(a) What is osmoregulation and
why is it needed?
Homeostasis control in the human body
- regulating water content
Homeostasis is a word that is
sometimes used to describe your bodily functions that try to maintain a
stable constant internal environment including the factors listed above.
Know that internal conditions that are
controlled in the body include water control, ion and urea concentrations.
Know and understand that water and
ion content, body temperature and blood glucose levels must be kept within
very narrow ranges.
Know and understand that waste products
that have to be removed from the body include:
Carbon dioxide, produced by
respiration and removed via the lungs when we breathe out.
Urea, produced in the liver
by the breakdown of amino acids and removed by the kidneys in the urine,
which is temporarily stored in the bladder.
The body's control of water content
and ion concentrations is called osmoregulation.
It is most important that the
water content of the blood is controlled to keep cells functioning
correctly.
Body cells are surrounded by
tissue fluid.
Tissue fluid usually has a
different water potential to the fluid inside cells (mainly the
cytoplasm).
Tissue fluid moves out of the
blood capillaries to supply all the needs of cells.
If the water potentials are
different, then water will move in or out of cells by osmosis.
If the water concentration in the
tissue fluid is too high, water is diffuses into the cells by
osmosis because of the higher water potential of the tissue fluid -
this is because the concentration of dissolved substances in the
cells is greater than that in the tissue fluid.
If too much water is absorbed the cells can burst and die
quite easily (the process is called lysis) - they don't have rigid cell walls like plants, which
can withstand much greater changes in water concentration changes.
If the water concentration of the
blood is too low, water is lost from cells by osmosis causing the
cells to shrink and possibly malfunctioning.
In other words if the water
potential of the cells is higher than the surrounding fluid,
water will diffuse out of the cells by osmosis - this is because
the concentration of dissolved substances in the tissue fluids
is greater than that in the cells.
If the water potential of the
cell fluids equals the water potential of the surrounding fluids
(similar concentrations of dissolved substances), there is no net
transfer of water and the cells are quite stable, won't change size
and can function normally.
Therefore it is obviously
important to keep the water content of the body at the right level
to keep the cells in a 'healthy' state and functioning correctly.
Even the loss of a few % of
our body's weight in water can make us feel thirsty and
uncomfortable - mild dehydration.
With ~5% water loss we lose
our ability to function properly e.g. doing our job at work.
If the water loss of the
body's weight reaches over 10%, the situation is life
threatening.
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and sub-index
(b)
The three functions of the kidneys
The
kidneys are bean-shaped organs found in the abdomen, one on either side
of the aorta.
The kidneys are really important in
...
(i) maintaining water balance by helping to regulate the water content of the body by controlling
how much water is reabsorbed and how much is lost in urine.
(ii) The kidneys are also an important
high pressure 'filtration' system for removing unwanted materials and waste products out
of the blood passing through the kidneys, which would otherwise cause harmful effects on the body if
not removed.
The kidneys filter out these
substances as the blood through - hence the process name of filtration.
Glucose, ions and other useful
substances and appropriate amount of water are re-absorbed back into the
blood stream - this process is called selective reabsorption.
This process is essential to
adjust the levels of ions to their appropriate concentration.
(iii) What does the kidney remove to maintain
the right balance of various substances?
After the filtration, the
principal waste substance removed by the body in
the urine is urea from the blood.
Urea is made from excess
nitrogen/amino acids from metabolised food and is less toxic than
ammonia.
The details of the processes
involved are described in the next section.
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and sub-index
(c) Some
details on how the kidney works in conjunction with the urinary system
The urinary system (simplified
diagram on the right)
The 'dirty' blood' i.e. 'used'
blood that need's processing enters the kidneys from the renal
artery - it is also supplying the kidneys with oxygen.
The purified or 'cleaned' blood
exits the kidney from the renal vein, but it is also depleted
in oxygen - used by the kidney cells respiring to power all the
filtration and transport processes.
Apart from the processed blood,
the waste fluid from the kidney filtration process, urine,
containing mainly urea, passes into the ureter tube
and stored in the bladder prior to excretion.
