# Anatomy and Physiology of Animals/The Gut and Digestion

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## Objectives

After completing this section, you should know:

-   what is meant by the terms: ingestion, digestion, absorption,
    assimilation, egestion, peristalsis and chyme
-   the characteristics, advantages and disadvantages of a herbivorous,
    carnivorous and omnivorous diet
-   the 4 main functions of the gut
-   the parts of the gut in the order in which the food passes down

## The Gut And Digestion

Plant cells are made of organic molecules using energy from the sun.
This process is called **photosynthesis**. Animals rely on these
ready-made organic molecules to supply them with their food. Some
animals (herbivores) eat plants; some (carnivores) eat the herbivores.

## Herbivores

Herbivores\'\' eat plant material. While no animal produces the
digestive enzymes to break down the large **cellulose** molecules in the
plant cell walls, micro-organisms\' like bacteria, on the other hand,
can break them down. Therefore herbivores employ micro-organisms to do
the job for them.

There are two types of herbivore:

:   The first, **ruminants** like cattle, sheep and goats, house these
    bacteria in a special compartment in the enlarged stomach called the
    **rumen**.

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:   The second group has an enlarged large intestine and caecum, called
    a **functional caecum**, occupied by cellulose digesting
    micro-organisms. These non-ruminant herbivores include the horse,
    rabbit and rat.

Plants are a primary pure and good source of nutrients, however they
aren\'t digested very easily and therefore herbivores have to eat large
quantities of food to obtain all they require. Herbivores like cows,
horses and rabbits typically spend much of their day feeding. To give
the micro-organisms access to the cellulose molecules, the plant cell
walls need to be broken down. This is why herbivores have teeth that are
adapted to crush and grind. Their guts also tend to be lengthy and the
food takes a long time to pass through it.

Eating plants have other advantages. Plants are immobile so herbivores
normally have to spend little energy collecting them. This contrasts
with another main group of animals - the carnivores that often have to
chase their prey.

## Carnivores

**Carnivorous animals** like those in the cat and dog families, polar
bears, seals, crocodiles and birds of prey catch and eat other animals.
They often have to use large amounts of energy finding, stalking,
catching and killing their prey. However, they are rewarded by the fact
that meat provides a very concentrated source of nutrients. Carnivores
in the wild therefore tend to eat distinct meals often with long and
irregular intervals between them. Time after feeding is spent digesting
and absorbing the food.

The guts of carnivores are usually shorter and less complex than those
of herbivores because meat is easier to digest than plant material.
Carnivores usually have teeth that are specialised for dealing with
flesh, gristle and bone. They have sleek bodies, strong, sharp claws and
keen senses of smell, hearing and sight. They are also often cunning,
alert and have an aggressive nature.

## Omnivores

Many animals feed on both animal and vegetable material -- they are
**omnivorous.** There are currently two similar definitions of
omnivorism:

1\. Having the ability to derive energy from plant and animal material.

2\. Having characteristics which are optimized for acquiring and eating
both plants and animals.

Some animals fit both definitions of omnivorism, including bears,
raccoons, dogs, and hedgehogs. Their food is diverse, ranging from plant
material to animals they have either killed themselves or scavenged from
other carnivores. They are well equipped to hunt and tear flesh (claws,
sharp teeth, and a strong, non-rotational jaw hinge), but they also have
slightly longer intestines than carnivores, which has been found to
facilitate plant digestion. The examples also retain an ability to taste
amino acids, making unseasoned flesh palatable to most members of the
species.

Classically, humans and chimpanzees are classified as omnivores.
However, further research has shown chimpanzees typically consume 95%
plant matter (the remaining mass is largely termites), and their teeth,
jaw hinge, stomach pH, and intestinal length closely matches herbivores,
which many suggest classified them as herbivores. Humans, conversely,
have chosen to eat meat for much of the archaeological record, although
their teeth, jaw hinge, and stomach pH, and intestinal lengths also
closely match other herbivores.

