from the
mouth to help identify it.
1
3
If the thorax of your animal is not already opened,
make a longitudinal cut with heavy scissors through the
ribs just to the animal's right of the sternum, or breastbone. Always keep the lower scissor tip pointed upward
against the inside of the sternum to avoid catching and cut-
28-9
ting internal structures. Be careful! Several major blood
vessels are under the sternum and should not be damaged.
The diaphragm is a sheet of muscle that separates the
abdominal cavity from the thoracic cavity. The thoracic
cavity is divided into three areas by membranes: the right
and left pleural cavities, which surround the lungs, and the
pericardia! cavity where the heart is located.
If the pleural membranes are removed, the lung structure can be seen. The trachea, when it enters the thorax, divides into two bronchi, which are hidden from direct view
beneath the heart and blood vessels. These bronchi, in turn,
divide into progressively smaller bronchioles, which finally end in clusters of microscopic air sacs called alveoli.
Alveoli have walls only a single cell layer thick and they
are covered by capillaries (fig. 28.8). In these air sacs, oxygen and carbon dioxide are exchanged between the blood
and the inhaled air. Have you ever known someone with
bronchitis? Where do you think the infection was?
Bronchioles have smooth muscles in their walls. Asthma
results from the spasmodic contraction of these muscles, preventing air flow to the alveoli.
Microscopic Examination of Mammalian Lung
emove a piece of the lung and put it in a small bowl of
water. Observe it with a dissecting microscope and find the
alveoli and bronchioles. Also look at the demonstration slide
of a section across several alveoli. How many cell thicknesses separate the air in a mammalian lung from the red
blood cells in the capillaries?
. The inset in figure 28.8
gives you the distances in micrometers between an erythrocyte in a capillary and the air in an alveolus. What is the diffusion distance for a molecule of oxygen in the alveolus to
an erythrocyte and its hemoglobin?
.
Figure 28.9 contains a scanning electron micrograph of
an alveolus and surrounding capillaries. The alveoli form an
interconnecting system of chambers, and macrophages in the
alveoli scavenge for microorganisms and particulate material.
Lung Ventilation Mechanism
Air enters the lungs as a result of the combined effects of
the contraction of the diaphragm and several sets of muscles. When the external intercostal (between the ribs) muscles contract, the volume of the thoracic cavity increases as
the rib cage elevates and the diaphragm depresses, causing
the air pressure in the cavity to decrease. Air rushes in
through the respiratory passageways and expands the alveoli. This causes the pressure between the atmosphere and
the pleural cavities to equilibrate. When the diaphragm and
intercostal muscles relax, the rib cage drops and the diaphragm rises decreasing the volume of the thoracic cavity.
Investigating Digestive and Gas Exchange Systems
387
I
Alveolus on
respiratory
bronchiole
Respiratory
bronchiole
Terminal
bronchiole
x
Smooth
muscle
Figure 28.8
(a) Microscopic structure of lung and close
relationship of alveoli and capillaries; (fa) Inspiration and
expiration mechanics. Contraction of intercostal muscles lifts rib
cage and contraction of diaphragm muscles flattens diaphragm:
both enlarge chest cavity and cause its pressure to drop. During
resting expiration, elastic recoil of lung tissues, together with
dropping of the rib cage and recoil of diaphragm because of
organ pressure, decrease size of chest cavity.
Intercostal
muscles
Diaphragm
down
Intercostal
muscles
Diaphragm
up
Erythrocyte
Capillary
endothelium
(0.04-0.2 urn)
Interstitial
space
(0.02-0.2 urn)
Alveolar
epithelium
(0.05-0.3 urn)
Surface lining'
(0.01 urn)
Inhalation
External intercostal muscles
contract, pull rib cage upward.
(a)
Exhalation
Internal intercostal muscles
contract, help pull rib cage
downward.
(b)
Consequently, the pressure in the pleural cavity increases
and collapses the alveoli, driving air out of the "respiratory
tree." Figure 28.8/7 illustrates these mechanics in humans.
15
Note the convex shape of the diaphragm in your pig
and imagine how contraction of the diaphragm increases
the volume of the thoracic cavity. Push the sternum up to
mimic the contraction of the involved muscles and note the
expansion of the chest cavity.
You are now finished with the fetal pig for this lab.
Return it to the storage area according to the directions
given by your lab instructor.
Figure 28.9
Scanning electron micrograph of an
alveolus, showing thickness of alveolar wall.