Nephrons and the kidney ultrafiltration
system
How does the kidney filter out
the 'unwanted stuff' and reabsorb useful substances still in the
blood?
It's extremely complicated, and I
hope my 'simplified' diagram plus text explain how it all works!
The out structure of the kidney,
namely the cortex and medulla, are not shown on the above diagram.
The part of the kidney that does
the actual separation is called a nephron - the filtration
unit.
Nephrons are sometimes called
kidney tubules - there are million of them in each
kidney!
Nephrons are the functional
units in the kidney, each consisting of a glomerulus,
Bowman's capsule and
its associated very long thin tubule, through which the
glomerulus filtrate passes before emerging as urine.
Overall the nephron is like a
long thin
tubular membrane surrounded by tiny arteries of blood - into
which reabsorbed materials will pass. - one grey 'squiggly'
artery is shown on the diagram.
A glomerulus is a tiny
ball-shaped structure composed of capillary blood vessels
actively involved in the filtration of the blood to form
urine. The glomerulus is one of the key structures that make up
the nephron, the functional unit of the kidney.
Bowman's capsule is a
cup-like sack at the beginning of the tubular component of a
nephron in the mammalian kidney that performs the first step
in the filtration of blood to form urine. A glomerulus is
enclosed in the sac.
The parts of the nephron
after the renal artery are as follows (not shown in detail on
the above simplified diagram):
tiny arteries of the
glomerulus membrane sac ==> Bowman's capsule (filter) ==>
proximal convoluted loop ==> loop of Hele ==> distal
convoluted loop (reabsorption sections)
In the nephron loop
sections the selective reabsorption take place.
Adaptation note:
Animals inhabiting areas which suffer from drought
conditions have an extended Henle loop (part of a nephron)
to reabsorb more water.
(i) The liquid part of the blood
from the renal artery, contains glucose, ions (e.g. sodium Na+
from salt - sodium chloride), water, urea and other
substances, is forced and filtered under high pressure through the glomerulus
into the tissue of the Bowman's capsule - this is where the main filtration
process occurs.
Larger molecules like
proteins or even larger blood cells cannot pass through the
membranes of the Bowman's capsule into the extended length of
the nephron - the residue remains in the bloodstream - the tiny
arteries of the glomerulus - this is the
ultrafiltration
process.
The smaller molecules/ions
like water, sodium chloride, urea and sugars pass through
the membrane filters of the Bowman's capsule from the blood into
the nephron of the kidney.
(ii) As the filtered liquid flows
through the nephron useful substances are
selectively reabsorbed.
All the glucose is selectively
reabsorbed back into the bloodstream against the
concentration gradient - so energy is needed for 'active
transport', since spontaneous diffusion will not take place in
the desired direction for reabsorption.
Any residual ions
required
are also reabsorbed back into the bloodstream e.g.
sufficient salt that is needed, any excess is passed on and eventually
excreted in urine.
Water is reabsorbed, but
the amount dictated by the level of the
ADH
hormone. (feedback system described in a later
section on this page).
Adaptation note: Animals
living in very dry regions, prone to drought, have longer
loops of Henle to reabsorb more water.
(iii) The residual waste substances
e.g. excess water, excess ions and urea,
which are not reabsorbed, flow out of the nephrons as urine, through
collecting ducts which all merge into the ureter and down into the
storage bladder. The urine is eventually excreted from
the bladder through the urethra.
|
Comparison table of typical
concentrations of substances in the blood, kidney tubule
filtrate and urine (g/100 cm3) |
|
Substance |
blood plasma |
kidney tubule filtrate |
urine |
|
water, H2O |
~92 |
~99 |
~96 |
|
proteins |
7.0 |
0.0
(2) |
0.0
(2) |
|
glucose, C6H12O6 |
0.1 |
0.1
(3) |
0.0
(1) |
|
urea, O=C(NH2)2 |
0.03 |
0.03 |
2.0 |
|
ions, e.g. Na+, K+,
Cl- |
0.75 e.g. 0.3 Na+ |
0.75 e.g. 0.3 Na+
(3) |
1.5 e.g. 0.6 Na+ |
Notes:
(1) Glucose might not be zero for a diabetic person.
