# Anatomy and Physiology of Animals/Urinary System

## Objectives

After completing this section, you should know:

-   Understand the parts of the urinary system.
-   The structure and function of a kidney.
-   The structure and function of a kidney tubule or nephron.
-   The processes of filtration, reabsorption, secretion and
    concentration that convert blood to urine in the kidney tubule.
-   The function of antidiuretic hormone in producing concentrated
    urine.
-   The composition, storage and voiding of normal urine.
-   Abnormal constituents of urine and their significance.
-   The functions of the kidney in excreting nitrogenous waste,
    controlling water levels and regulating salt concentrations and
    acid-base balance.
-   That birds do not have a bladder.

## Homeostasis

It is defined as the processes in which the animals or humans regulate
their internal temperature. Homeostasis is the maintenance of a stable
internal environment. Homeostasis is a term coined in 1959 to describe
the physical and chemical parameters that an organism must maintain to
allow proper functioning of its component cells, tissues, organs, and
organ systems.

Recall that enzymes function best when within a certain range of
temperature and pH, and that cells must strive to maintain a balance
between having too much or too little water in relation to their
external environment. Both situations demonstrate homeostasis. Just as
we have a certain temperature range (or comfort zone), so our body has a
range of environmental (internal as well as external) parameters within
which it works best. Multicellular organisms accomplish this by having
organs and organ systems that coordinate their homeostasis. In addition
to the other functions that life must perform (recall the discussion in
our Introduction chapter), unicellular creatures must accomplish their
homeostasis within but a single cell!

Single-celled organisms are surrounded by their external environment.
They move materials into and out of the cell by regulation of the cell
membrane and its functioning. Most multicellular organisms have most of
their cells protected from the external environment, having them
surrounded by an aqueous internal environment. This internal environment
must be maintained in such a state as to allow maximum efficiency. The
ultimate control of homeostasis is done by the nervous system. Often
this control is in the form of negative feedback loops. Heat control is
a major function of homeostatic conditions that involves the integration
of skin, muscular, nervous, and circulatory systems.

The difference between homeostasis as a single cell performs it and what
a multicelled creature does derives from their basic organizational
plan: a single cell can dump wastes outside the cell and just be done
with it. Cells in a multicelled creature, such as a human or cat, also
dump wastes outside those cells, but like the trash can or dumpster
outside my house/apartment, those wastes must be carted away. The
carting away of these wastes is accomplished in my body by the
circulatory system in conjunction with the excretory system. For my
house, I have the City of Phoenix sanitation department do that (and get
to pay each month for their service!).

The ultimate control of homeostasis is accomplished by the nervous
system (for rapid responses such as reflexes to avoid picking up a hot
pot off the stove) and the endocrine system (for longer-term responses,
such as maintaining the body levels of calcium, etc.). Often this
homeostatic control takes the form of negative feedback loops. There are
two types of biological feedback: positive and negative. Negative
feedback turns off the stimulus that caused it in the first place. Your
house's heater (or cooler for those of us in the Sun Belt) acts on the
principle of negative feedback. When your house cools off below the
temperature set by your thermostat, the heater is turned on to warm air
until the temperature is at or above what the thermostat is set at. The
thermostat detects this rise in temperature and sends a signal to shut
off the heater, allowing the house to cool of until the heater is turned
on yet again and the cycle (or loop) continues.

## Water In The Body

Water is essential for living things to survive because all the chemical
reactions within a body take place in a solution of water. An animal's
body consists of up to 80% water. The exact proportion depends on the
type of animal, its age, sex, health and whether or not it has had
sufficient to drink. Generally animals do not survive a loss of more
than 15% of their body water.

In vertebrates almost 2/3rd of this water is in the cells
(**intracellular fluid**). The rest is outside the cells
(**extracellular fluid**) where it is found in the spaces around the
cells (**tissue fluid**), as well as in the blood and lymph. Water is
considered to be the source of life. It is important for animal life
because of the following reasons:

\(i\) Water is vital body fluid which is essential for regulating the
processes such as , digestion , transport of nutrients and excretion.
Water dissolves ionic and large number of polar organic compounds. Thus,
it transports the products of digestion to the place of requirement of
the body.

\(ii\) Water regulates the body temperature by the process of sweating
and evaporation.

\(iii\) Water is a medium for all metabolic reactions in the body. All
metabolic reactions in the body take place in solution phase.