Measuring Human Respiratory Volumes
Respiratory volumes vary with a person's size, age, gender,
and physical condition. Normal breathing results in approximately 500 ml of air moving into and out of the lungs with
each breath. This is called the tidal volume. Each of us can
draw much more into our lungs because we have an inspiratory reserve volume as well as force more from our
lungs, our expiratory reserve volume. However, regardless
of how much we exhale, there is always a residual volume
of air in the lungs because the alveoli do not completely collapse. These concepts are summarized in figure 28.10.
388
Investigating Digestive and Gas Exchange Systems
28-10
Figure 28.10
Approximate capacities of human lungs.
n
5000
Inspiratory
reserve
volume
21 00-3000 ml
E
§
Tidal volume
500ml
:
:
n /\
.
Expiratory
reserve
volume
800-1 200 ml
m
n
A/I
I
A/I
Vital
' capacity
Total lung
capacity
4400-5900 ml
u
Residual
volume
1000-1 200 ml
Time •
Expiratory reserve volume is the amount of air that can
In this section of the exercise, you will measure these 18
volumes with a spirometer. Your instructor will explain
be forcibly exhaled after a normal expiration. Devise a prohow the instrument works. You may work in pairs in this
cedure to measure this and repeat the measurements three
part of the exercise, or your instructor may choose to do it W times. Calculate an average. What is your expiratory reas a demonstration.
serve volume?
Set the spirometer to zero and install a new disposable mouthpiece, or sterilize the old one by dipping it in
70% alcohol.
To measure tidal volume, sit in a chair for three minutes.
Your vital capacity is the sum of the three values you
Then, breathe in and out at a normal rate five or six times.
just measured. What is your vital capacity?
On the last breath, exhale as you normally would through the
spirometer mouthpiece. Repeal this measurement three times
and calculate an average volume. What is your tidal volume?
To measure inspiratory reserve volume, again breathe
normally five or six times, then make the deepest possible
inspiration. Now breathe out through the respirometer, exhaling only to the normal expiratory end point. Do not
force the expiration beyond where you would normally stop
exhaling. Repeat the measurement three times and calculate
an average. To obtain the actual inspiratory reserve volume,
subtract the tidal volume from the average of the just mea& sured volumes. What is your inspiratory reserve volume?
28-11
Total lung capacity involves a fourth volume that cannot be easily measured. (See figure 28.10.) What is it?
Comparative Summary
To gain an understanding of comparative anatomy, fill in
tables 28.1 to 28.4. Write a brief description of each structure and its function. Include a summary statement of the
organ system for each animal.
Investigating Digestive and Gas Exchange Systems
389
TABLE 28.3
Insects—gas exchange system summary
Spiracles
Trachea
Tracheoles
Tissues
Air sacs
Description of gas exchange
TABLE 28.4
stal pig—mammal-
summary
External nares
Nasopharynx
Larynx
Trachea
Bronchi
Bronchioles
Alveoli
Blood
Diaphragm
Rib cage muscles
Description of gas exchange
28-13
Investigating Digestive and Gas Exchange Systems
391
Learning Biology by Writing
Your instructor may ask you to answer the Lab Summary
and Critical Thinking Questions that follow.
Internet Sources
The World Wide Web has become an important
resource for those who work in the medical field. You
can explore this resource by using GOOGLE to search
for information on "pulmonary pathology." When the
list of citations is returned, look for those that have
images of black lung disease, silicosis, and asbestos
accumulation. From your observations of the photos,
write a short paragraph describing how these
diseases affect the lung. Be sure to include the URL for
future reference.
Lab Summary Questions
1. Create a flowchart that traces the pathway of food
through the mammalian digestive system, and indicate
to the right of the chart the functions of each organ.
2. The digestive system of Hydra is saclike, whereas that
of mammals and many other animals is tubelike. What
are the advantages of tubular digestive systems?
3. Outline the pathway of air from the nostrils to the
alveoli in the fetal pig.
4. Describe how the diaphragm and rib cage function in
moving air into and out of a mammal's lungs.
5. Contrast how cells in a mammal receive oxygen with
how cells in an insect receive oxygen.
6. Explain what is meant by term vital capacity of the
lungs. How is it different from total capacity?
Critical Thinking Questions
1. Explain why the small intestine is the longest part of
the mammalian digestive system.
2. Consider air and water as respiratory media. Which
contains a greater concentration of oxygen? Which
takes more energy to move over the respiratory
surface? Which is potentially more damaging to the
respiratory surface?
3. What affect does the giraffe's long neck have on
breathing? (Consider volume of trachea.)
•
=
a
.
*
<*!
392
Investigating Digestive and Gas Exchange Systems
28-14
I