The dispute of human/ chimps classifications is caused by two things.
First, there is research that both plant-only and some-animal diets
promote health (longevity and freedom from disease) in humans. Second,
well-off humans have often chosen to eat meat and dairy products
throughout written history, which some argue shows that we prefer meat
and dairy by latent instinct.

Per the classical definition, omnivores lack the specialized teeth and
guts of carnivores and herbivores but are often highly intelligent and
adaptable reflecting their varied diet.

## Treatment Of Food

Whether an animal eats plants or flesh, the **carbohydrates**, **fats**
and **proteins** in the food it eats are generally giant molecules (see
chapter 1). These need to be split up into smaller ones before they can
pass into the blood and enter the cells to be used for energy or to make
new cell constituents.

For example:

:   **Carbohydrates** like cellulose, starch, and glycogen need to be
    split into **glucose** and other **monosaccharides**;
:   **Proteins** need to be split into **amino acids**;
:   **Fats** or **lipids** need to be split into **fatty acids** and
    **glycerol**.

## The Gut

The **digestive tract, alimentary canal** or **gut** is a hollow tube
stretching from the mouth to the anus. It is the organ system concerned
with the treatment of foods.

At the mouth the large food molecules are taken into the gut - this is
called **ingestion**. They must then be broken down into smaller ones by
digestive enzymes - **digestion**, before they can be taken from the gut
into the blood stream - **absorption**. The cells of the body can then
use these small molecules - **assimilation**. The indigestible waste
products are eliminated from the body by the act of **egestion** (see
diagram 11.1).

![](Anatomy_and_physiology_of_animals_From_ingestion_to_egestion.jpg "Anatomy_and_physiology_of_animals_From_ingestion_to_egestion.jpg")

Diagram 11.1 - From ingestion to egestion

The 4 major functions of the gut are:

:   1\. Transporting the food;
:   2\. Processing the food physically by breaking it up (chewing),
    mixing, adding fluid etc.
:   3\. Processing the food chemically by adding digestive enzymes to
    split large food molecules into smaller ones.
:   4\. Absorbing these small molecules into the blood stream so the
    body can use them.

The regions of a typical mammals gut (for example a cat or dog) are
shown in diagram 11.2.

![](Anatomy_and_physiology_of_animals_Typical_mammalian_gut.jpg "Anatomy_and_physiology_of_animals_Typical_mammalian_gut.jpg")

Diagram 11.2 - A typical mammalian gut

The food that enters the **mouth** passes to the **oesophagus**, then to
the **stomach**, **small intestine**, **cecum**, **large intestine**,
**rectum** and finally undigested material exits at the **anus**. The
**liver** and **pancreas** produce secretions that aid digestion and the
**gall bladder** stores **bile**. Herbivores have an appendix which they
use for the digestion of cellulose. Carnivores have an appendix but is
not of any function anymore due to the fact that their diet is not based
on cellulose anymore.

## Mouth

The mouth takes food into the body. The lips hold the food inside the
mouth during chewing and allow the baby animal to suck on its mother's
teat. In elephants the lips (and nose) have developed into the trunk
which is the main food collecting tool. Some mammals, e.g. hamsters,
have stretchy cheek pouches that they use to carry food or material to
make their nests.

The sight or smell of food and its presence in the mouth stimulates the
**salivary glands** to secrete **saliva**. There are four pairs of these
glands in cats and dogs (see diagram 11.3). The fluid they produce
moistens and softens the food making it easier to swallow. It also
contains the enzyme, **salivary amylase**, which starts the digestion of
starch.

The **tongue** moves food around the mouth and rolls it into a ball
called a bolus for swallowing. **Taste buds** are located on the tongue
and in dogs and cats it is covered with spiny projections used for
grooming and lapping. The cow's tongue is prehensile and wraps around
grass to graze it.