\(iv\) Water provides habitat for various animals in the form of ponds
and rivers, sea, etc.

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

Diagram 12.1 - Water in the body

## Maintaining Water Balance

Animals lose water through their skin and lungs, in the faeces and
urine. These losses must be made up by water in food and drink and from
the water that is a by-product of chemical reactions. If the animal does
not manage to compensate for water loss the dissolved substances in the
blood may become so concentrated they become lethal. To prevent this
happening various mechanisms come into play as soon as the concentration
of the blood increases. A part of the brain called the **hypothalamus**
is in charge of these homeostatic processes. The most important is the
feeling of thirst that is triggered by an increase in blood
concentration. This stimulates an animal to find water and drink it.

The kidneys are also involved in maintaining water balance as various
hormones instruct them to produce more concentrated urine and so retain
some of the water that would otherwise be lost (see later in this
Chapter and Chapter 16).

### Desert Animals

Coping with water loss is a particular problem for animals that live in
dry conditions. Some, like the camel, have developed great tolerance for
dehydration. For example, under some conditions, camels can withstand
the loss of one third of their body mass as water. They can also survive
wide daily changes in temperature. This means they do not have to use
large quantities of water in sweat to cool the body by evaporation.

Smaller animals are more able than large ones to avoid extremes of
temperature or dry conditions by resting in sheltered more humid
situations during the day and being active only at night.

The kangaroo rat is able to survive without access to any drinking water
at all because it does not sweat and produces extremely concentrated
urine. Water from its food and from chemical processes is sufficient to
supply all its requirements.

## Excretion

Animals need to excrete because they take in substances that are excess
to the body's requirements and many of the chemical reactions in the
body produce waste products. If these substances were not removed they
would poison cells or slow down metabolism. All animals therefore have
some means of getting rid of these wastes.

The major waste products in mammals are carbon dioxide that is removed
by the lungs, and urea that is produced when excess amino acids (from
proteins) are broken down. Urea is filtered from the blood by the
kidneys.

![](Urinary_System_of_Dog.JPG‎ "Urinary_System_of_Dog.JPG‎")

Diagram 12.2 - The position of the organs of the urinary system in a dog

BY GIZAW MEKONNEN

\'\'\'The Kidneys And Urinary System==

The urinary system, also known as the renal system or urinary tract,
consists of the kidneys, ureters, bladder, and the urethra. The purpose
of the urinary system is to eliminate waste from the body, regulate
blood volume and blood pressure, control levels of electrolytes and
metabolites, and regulate blood pH. The urinary tract is the body\'s
drainage system for the eventual removal of urine.\[1\] The kidneys have
an extensive blood supply via the renal arteries which leave the kidneys
via the renal vein. Each kidney consists of functional units called
nephrons. Following filtration of blood and further processing, wastes
(in the form of urine) exit the kidney via the ureters, tubes made of
smooth muscle fibres that propel urine towards the urinary bladder,
where it is stored and subsequently expelled from the body by urination
(voiding). The female and male urinary system are very similar,
differing only in the length of the urethra.

Urine is formed in the kidneys through a filtration of blood. The urine
is then passed through the ureters to the bladder, where it is stored.
During urination, the urine is passed from the bladder through the
urethra to the outside of the body.

800--2,000 milliliters (mL) of urine are normally produced every day in
a healthy human. This amount varies according to fluid intake and kidney
function.

The **kidneys** in mammals are bean-shaped organs that lie in the
abdominal cavity attached to the dorsal wall on either side of the spine
(see diagram 12.2). An artery from the dorsal aorta called the **renal
artery** supplies blood to them and the **renal vein** drains them.

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

Diagram 12.3 - The urinary system

To the naked eye kidneys seem simple enough organs. They are covered by
a fibrous coat or capsule and if cut in half lengthways (longitudinally)
two distinct regions can be seen - an inner region or **medulla** and
the outer **cortex**. A cavity within the kidney called the **pelvis**
collects the urine and carries it to the **ureter**, which connects with
the **bladder** where the urine is stored temporarily. Rings of muscle
(**sphincters**) control the release of urine from the bladder and the
urine leaves the body through the **urethra** (see diagrams 12.3 and
12.4).

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

Diagram 12.4 - The dissected kidney

## Kidney Tubules Or Nephrons

It is only when you examine kidneys under the microscope that you find
that their structure is not simple at all. The cortex and medulla are
seen to be composed of masses of tiny tubes. These are called **kidney
tubules** or **nephrons** (see diagrams 12.5 and 12.6). A human kidney
consists of over a million of them.

