Lesson 5- The Digestive System
Assignment:
• Read Chapter 7 in the textbook.
• Read and study the lesson discussion.
• Complete the Lesson 5 worksheet
Objectives: After you have completed this lesson, you will be able to:
• Explain digestion in monogastrics, including exocrine secretions and function,
digestive tract function, digestive tract absorption, and the role of the liver in
digestion and metabolism.
• Compare and contrast the specialization of dentition and digestive tracts found in
the various domestic species.
• Define symbiosis and its significance in the ruminant.
• Discuss the clinical significance of the academic material in this chapter.
• Identify common digestive system disorders.
The Digestive System
When discussing the digestive systems of animals, it is important to note that there
are three different types of digestive systems veterinarians encounter—the
monogastric, the ruminant, and the avian. All three digestive systems are based on
the same premise and serve the same functions, designed with the purpose of taking
in food to be broken down through chemical and physical means to be absorbed by
the body for its nutritional requirements. In the textbook, there is a great discussion
about the digestive system; be sure to make yourself familiar with the concepts
presented.
The Monogastric System
Monogastric animals are those that have one stomach (a one-compartment
stomach). Pigs, dogs, cats, rabbits, and humans are examples of animals that have
one stomach compartment. Animals with one stomach must have more grains and
fewer roughages in their diet. This is due to the "simpler" digestive tract involved in
the monogastric animal. You will understand more about this concept after the
discussion on ruminants.
According to information obtained from The University of Florida IFAS Extension,
Digestion is the breakdown of food occurring along the digestive tract. The
digestive tract may be thought of as a long tube through which food passes. As food
passes through the digestive tract, it is broken down into smaller and smaller units.
These small units of food are absorbed as nutrients or pass out of the body as urine
and feces. The digestive tract of [monogastric animals] has five main parts—the
mouth, esophagus, stomach, small intestines, and large intestines.
The mouth is
where food
enters the
digestive tract
and where
mechanical
breakdown of
food begins.
The teeth chew
and grind food
into smaller
pieces. Saliva,
produced by
the mouth, acts
to soften and
moisten the
small food
particles. Saliva
also contains an
enzyme which
starts the
digestion of starch. The tongue helps by pushing the food toward the esophagus.
The esophagus is a tube which carries the food from the mouth to the stomach. A
series of muscle contractions push the food toward the stomach. Swallowing is the
first of these contractions. At the end of the esophagus is the cardiac valve, which
prevents food from passing from the stomach back into the esophagus.
The stomach is the next part of the digestive tract. It is a reaction chamber where
chemicals are added to the food. Certain cells along the stomach wall secrete
hydrochloric acids and enzymes. These chemicals help break down food into small
particles of carbohydrates, protein, and fats. Some particles are absorbed from the
stomach into the bloodstream. Other particles which the stomach cannot absorb
pass on to the small intestine through the pyloric valve.
The small intestine is a complex tube which lies in a spiral, allowing it to fit in a
small space. Its wall has many tiny finger-like projections known as villi, which
increase the absorptive area of the intestine. The cells along the small intestine’s
wall produce enzymes that aid in digestion and absorb digested foods. In the first
section of the small intestine, the duodenum, secretions from the liver and pancreas
are added. Secretions from the liver are stored in the gall bladder and pass into the
intestine through the bile duct. These bile secretions aid in the digestion of fats.
Digestive juices from the pancreas pass through the pancreatic duct into the small
intestine. These secretions contain enzymes that are vital to the digestion of fats,
carbohydrates, and proteins. Most food nutrients are absorbed in the second and
third parts of the small intestine, called the jejunum and the ileum. Undigested
nutrients and secretions pass on to the large intestine through the ileocecal valve.
A "blind gut," or cecum, is located at the beginning of the large intestine. In most
animals, the cecum has little function. However, in animals such as the horse and
rabbit, the cecum is very important in the digestion of fibrous feeds.
The last major part of the digestive tract, the large intestine, is shorter, but larger in
diameter than the small intestine. Its main function is the absorption of water. The
large intestine is a reservoir for waste materials that make up the feces. Some
digestion takes place in the large intestine. Mucous is added to the remaining food in
the large intestine, which acts as a lubricant to make passage easier. Muscle
contractions push food through the intestines. The terminal portion of the large
intestine is called the rectum. The anus is an opening through which undigested
food passes out of the body. Food that enters the mouth and is not digested or
absorbed as it passes down the digestive tract is excreted through the anus as feces
(Rowan et al).
VIEW THIS ANIMATION WHICH ILLUSTRATES AND EXPLAINS THE STEPS OF THE
MONOGASTRIC SYSTEM .
The Ruminant System
For the most part, the digestive system of ruminants is very similar to that of other
mammals, but the stomach is considerably different from the monogastric. The
function of the ruminant system overall is the same; it is just a different way to get
to the end result. Cows, llamas, sheep, and deer are all examples of animals with the
ruminant digestive system.