Swallowing is a complex reflex involving 25 different muscles. It pushes
food into the oesophagus and at the same time a small flap of tissue
called the **epiglottis** closes off the windpipe so food doesn't enter
the trachea and choke the animal (see diagram 11.4).

![](Anatomy_and_physiology_of_animals_Salivary_glands.jpg "Anatomy_and_physiology_of_animals_Salivary_glands.jpg")

Diagram 11.3 - Salivary glands

![](Anatomy_and_physiology_of_animals_Section_through_head_of_a_dog.jpg "Anatomy_and_physiology_of_animals_Section_through_head_of_a_dog.jpg")

Diagram 11.4 - Section through the head of a dog

## Teeth

Teeth seize, tear and grind food. They are inserted into sockets in the
bone and consist of a crown above the gum and root below. The crown is
covered with a layer of **enamel**, the hardest substance in the body.
Below this is the **dentine**, a softer but tough and shock resistant
material. At the centre of the tooth is a space filled with **pulp**
which contains blood vessels and nerves. The tooth is cemented into the
**socket** and in most teeth the tip of the root is quite narrow with a
small opening for the blood vessels and nerves (see diagram 11.5).

In teeth that grow continuously, like the incisors of rodents, the
opening remains large and these teeth are called **open rooted teeth**.
Mammals have 2 distinct sets of teeth. The first set, the **milk
teeth**, are replaced by the **permanent teeth**.

![](Anatomy_and_physiology_of_animals_Stucture_of_tooth.jpg "Anatomy_and_physiology_of_animals_Stucture_of_tooth.jpg")

Diagram 11.5 - Structure of a tooth

### Types Of Teeth

All the teeth of fish and reptiles are similar but mammals usually have
four different types of teeth.

The **incisors** are the chisel-shaped 'biting off' teeth at the front
of the mouth. In rodents and rabbits the incisors never stop growing
(open-rooted teeth). They must be worn or ground down continuously by
gnawing. They have hard enamel on one surface only so they wear unevenly
and maintain their sharp cutting edge.

The largest incisors in the animal kingdom are found in elephants, for
tusks are actually giant incisors. Sloths have no incisors at all, and
sheep have no incisors in the upper jaw (see diagram 11.6). Instead
there is a horny pad against which the bottom incisors cut.

The **canines** or 'wolf-teeth' are long, cone-shaped teeth situated
just behind the incisors. They are particularly well developed in the
dog and cat families where they are used to hold, stab and kill the prey
(see diagram 11.7).

The tusks of boars and walruses are large canines while rodents and
herbivores like sheep have no (or reduced) canines. In these animals the
space where the canines would normally be is called the **diastema**. In
rodents like the rat and beaver it allows the debris from gnawing to be
expelled easily.

The cheek teeth or **premolars** and **molars** crush and grind the
food. They are particularly well developed in herbivores where they have
complex ridges that form broad grinding surfaces (see diagram 11.6).
These are created from alternating bands of hard enamel and softer
dentine that wear at different rates.

In carnivores the premolars and molars slice against each other like
scissors and are called **carnassial** teeth see diagram 11.7). They are
used for shearing flesh and bone.

### Dental Formula

The numbers of the different kinds of teeth can be expressed in a
**dental formula**. This gives the numbers of incisors, canines,
premolars and molars in **one half** of the mouth. The numbers of these
four types of teeth in the left **or** right **half of the upper jaw**
are written above a horizontal line and the four types of teeth in the
right **or** left **half of the lower jaw** are written below it.

Thus the dental formula for the sheep is:

:   

    :   0.0.3.3
    :   3.1.3.3

It indicates that in the upper right (or left) **half** of the jaw there
are no incisors or canines (i.e. there is a **diastema**), three
premolars and three molars. In the lower right (or left) **half** of the
jaw are three incisors, one canine, three premolars and three molars
(see diagram 11.6).