(2)
The large protein molecules have been filtered out.
(3)
Useful small molecules like glucose and 'non-excess' ions are
reabsorbed.
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and sub-index
(d) More on controlling
water content - water input and output
"You can survive for several weeks
without food, but only a few days without water"!
Water is extremely important for a
healthy life for the following reasons ....
-
65% of our body is water, its
in every cell and makes up most of the blood.
-
It is a versatile solvent -
nutrients like glucose, salts, amino acids are all soluble in
blood, carbon dioxide and oxygen are exchanged via blood and
other waste products like urea are dissolved and removed in
urine.
-
It is essential for enzyme
action - water is the medium for their substrates and products.
-
Water has a specific high
heat capacity, that is important for temperature regulation -
thermoregulation.
Typical daily human input and
output of volumes of water - which must be constantly kept in balance
Volume taken in cm3:
drink 1050; in food 600; from respiration 250; total 1900
cm3
Volume given out cm3:
urine 1000; exhaled air 350; skin 400; faeces 150;
total 1900 cm3
There is considerable
variation in these values from person to person and textbook to
textbook!
If the water or
ion content of the body is wrong, too much water may move into or out of the
cells and damage them.
If your blood becomes too
concentrated in e.g. ions (salts), water is drawn out of the cells and tissues by osmotic
action and they become dehydrated. Under these circumstances enzymes
cease to work efficiently and so do the cells.
If the concentration of e.g.
sodium ions in the blood increases, the concentration of water
in the blood decreases too. The brain will detect the blood
needs more water and the pituitary gland releases more ADH, so
more water will be reabsorbed. from the collecting ducts in the
kidney, so the water content in the blood will rise. See
ADH negative feedback
mechanism for water balance
If the blood becomes too dilute
the reverse osmotic action happens. Water will collect in the
tissues and the cells swell up - again a situation of imbalance.
The water content of the blood is
continually influenced by the temperature around us (how much we
sweat), our diet and the amount of water we lose (see below).
The water content of the body –
our skin keeps us waterproof but water leaves the body
via ...
(i) the lungs when we breathe out
(you see the condensed water from our breath in the winter),
(ii) the skin when we sweat to cool us
down,
(iii) excess water is lost via the kidneys in the urine,
(iv) and in our faeces.
and this 'output' is balanced by
the 'input' of water we take in from food and drinks,
BUT, we can't control how much we
lose in the ways described above, so we need a balancing system
between the amount of water we consume and the amount of water
removed by the kidneys in urine.
We need to, and are continually
taking in water via drinks and food.
Any loss needs to be replaced,
more so in the summer when we sweat more than in the winter.
On cold days you sweat less and
pass more pale dilute urine, and on warm days you sweat more and pass more
darker coloured more concentrated urine (assuming a similar fluid intake
each day), either way the water balance is maintained.
After vigorous exercise (keeping
fit or sport) you will have lost water and ions through sweat as well as
burning up more glucose than usual.
Sports drinks contain water, ions and
sugar to replace those lost but there are many products on the market
competing for this lucrative revenue stream and each claiming to be just the
right drink to take!
I gather from a TV program using
a cheap, 'to dilute' fruit juice drink with a spoonful of sugar
dissolved in it,
is just as effective - but my doctorate is in chemistry!
The control of the body's water
balance via the ADH hormone negative feedback mechanism is described in the
next section.
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and sub-index
(e) Hormone controlled concentration of the blood and urine -
ADH negative feedback
mechanism
Urine volume and concentration is
regulated through the same processes that regulate blood volume.
Antidiuretic hormone (ADH), produced by the pituitary gland, increases the
amount of water reabsorbed in the kidney tubules.
The concentration of urine in your
body fluids is controlled by the anti-diuretic hormone (ADH)
- which makes the kidney tubules more permeable - so more water can be
reabsorbed into the bloodstream.
In effect, the ADH controls
the permeability of the urine collecting duct in the kidney.