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

Diagram 12.5 - Several kidney tubules or nephrons

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

Diagram 12.6 - A kidney tubule or nephron

At one end of each nephron, in the cortex of the kidney, is a cup shaped
structure called the (**Bowman's** or **renal**) **capsule**. It
surrounds a tuft of capillaries called the **glomerulus** that carries
high-pressure blood. Together the glomerulus and capsule act as a
blood-filtering device (see diagram 12.7). The holes in the filter allow
most of the contents of the blood through except the _red and
white cells_ and _large protein
molecules_. The fluid flowing from the capsule into the rest
of the kidney tubule is therefore very similar to blood plasma and
contains many useful substances like water, glucose, salt and amino
acids. It also contains waste products like **urea**.

### Processes Occurring In The Nephron

After entering the glomerulus the filtered fluid flows along a coiled
part of the tubule (the **proximal convoluted tubule**) to a looped
portion (the **Loop of Henle**) and then to the **collecting tube** via
a second length of coiled tube (the **distal convoluted tubule**) (see
diagram 12.6). From the collecting ducts the urine flows into the
**renal pelvis** and enters the **ureter**.

Note that the glomerulus, capsule and both coiled parts of the tubule
are all situated in the cortex of the kidney while the loops of Henle
and collecting ducts make up the medulla (see diagram 12.5).

As the fluid flows along the proximal convoluted tubule useful
substances like glucose, water, salts, potassium ions, calcium ions and
amino acids are **reabsorbed** into the blood capillaries that form a
network around the tubules. Many of these substances are transported by
active transport and energy is required.

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

Diagram 12.7 - Filtration in the glomerulus and capsule

In a separate process, some substances, particularly potassium, ammonium
and hydrogen ions, and drugs like penicillin, are actively **secreted**
into the distal convoluted tubule.

By the time the fluid has reached the collecting ducts these processes
of absorption and secretion have changed the fluid originally filtered
into the Bowman's capsule into urine. The main function of the
collecting ducts is then to remove more water from the urine if
necessary. These processes are summarised in diagram 12.8.

**Normal urine** consists of water, in which waste products such as urea
and salts such as sodium chloride are dissolved. Pigments from the
breakdown of red blood cells give urine its yellow colour.

### The Production Of Concentrated Urine

Because of the high pressure of the blood in the glomerulus and the
large size of the pores in the glomerulus/capsule-filtering device, an
enormous volume of fluid passes into the kidney tubules. If this fluid
were left as it is, the animal's body would be drained dry in 30
minutes. In fact, as the fluid flows down the tubule, over 90% of the
water in it is reabsorbed. The main part of this reabsorption takes
place in the collecting tubes.

The amount of water removed from the collecting ducts is controlled by a
hormone called **antidiuretic hormone (ADH)** produced by the
**pituitary gland**, situated at the base of the brain. When the blood
becomes more concentrated, as happens when an animal is deprived of
water, ADH is secreted and causes more water to be absorbed from the
collecting ducts so that concentrated urine is produced. When the animal
has drunk plenty of water and the blood is dilute, no ADH is secreted
and no or little water is absorbed from the collecting ducts, so dilute
urine is produced. In this way the concentration of the blood is
controlled precisely.

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

Diagram 12.8 - Summary of the processes involved in the formation of
urine

## Water Balance In Fish And Marine Animals

### Fresh Water Fish

Although the skin of fish is more or less waterproof, the gills are very
porous. The body fluids of fish that live in fresh water have a higher
concentration of dissolved substances than the water in which they swim.
In other words the body fluids of fresh water fish are **hypertonic** to
the water (see chapter 3). Water therefore flows into the body by
**osmosis**. To stop the body fluids being constantly diluted fresh
water fish produce large quantities of dilute urine.

### Marine Fish

Marine fish like the sharks and dogfish have body fluids that have the
same concentration of dissolved substances as the water (**isotonic**)
have little problem with water balance. However, marine bony fish like
red cod, snapper and sole, have body fluids with a lower concentration
of dissolved substances than seawater (they are **hypotonic** to
seawater). This means that water tends to flow out of their bodies by
osmosis. To make up this fluid loss they drink seawater and get rid of
the excess salt by excreting it from the gills.