Have you ever heard someone say that cows have "four stomachs?" If so, you should
know that, in an anatomical sense, this is incorrect.
According to Dr. Thomas Caceci, a professor from the Virginia—Maryland Regional
College of Veterinary Medicine,
… There really is only one stomach, but it does have [four] divisions …. In
monogastric animals, the stomach's functions are limited to temporary storage and
preliminary [breakdown] of food into a liquid mass; little or no absorption of
nutrients takes place. In ruminants, however, the stomach has an absorption
function in addition to the usual functions of mastication (breakdown) and
acidification. The evolutionary success of the ruminants as a group is based on the
efficiency of their digestive system in extracting nutrition from low-quality food.
This is why cows are able to be fed low-quality feedstuffs. Special microorganisms
that thrive in the ruminant system are able to break down large amounts of lowquality roughage high in cellulose. Cellulose is basically the "fatty tissue" in plants
that all other herbivores, except ruminants, cannot utilize. Yet, the fat released from
cellulose in the ruminants’ system is a large part of their dietary budget (Caceci).
The four divisions of the ruminant stomach are the rumen, the reticulum, the
omasum, and the abomasum.
The rumen is an enormous space filled with chewed and half-chewed materials the
cow has ingested, swallowed, regurgitated, and swallowed again (often several
times.) A combination of mechanical mastication and enzymatic action on the hard
cellulose material of the cow's diet permits the breakdown of these otherwise
indigestible materials. A [ruminant who is] "chewing her cud," is methodically
grinding the food into smaller and smaller bits (Caceci).
The reticulum is oftentimes referred to as "the honeycomb." Again, very little
absorption takes place in this compartment. There is an additional amount of
chemical breakdown that occurs here. The reticulum is also the compartment that
traps any unwanted materials such as metal objects. Wire and nails are two very
common metal objects that cows and other ruminants tend to eat, usually by
accident. These indigestible, sharp objects can easily cause serious problems when
they puncture the wall of the stomach.
Dr. Caceci explains treatment for this problem:
One way to deal with this is to have the cow swallow a magnet, which attracts and
holds the metals and prevents them from migrating to a place where they can do
damage as they slosh around in the vast caverns of the stomach. There is a brisk
trade in recycling these magnets which are recovered at slaughter and reused. The
nickname for injuries and illness caused by ingested material is "hardware
disease."
The food material then enters the omasum. This stomach has folds that resemble
the pages of a book. It is in the omasum that water and other nutrients are absorbed
and the ingesta (swallowed food) begins to turn into fecal matter.
The abomasum is called the "true stomach" because it most resembles the
functions of the stomach in the monogastric. Most of the physical and chemical
breakdown is finished in this compartment before it is sent on to the small intestine
(Caceci).
The remainder of the digestive system of the ruminant—the small and large
intestines, the cecum, and the anus—all function very similarly to these organs of
the monogastric system.
The Avian System
To a lesser degree than the previous two, you will see the avian digestive system in
the vet clinic. This system has a substantially different set-up than the other two
systems, but the fundamental principle of this system is the same—to turn foodstuff
into usable nutrients for the body while excreting fecal waste material.
In the article "Avian Digestive System," author Sherri Carpenter notes that as the
bird swallows food, it mixes with saliva so that it can slide easily down the
esophagus. Remember, birds do not have teeth so there is no physical breakdown of
food in the mouth. The food ends up in the crop at the end of the esophagus where it
sits and softens before entering the stomach.
Birds have a two-part
stomach; one is glandular
and one is muscular. The first
is the glandular stomach,
which is called the
proventriculus. This is
where gastric juices are
secreted and are absorbed
into the feed. These juices
are composed of
hydrochloric acid, enzymes,
and mucous. It is the
hydrochloric acid that works
to activate the enzymes. It is
here that the chemical
digestion of food begins. The
food then moves to the
muscular stomach, the
gizzard. The gizzard is extremely thick. If you have ever butchered chickens and
cleaned gizzards, you know that they are full of feedstuff and rocks. Rocks are used
to aid the gizzard in the mechanical breakdown of the food. You may notice that
birds constantly peck at the ground; this is because they are swallowing rocks for
the gizzard. Food
may pass back and
forth from the
proventriculus to
the gizzard to aid
the breakdown of
food.
Next, the food
enters the small
intestine through a
valve called the
pylorus. The
pylorus only
allows a small
amount of food to
pass through at a time. The small intestine is lined with villi and microvilli. These
villi continue to break down the food. The three sections of the small intestine are
the same as described in the monogastric and ruminant system (duodenum,
jejunum, and ileum). Carpenter continues,
The liver and the pancreas secrete their fluids to the duodenum through a common
duct. The pancreas produces enzymes that break down all categories of digestible
foods. The enzymes will complete the digestion [process].