![](Anatomy_and_physiology_of_animals_Sheeps_skull.jpg "Anatomy_and_physiology_of_animals_Sheeps_skull.jpg")

Diagram 11.6 - A sheep's skull

The dental formula for a dog is:

:   

    :   3.1.4.2
    :   3.1.4.3

The formula indicates that in the right (or left) **half** of the upper
jaw there are three incisors, one canine, four premolars and two molars.
In the right (or left) **half** of the lower jaw there are three
incisors, one canine, four premolars and three molars (see diagram
11.7).

![](Anatomy_and_physiology_of_animals_Dogs_skull.jpg "Anatomy_and_physiology_of_animals_Dogs_skull.jpg")

Diagram 11.7 - A dog's skull

## Esophagus

The **Esophagus** transports food to the stomach. Food is moved along
the esophagus, as it is along the small and large intestines, by
contraction of the smooth muscles in the walls that push the food along
rather like toothpaste along a tube. This movement is called
**peristalsis** (see diagram 11.8).

![](Anatomy_and_physiology_of_animals_Peristalis.jpg "Anatomy_and_physiology_of_animals_Peristalis.jpg")

Diagram 11.8 - Peristalsis

## Stomach

The **stomach** stores and mixes the food. Glands in the wall secrete
**gastric juice** that contains enzymes to digest protein and fats as
well as **hydrochloric acid** to make the contents very acidic. The
walls of the stomach are very muscular and churn and mix the food with
the gastric juice to form a watery mixture called **chyme** (pronounced
kime). Rings of muscle called **sphincters** at the entrance and exit to
the stomach control the movement of food into and out of it (see diagram
11.9).

![](Anatomy_and_physiology_of_animals_Stomach.jpg "Anatomy_and_physiology_of_animals_Stomach.jpg")

Diagram 11.9 - The stomach

## Small Intestine

Most of the breakdown of the large food molecules and absorption of the
smaller molecules take place in the long and narrow small intestine. The
total length varies but it is about 6.5 metres in humans, 21 metres in
the horse, 40 metres in the ox and over 150 metres in the blue whale.

It is divided into 3 sections: the duodenum (after the stomach), jejunum
and ileum. The duodenum receives 3 different secretions:

:   1\) **Bile** from the liver;
:   2\) **Pancreatic juice** from the pancreas and
:   3\) **Intestinal juice** from glands in the intestinal wall.

These complete the digestion of starch, fats and protein. The products
of digestion are absorbed into the blood and lymphatic system through
the wall of the intestine, which is lined with tiny finger-like
projections called **villi** that increase the surface area for more
efficient absorption (see diagram 11.10).

![](Anatomy_and_physiology_of_animals_Wall_of_small_intestine_showing_villi.jpg "Anatomy_and_physiology_of_animals_Wall_of_small_intestine_showing_villi.jpg")

Diagram 11.10 - The wall of the small intestine showing villi

## The Rumen

In ruminant herbivores like cows, sheep and antelopes the stomach is
highly modified to act as a "fermentation vat". It is divided into four
parts. The largest part is called the **rumen**. In the cow it occupies
the entire left half of the abdominal cavity and can hold up to 270
litres. The **reticulum** is much smaller and has a honeycomb of raised
folds on its inner surface. In the camel the reticulum is further
modified to store water. The next part is called the **omasum** with a
folded inner surface. Camels have no omasum. The final compartment is
called the **abomasum**. This is the 'true' stomach where muscular walls
churn the food and gastric juice is secreted (see diagram 11.11).

![](Anatomy_and_physiology_of_animals_The_rumen.jpg "Anatomy_and_physiology_of_animals_The_rumen.jpg")

Diagram 11.11 - The rumen

Ruminants swallow the grass they graze almost without chewing and it
passes down the oesophagus to the rumen and reticulum. Here liquid is
added and the muscular walls churn the food. These chambers provide the
main fermentation vat of the ruminant stomach. Here bacteria and
single-celled animals start to act on the cellulose plant cell walls.
These organisms break down the cellulose to smaller molecules that are
absorbed to provide the cow or sheep with energy. In the process, the
gases methane and carbon dioxide are produced. These cause the "burps"
you may hear cows and sheep making.