### Marine Birds

Marine birds that eat marine fish take in large quantities of salt and
some only have access to seawater for drinking. Bird's kidneys are
unable to produce very concentrated urine, so they have developed a salt
gland. This excretes a concentrated salt solution into the nose to get
rid of the excess salt.

## Diabetes And The Kidney

There are two types of diabetes. The most common is called sugar
diabetes or **diabetes mellitus** and is common in cats and dogs
especially if they are overweight. It is caused by the pancreas
secreting insufficient **insulin**, the hormone that controls the amount
of glucose in the blood. If insulin secretion is inadequate, the
concentration of glucose in the blood increases. Any increase in the
glucose in the blood automatically leads to an increase in glucose in
the fluid filtered into the kidney tubule. Normally the kidney removes
all the glucose filtered into it, but these high concentrations swamp
this removal mechanism and urine containing glucose is produced. The
main symptoms of this type of diabetes are the production of large
amounts of dilute urine containing glucose, and excessive thirst.

The second type of diabetes is called **diabetes insipidus**. The name
comes from the main symptom, which is the production of large amounts of
very dilute and "tasteless" urine. It occurs when the pituitary gland
produces insufficient ADH, the hormone that stimulates water
re-absorption from the kidney tubule. When this hormone is lacking,
water is not absorbed and large amounts of dilute urine are produced.
Because so much water is lost in the urine, animals with this form of
diabetes can die if deprived of water for only a day or so.

## Other Functions Of The Kidney

The excretion of urea from the body and the maintenance of water
balance, as described above, are the main functions of the kidney.
However, the kidneys have other roles in keeping conditions in the body
stable i.e. in maintaining homeostasis. These include:

:\* controlling the concentration of salt ions (Na+, K+, Cl-) in the
blood by adjusting how much is excreted or retained;

:\* maintaining the correct acidity of the blood. Excess acid is
constantly being produced by the normal chemical reactions in the body
and the kidney eliminates this.

## Normal Urine

Normal urine consists of water (95%), urea, salts (mostly sodium
chloride) and pigments (mostly from bile) that give it its
characteristic colour.

## Abnormal Ingredients Of Urine

If the body is not working properly, small amounts of substances not
normally present may be found in the urine or substances normally
present may appear in abnormal amounts.

:\* The presence of **glucose** may indicate diabetes (see above).

:\* Urine with red blood cells in it is called **haematuria**, and may
indicate inflammation of the kidney,or urinary tract, cancer or a blow
to the kidneys.

:\* Sometimes free **haemoglobin** is found in the urine. This indicates
that the red blood cells in the blood have **haemolysed** (the membrane
has broken down) and the haemoglobin has passed into the kidney tubules.

:\* The presence of **white blood cells** in the urine indicates there
is an infection in the kidney or urinary tract.

:\* **Protein molecules** are usually too large to pass into the kidney
tubule so no or only small amounts of proteins like **albumin** is
normally found in urine. Large quantities of albumin indicate that the
kidney tubules have been injured or the kidney has become diseased. High
blood pressure also pushes proteins from the blood into the tubules.

:\* **Casts** are tiny cylinders of material that have been shed from
the lining of the tubules and flushed out into the urine.

:\* **Mucus** is not usually found in the urine of healthy animals but
is a normal constituent of horses' urine, giving it a characteristic
cloudy appearance.

Tests can be carried out to identify any abnormal ingredients of urine.
These tests are normally done by "**stix**", which are small plastic
strips with absorbent ends impregnated with various chemicals. A colour
change occurs in the presence of an abnormal ingredient.

## Excretion In Birds

Birds' high body temperature and level of activity means that they need
to conserve water. Birds therefore do not have a bladder and instead of
excreting urea, which needs to be dissolved in large amounts of water,
birds produce uric acid that can be discharged as a thick paste along
with the feces. This is the white chalky part of the bird droppings that
land on you or your car.