The liver's primary function is to produce bile, a watery solution that aids in more
digestive processes. There are also certain cells in the liver that destroy any bacteria
that have gotten through the walls of the digestive tract and into the blood. The liver
also detoxifies drugs, degrades hormones, and makes many substances that are
necessary for metabolism. One final function of the liver is to maintain the glucose
(sugar) level in the blood (Carpenter).
In addition, Carpenter explains that,
The gallbladder is the storage
house of the bile and is connected to
the duodenum through the cystic
duct. The bile is concentrated by the
removal of water and is made
available to the duodenum when
fatty foods stimulate a hormone
response to the gallbladder to send
out the stored bile … .
The large intestine (colon) is
relatively shorter in birds compared
to mammals. This helps with quicker
elimination to prepare for flight. The
large intestine's main function is to
absorb water, dry out the
indigestible foods, and eliminate
waste products … .
The large intestine also contains
bacteria that collect the remaining
nutrients.
The large intestine joins the cloaca,
which is where the feces meet with
urine from the kidneys.
Digestive Disorders
When an animal's digestive system is running smoothly, all is well. However, just as
in any other system of the body, when abnormalities arise, severe conditions can
result. During the digestive process, various organs reduce food into nutrients that
the animal's body can absorb and utilize to maintain health. There are several
conditions of the digestive disorder than can occur in all species of animals. Two of
the most common conditions are vomiting and diarrhea.
According to an article obtained from Pets Health.com, "Food moves from the mouth
to the stomach via the esophagus. Inflammation or obstruction of the food tube is
referred to as megaesophagus." In this condition, there is a weakening and dilation
of esophageal muscles that will cause the animal to regurgitate food before it even
reaches the stomach. As the stomach churns food into a thick liquid (chyme),
special glands secrete enzymes that break down proteins, hydrochloric acid that
aids those enzymes, and mucus that protects the stomach from digesting itself.
Vomiting is the most obvious sign of stomach inflammation.
The article continues and indicates that "most of the digestion and nutrient
absorption occurs in the small intestine, where more enzymes and mucus are
added." Once the small intestine has begun digestion, the food mass continues to the
large intestine where water continues to be absorbed. Finally, the large intestine
pushes the now-digested food, in the form of feces, to the anus.
Difficulties in digestion can often be diagnosed by a visual stool examination. For
example, if an animal has "watery diarrhea with no blood or mucus, does not strain
when defecating, and eliminates on a normal schedule, its small intestine is
probably inflamed."
Conversely, if the animal has "mucousy diarrhea tinged with fresh blood, strains
when defecating, and has frequent urges to move its bowels, its large intestine is
probably inflamed." There are
many causes of digestive
disorders with the most common
being a change in the diet. Other
causes include toxins, food
allergies, infection, cancer,
foreign bodies, metabolic
diseases, organ failure, and
parasites. These ailments usually
affect the animal by stalling "the
digestive processes, causing
constipation while others rush
food through the system too
quickly, resulting in minimal
nutrient and water absorption
and a large volume of loose feces."
If your animal experiences any digestive disorders, call your veterinarian to discuss
the situation. Depending on the seriousness of the problem and the demeanor of
your pet, a trip to your pet's doctor and an examination may be required.
Here are some tips to help protect your animal from digestive disorders:
• feed your animal a consistent diet appropriate for its age, weight, and overall
health
• make sure your animal's vaccinations are up to date
• have your animal's fecal material checked regularly for parasites (twice per year)
• limit your animal's access to foreign objects that can harm its digestive tract
• prevent your animal from scavenging garbage cans or compost piles
• keep toxic substances out of the animal's reach (PetsHealth.com).
Summary
Digestive tract differences among animal species present veterinarians with many
challenges. Even though the process of digestion is the same for all animals, the
systems vary considerably. Ruminants have multiple-compartment stomachs, dogs
have monogastric stomachs, and horses have monogastric stomachs, accompanied
with a specialized cecum to allow them to eat more feed. It is these unique features
and the conditions associated with them that keep veterinarians on their toes.
Sources Cited:
Caceci, Dr. Thomas. "Urinary System." Veterinary Histology. 1998. 28 Dec. 2006
<http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab21/LAB21.HTM>.
Carpenter, Sherri. "The Avian Digestive System." HolisticBird Newsletter. 3.1(Winter 2003). 28 Dec. 2006
<http://www.holisticbirds.com/pages/digestive0203.htm>.
Columbia Animal Hospital. "Digestive Disturbances in Dogs." Pets Health.com. 28 Dec. 2006
<http://www.petshealth.com/dr_library/digestdistdogs.html>.
Oracle Education Foundation. "The Respiratory System." ThinkQuest. 1996. 28 Dec. 2006.
<http://library.thinkquest.org/2935/Natures_Best/Nat_Best_Low_Level/Respiratory_page.L.html>.
Rowan, J.P. et. al. "The Digestive Tract of the Pig." University of Florida IFAS Extension. Dec. 1996. 28 Dec. 2006
<http://edis.ifas.ufl.edu/AN012>.