ADH is released into the blood by the
pituitary gland in the brain.
The brain's hypothalamus monitors the
water content of the blood via osmotic pressure changes -
osmoregulation.
If the water content is not right,
the brain instructs the pituitary gland to release or not release ADH into the
bloodstream depending on how much water is needed.
The hormone ADH makes the
collecting duct membranes of the nephrons more permeable so
that more water can be reabsorbed into the blood.
This process is controlled by a
negative feedback system so that if the water content of your body gets
too high or too low the ADH mechanism is initiated to bring the water
levels to normal.
(i) If you drink a lot of
water it may dilute the blood too much. If this happens, little, if any
ADH is released into the bloodstream. This results in less water being
reabsorbed by the nephrons in the kidney and more water passes through the tubules to
produce urine.
In passing (if you will pardon
the pun), drinking alcohol inhibits the production of ADH so you
produce much more urine, so heavy drinking of alcoholic beverages can produce dehydration.
(ii) If the blood starts to
become too concentrated it makes you feel thirsty and the brain
instructs the pituitary gland to release antidiuretic hormone ADH
into the bloodstream. ADH stimulates the nephrons in the kidneys to reabsorb water into
the blood from the urine. Reducing water loss in the urine helps to
conserve water and reduce the likelihood of dehydration.
(iii) Note that the more water
your kidneys reabsorb, the smaller the volume of water you excrete as
urine.
However, you will still excrete
all the waste products produced by the kidney, but your urine will
be more concentrated and probably a darker colour than the usual
pale straw colour.
If your water intake is in
excess of what is need for a balanced water content, the
hypothalamus detects this increase 'above normal' and instructs the
pituitary gland to secrete less ADH and this instructs the kidneys
to reabsorb less water. This produces a greater volume of more
dilute urine - more water lost to restore the water content
balance in the body.
If you take in a lot of salt,
the kidney responds by excreting more salt in concentrated urine.
|
A description of a
body's homeostasis negative feedback system for controlling
water-urine content via ADH |
|
(a) The negative feedback in action for too high level of water
content in the body e.g. after drinking a large volume of water |
(b) The negative feedback in action for too low level of water
content in the body e.g. hot and thirsty due to high outside
temperature or physical exercise |
| 1. A receptor
in the brain's hypothalamus detects a stimulus that the water
level is too high
- blood solute concentrations too low - too dilute - detection is to
do with osmotic pressure (osmoregulators). |
1. A receptor
in the brain's hypothalamus
detects a stimulus that the water level is too low -
blood solute concentrations too high - too concentrated - detection
is to do with osmotic pressure (osmoregulators). |
| 2. The hypothalamus
coordination centre in the brain receives and processes the stimulus
information and then organises a response by the effector -
pituitary gland. |
2. The hypothalamus
coordination centre in the brain receives and processes the stimulus
information and then organises a response by the effector -
pituitary gland. |
| 3. The effector, the pituitary gland, responds by
releasing none or
very little ADH - so less water is reabsorbed by the blood
from the kidney tubules. This results in larger volumes of
dilute urine - pale yellow colour. |
3. The effector, the pituitary gland, responds by
releasing more ADH
into the blood stream - so more water is reabsorbed into the
blood from the kidney tubules . This results in a smaller volume
of concentrated urine - dark yellow colour. |
| 4. The
pituitary gland will carry on producing less of the ADH - the 'water reducing' response, as long as
the coordination centre is stimulated by the receptors are saying
,too much water'. Eventually the correct water balance is reached. |
4. The
pituitary gland will carry on producing the increased ADH 'water increasing' response as long
as the coordination centre is stimulated by the receptors are saying
'too little water'. Eventually the correct water balance is reached. |
| 5. If the
pituitary
effector response is more than required, and the water content becomes
too low, the receptors will detect
this, and the negative feedback will stimulate the effectors to
increase the water level (1. - 3. on the right). |
5. The
pituitary effector response is more than required, and the water
content becomes
too high, the receptors will detect
this and the negative feedback will stimulate the effectors to
decrease the water level (1. - 3. on the left). |
| This
is all automatically done by the organism's complex control
systems and enables the organism e.g. your body, to maintain as near
as possible the 'ideal' water content of your body. |
Schematic simplified diagram of the ADH negative
feedback system for controlling the water content of the body.