CONCLUSION

-   The excretory system consists of paired **kidneys** and associated
    blood supply. **Ureters** transport urine from the kidneys to the
    bladder and the **urethra** with associated sphincter muscles
    controls the release of urine.
-   The kidneys have an important role in maintaining **homeostasis** in
    the body. They excrete the waste product urea, control the
    concentrations of water and salt in the body fluids, and regulate
    the acidity of the blood.
-   A kidney consists of an outer region or **cortex,** inner
    **medulla** and a cavity called the **pelvis** that collects the
    urine and carries it to the ureter.
-   The tissue of a kidney is composed of masses of tiny tubes called
    **kidney tubules** or **nephrons**. These are the structures that
    make the urine.
-   High-pressure blood is supplied to the nephron via a tuft of
    capillaries called the **glomerulus**. Most of the contents of the
    blood except the cells and large protein molecules filter from the
    glomerulus into the (**Bowmans) capsule.** This fluid flows down a
    coiled part of the tubule (**proximal convoluted tubule**) where
    useful substances like glucose, amino acids and various ions are
    reabsorbed. The fluid flows to a looped portion of the tubule called
    the **Loop of Henle** where water is reabsorbed and then to another
    coiled part of the tubule (**distal convoluted tubule**) where more
    reabsorbtion and secretion takes place. Finally the fluid passes
    down the **collecting duct** where water is reabsorbed to form
    concentrated urine.

`                                                                                                               BY GIZAW MEKONNEN`

## Worksheet

Use this Excretory System
Worksheet to
help you learn the parts of the urinary system, the kidney and kidney
tubule and their functions.

## Test Yourself

The Urinary System Test Yourself can then be used to see if you
understand this rather complex system.

1\. Add the following labels to the diagram of the excretory system
shown below. Bladder \| ureter \| urethra \| kidney \| dorsal aorta \|
vena cava \| renal artery \| vein

![](Anatomy_and_physiology_of_animals_diagram_of_urinary_system_unlabeled.JPG "Anatomy_and_physiology_of_animals_diagram_of_urinary_system_unlabeled.JPG")

2\. Using the words/phrases in the list below fill in the blanks in the
following statements.

:   

    :   \| cortex \| amino acids \| renal \| glucose \| water
        reabsorption \| large proteins \|
    :   \| bowman's capsule \| diabetes mellitus \| secreted \|
        antidiuretic hormone (ADH) \| blood cells \|
    :   \| glomerulus \| concentration of the urine \| medulla \|
        nephron \|

a\) Blood enters the kidney via the \...\...\...\...\...\...\...\....
artery.

b\) When cut across the kidney is seen to consist of two regions, the
outer\...\...\...\..... and the inner\...\...\...\.....

c\) Another word for the kidney tubule is
the\...\...\...\...\...\...\...\...\...\....

d\) Filtration of the blood occurs in
the\...\...\...\...\...\...\...\...\...\...

e\) The filtered fluid (filtrate) enters
the\...\...\...\...\...\...\...\...\.....

f\) The filtrate entering the e) above is similar to blood but does not
contain\...\...\...\...\...\... or\...\...\...\...\...\.....

g\) As the fluid passes along the first coiled part of the kidney
tubule\...\...\...\...\...\... and\...\...\...\...\...\..... are
removed.

h\) The main function of the loop of Henle
is\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\....

i\) Hydrogen and potassium ions
are\...\...\...\...\...\...\...\...\...\... into the second coiled part
of the tubule.

j\) The main function of the collecting tube
is\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....

k\) The hormone\...\...\...\...\...\...\...\...\...\...\...\..... is
responsible for controlling water reabsorption in the collecting tube.

**Write short answer for following question** l) When the pancreas
secretes inadequate amounts of the hormone insulin the condition known
as\...\...\...\...\...\...\...\...\...\.... results. This is most easily
diagnosed by testing for\...\...\...\...\...\...\...\...\...\..... in
the urine.What is Homeostasis?

2\. Give 2 examples of homeostasis.

3\. List 3 ways in which animals keep their body temperature constant
when the weather is hot.

4\. How does the kidney compensate when an animal is deprived of water
to drink?

6\. Describe how panting helps to reduce the acidity of the blood.

/Test Yourself Answers/

## Websites

-   <http://www.biologycorner.com/bio3/nephron.html> Biology Corner. A
    fabulous drawing of the kidney and nephron to print off, label and
    colour in with clear explanation of function.
-   <http://health.howstuffworks.com/adam-200032.htm> How Stuff Works.
    This animation traces the full process of urine formation and
    reabsorption in the kidneys, its path down the ureter to the
    bladder, and its excretion via the urethra. Needs Shockwave.
-   <http://en.wikipedia.org/wiki/Nephron> Wikipedia. A bit more detail
    than you need but still good clear explanations and lots of
    information.

## Glossary

-   Link to
    Glossary