Not only do the micro-organisms break down the cellulose but they also
produce the **vitamins E, B and K** for use by the animal. Their
digested bodies provide the ruminant with the majority of its protein
requirements.

In the wild grazing is a dangerous activity as it exposes the herbivore
to predators. They crop the grass as quickly as possible and then when
the animal is in a safer place the food in the rumen can be regurgitated
to be chewed at the animal's leisure. This is 'chewing the cud' or
**rumination**. The finely ground food may be returned to the rumen for
further work by the microorganisms or, if the particles are small
enough, it will pass down a special groove in the wall of the oesophagus
straight into the omasum. Here the contents are kneaded and water is
absorbed before they pass to the abomasum. The abomasum acts as a
"proper" stomach and gastric juice is secreted to digest the protein.

## Large Intestine

The **large intestine** consists of the **caecum**, **colon** and
**rectum**. The chyme from the small intestine that enters the colon
consists mainly of water and undigested material such as cellulose
(fibre or roughage). In omnivores like the pig and humans the main
function of the colon is absorption of water to give solid faeces.
Bacteria in this part of the gut produce vitamins B and K.

The caecum, which forms a dead-end pouch where the small intestine joins
the large intestine, is small in pigs and humans and helps water
absorption. However, in rabbits, rodents and horses, the caecum is very
large and called the **functional caecum**. It is here that cellulose is
digested by micro-organisms. The **appendix**, a narrow dead end tube at
the end of the caecum, is particularly large in primates but seems to
have no digestive function.

## Functional Caecum

The caecum in the rabbit, rat and guinea pig is greatly enlarged to
provide a "fermentation vat" for micro-organisms to break down the
cellulose plant cell walls. This is called a **functional caecum** (see
diagram 11.12). In the horse both the caecum and the colon are enlarged.
As in the rumen, the large cellulose molecules are broken down to
smaller molecules that can be absorbed. However, the position of the
functional caecum after the main areas of digestion and absorption,
means it is potentially less effective than the rumen. This means that
the small molecules that are produced there can not be absorbed by the
gut but pass out in the faeces. The rabbit and rodents (and foals) solve
this problem by eating their own faeces so that they pass through the
gut a second time and the products of cellulose digestion can be
absorbed in the small intestine. Rabbits produce two kinds of faeces.
Softer night-time faeces are eaten directly from the anus and the harder
pellets you are probably familiar with, that have passed through the gut
twice.

![](Anatomy_and_physiology_of_animals_Gut_of_a_rabbit.jpg "Anatomy_and_physiology_of_animals_Gut_of_a_rabbit.jpg")

Diagram 11.12 - The gut of a rabbit

## The Gut Of Birds

Birds' guts have important differences from mammals' guts. Most
obviously, birds have a **beak** instead of teeth. Beaks are much
lighter than teeth and are an adaptation for flight. Imagine a bird
trying to take off and fly with a whole set of teeth in its head! At the
base of the oesophagus birds have a bag-like structure called a
**crop**. In many birds the crop stores food before it enters the
stomach, while in pigeons and doves glands in the crop secretes a
special fluid called **crop-milk** which parent birds regurgitate to
feed their young. The stomach is also modified and consists of two
compartments. The first is the true stomach with muscular walls and
enzyme secreting glands. The second compartment is the **gizzard**. In
seed eating birds this has very muscular walls and contains pebbles
swallowed by the bird to help grind the food. This is the reason why you
must always supply a caged bird with grit. In birds of prey like the
falcon the walls of the gizzard are much thinner and expand to
accommodate large meals (see diagram 11.13).

![](Anatomy_and_physiology_of_animals_Stomach_&_small_intestine_of_hen.jpg "Anatomy_and_physiology_of_animals_Stomach_&_small_intestine_of_hen.jpg")

Diagram 11.13 - The stomach and small intestine of a hen

## Digestion

During digestion the large food molecules are broken down into smaller
molecules by **enzymes**. The three most important groups of enzymes
secreted into the gut are:

:# **Amylases** that split carbohydrates like starch and glycogen into
monosaccharides like glucose.

