ANS 222:
ANATOMY AND PHYSIOLOGY
LECTURE
4: Comparative anatomy and Physiology of Urinary system
Objectives
After completing this section, you should know:
- 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
The cells of an animal can only remain healthy if
the conditions are just right. The processes that take place in them are upset
if the temperature is too high or too low, or if the fluid around or inside
them is too acid or alkaline. Homeostasis is the name given to the
processes that help keep the internal conditions constant even when external
conditions change. The word means, “Staying the same”.
There are
a number of organs in the body that play a part in maintaining homeostasis. For
example, the skin helps keep the internal temperature of bird and mammals
bodies within a narrow range even when the outside temperatures change; the
lungs control the amount of carbon dioxide in the blood; the liver and pancreas
work together to keep the amount of glucose in the blood within narrow limits
and the kidneys regulate the acidity and the concentration of water and salt in
the blood.
Hormones are chemicals that carry messages around the body
in the blood and are central to many of the homeostatic processes.
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.
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.
Desert Animals/Camel
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.
Diagram - The position of the
organs of the urinary system in mammal
The Kidneys and Urinary System
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. An artery from the dorsal aorta
called the renal artery supplies blood to them and the renal vein
drains them.
Diagram - The urinary system
Kidneys
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.
Diagram - The dissected kidney
Kidney Tubules or Nephrons
The cortex and medulla are seen to be composed of
masses of tiny tubes. These are called kidney tubules or nephrons.
Diagram - Several kidney tubules or nephrons
Diagram - A kidney tubule or nephron
Bowman’s or renal)
capsule
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. 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).
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. 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.
Diagram - 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.
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.
Diagram - 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. 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.
Other Functions of the Kidney
The excretion of urea from the body and the
maintenance of water balance. 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.
·
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.