:# **Proteases** that split proteins into amino acids.

:# **Lipases** that split lipids or fats into fatty acids and glycerol.

Glands produce various secretions which mix with the food as it passes
along the gut.

These secretions include:

:# **Saliva** secreted into the mouth from several pairs of **salivary
glands** (see diagram 11.3). Saliva consists mainly of water but
contains salts, mucous and salivary amylase. The function of saliva is
to lubricate food as it is chewed and swallowed and salivary amylase
begins the digestion of starch.

:# **Gastric juice** secreted into the stomach from glands in its walls.
Gastric juice contains **pepsin** that breaks down protein and
hydrochloric acid to produce the acidic conditions under which this
enzyme works best. In baby animals rennin to digest milk is also
produced in the stomach.

:# **Bile** produced by the liver. It is stored in the **gall bladder**
and secreted into the duodenum via the **bile duct** (see diagram
11.14). (Note that the horse, deer, parrot and rat have no gall
bladder). Bile is not a digestive enzyme. Its function is to break up
large globules of fat into smaller ones so the fat splitting enzymes can
gain access the fat molecules.

![](Anatomy_and_physiology_of_animals_Liver,_gall_bladder_&_pancreas.jpg "Anatomy_and_physiology_of_animals_Liver,_gall_bladder_&_pancreas.jpg")

Diagram 11.14 - The liver, gall bladder and pancreas

## Pancreatic juice

The **pancreas** is a gland located near the beginning of the duodenum
(see diagram 11.14). In most animals it is large and easily seen but in
rodents and rabbits it lies within the membrane linking the loops of the
intestine (the **mesentery**) and is quite difficult to find.
**Pancreatic juice** is produced in the pancreas. It flows into the
duodenum and contains **amylase** for digesting starch, **lipase** for
digesting fats and **protease** for digesting proteins.

## Intestinal juice

**Intestinal juice** is produced by glands in the lining of the small
intestine. It contains enzymes for digesting disaccharides and proteins
as well as mucus and salts to make the contents of the small intestine
more alkaline so the enzymes can work.

## Absorption

The small molecules produced by digestion are absorbed into the
**villi** of the wall of the **small intestine**. The tiny finger-like
projections of the villi increase the surface area for absorption.
Glucose and amino acids pass directly through the wall into the blood
stream by diffusion or active transport. Fatty acids and glycerol enter
vessels of the lymphatic system (**lacteals**) that run up the centre of
each villus.

## The Liver

The liver is situated in the abdominal cavity adjacent to the diaphragm
(see diagrams 2 and 14). It is the largest single organ of the body and
has over 100 known functions. Its most important digestive functions
are:

:# the production of **bile** to help the digestion of fats (described
above) and

:# the control of **blood sugar** levels

Glucose is absorbed into the capillaries of the villi of the intestine.
The blood stream takes it directly to the liver via a blood vessel known
as the **hepatic portal vessel** or **vein** (see diagram 11.15).

The liver converts this glucose into glycogen which it stores. When
glucose levels are low the liver can convert the glycogen back into
glucose. It releases this back into the blood to keep the level of
glucose constant. The hormone **insulin**, produced by special cells in
the **pancreas**, controls this process.

![](Anatomy_and_physiology_of_animals_Control_of_glucose_by_the_liver.jpg "Anatomy_and_physiology_of_animals_Control_of_glucose_by_the_liver.jpg")

Diagram 11.15 - The control of blood glucose by the liver

Other functions of the liver include:

:   3\. making **vitamin A**,
:   4\. making the **proteins** that are found in the **blood plasma**
    (**albumin, globulin** and **fibrinogen**),
:   5\. storing **iron**,
:   6\. removing **toxic substances** like alcohol and poisons from the
    blood and converting them to safer substances,
:   7\. producing **heat** to help maintain the temperature of the body.

![](Anatomy_and_physiology_of_animals_Summary_of_the_main_functions_of_the_different_regions_of_the_gut.jpg "Anatomy_and_physiology_of_animals_Summary_of_the_main_functions_of_the_different_regions_of_the_gut.jpg")

Diagram 11.16 - Summary of the main functions of the different regions
of the gut

## Summary

-   The **gut** breaks down plant and animal materials into nutrients
    that can be used by animals' bodies.
-   Plant material is more difficult to break down than animal tissue.
    The gut of **herbivores** is therefore longer and more complex than
    that of **carnivores**. Herbivores usually have a compartment (the
    **rumen** or **functional caecum**) housing micro-organisms to break
    down the **cellulose** wall of plants.
-   Chewing by the teeth begins the food processing. There are 4 main
    types of teeth: **incisors, canines, premolars** and **molars**. In
    dogs and cats the premolars and molars are adapted to slice against
    each other and are called **carnassial** teeth.
-   **Saliva** is secreted in the mouth. It lubricates the food for
    swallowing and contains an enzyme to break down starch.
-   Chewed food is swallowed and passes down the **oesophagus** by waves
    of contraction of the wall called **peristalsis.** The food passes
    to the stomach where it is churned and mixed with acidic **gastric
    juice** that begins the digestion of protein.
-   The resulting **chyme** passes down the small intestine where
    enzymes that digest fats, proteins and carbohydrates are secreted.
    **Bile** produced by the liver is also secreted here. It helps in
    the breakdown of fats. **Villi** provide the large surface area
    necessary for the absorption of the products of digestion.
-   In the **colon** and **caecum** water is absorbed and micro
    organisms produce some **vitamin B and K**. In rabbits, horses and
    rodents the caecum is enlarged as a **functional caecum** and
    micro-organisms break down cellulose cell walls to simpler
    carbohydrates. Waste products exit the body via the **rectum** and
    **anus**.
-   The **pancreas** produces **pancreatic juice** that contains many of
    the enzymes secreted into the small intestine.
-   In addition to producing bile the liver regulates blood sugar levels
    by converting glucose absorbed by the villi into glycogen and
    storing it. The liver also removes toxic substances from the blood,
    stores iron, makes vitamin A and produces heat.

## Worksheet

Use the Digestive System
Worksheet to
help you learn the different parts of the digestive system and their
functions.

## Test Yourself

Then work through the Test Yourself below to see if you have understood
and remembered what you learned.

1\. Name the four different kinds of teeth

2\. Give 2 facts about how the teeth of cats and dogs are adapted for a
carnivorous diet:

:   1\.
:   2\.

3\. What does saliva do to the food?

4\. What is peristalsis?

5\. What happens to the food in the stomach?

6\. What is chyme?

7\. Where does the chyme go after leaving the stomach?

8\. What are villi and what do they do?

9\. What happens in the small intestine?

10\. Where is the pancreas and what does it do?

11\. How does the caecum of rabbits differ from that of cats?

12\. How does the liver help control the glucose levels in the blood?

13\. Give 2 other functions of the liver:

:   1\.
:   2\.

/Test Yourself Answers/

## Websites

-   <http://www.second-opinions.co.uk/carn_herb_comparison.html> Second
    opinion. A good comparison of the guts of carnivores and herbivores
-   <http://www.chu.cam.ac.uk/~ALRF/giintro.htm> The gastrointestinal
    system. A good comparison of the guts of carnivores and herbivores
    with more advanced information than in the previous site.
-   <http://www.westga.edu/~lkral/peristalsis/index.html> Peristalsis
    animation.
-   <http://en.wikipedia.org/wiki/Digestion> Wikipedia on digestion with
    links to further information on most aspects. Like most websites
    this is mainly about human digestion but much is applicable to
    animals.

## Glossary

-   Link to
    Glossary