Its all about getting the
blood plasma concentration
normalised.
A simple graphical representation of the body's
'water content' homeostasis mechanism.
(iv) Ion concentration and the feedback
mechanism
If the concentration of sodium
ions increases in the bloodstream, the concentration of water will
fall.
Therefore, the brain detects that
the blood needs more water.
More ADH is released so more
water will be reabsorbed from the collecting ducts and the water
content of the blood will rise.
This can happen when you eat
'salty' food (NaCl salt gives sodium ions), you may feel thirsty
soon after if you haven't taken in enough water to be reabsorbed.
To much salt in your bloodstream
is bad for you. It draws water out of cells by osmosis and a high
salt diet is associated with high blood pressure.
Most experts think we have too
much salt in our diet.
(v) Sweating and dehydration
Losing a significant amount of
water can cause dehydration.
This most likely to happen in hot
weather or after vigorous exercise - both result in sweating
- loss of body fluid - mainly water containing salt (sodium
chloride).
As we have described already, the
reduction in water content is detected by receptors in the
hypothalamus (osmoregulators).
The hypothalamus registers the
change and instructs the pituitary gland to release more of the
hormone ADH.
This instructs the kidneys to
reabsorb more water so the volume of blood increases and a
smaller volume of more concentrated urine is made.
At the same time, the brain
triggers feelings of thirst when you are dehydrated.
Therefore your brain makes you
want to drink more and hence restore the water content balance in
your body.
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and sub-index
(f) More on controlling ion
concentrations and removing waste products
and the problems resulting from malfunctioning kidneys
and kidney failure
The ions essential for life are
absorbed into the blood stream in the gut from digested food.
If the water or
ion content of the body is wrong, too much water may move into or out of the
cells and damage them.
The ion
content of the body – ions are lost via the skin when we sweat (you can
taste it on your skin) and
excess ions are lost via the kidneys in the urine.
For example, the kidney controls
the sodium ion (Na+) concentration from digested food containing
salt.
Any excess of any ion needs to
be removed e.g. by the kidneys and subsequent excretion of urine.
Water and ions, such as sodium (Na+) enter the body when we eat and drink
and are then absorbed into the bloodstream.
Any imbalance in the ion
concentrations of the cells will result in the wrong quantity of water being
absorbed by osmosis and this reduces the correct functioning of the cell.
It is the kidney maintains the ion
concentration balance in your blood.
If the ion or water content of the
body is not right it can upset the balance between ions and water. It
can result in too little or too much water being absorbed into the cells
by osmosis. The wrong amount of water in cells damages them inhibiting
their normal function.
Ions are lost in sweat, but this is
not a regulated system, which is why the correct balance of ions is
maintained by the kidneys.
The appropriate amount of ions is
reabsorbed into the blood after filtration in the kidney and any excess
ions removed from the body in the urine.
As already described, humans need to remove waste
products from their bodies to keep their internal environment relatively
constant.
Know and understand that waste products that have to
be removed from the body include urea, produced in the liver
by the breakdown of amino acids and removed by the kidneys in the urine,
which is temporarily stored in the bladder.
We take in protein in our food e.g.
meat and cheese, which is broken down into amino acids (H2NCHRCOOH,
R is variable). The amino acids are then converted (via
DNA/RNA
coding) into the proteins we actually need.
Unfortunately,
we cannot store amino acids in our body, so the excess must be dealt
with in some way. The amino acids can be converted into fats and
carbohydrates (C, H and O compounds) which can be stored in the body,
but neither of these groups of molecules contain nitrogen - whose excess
must be removed in some way by the liver and kidneys.
This
conversion occurs in the liver (can also be done by the kidney) and involves a process called deamination.
Toxic ammonia (NH3) is produced as a
waste product of the process and this must be converted by the liver
into non-toxic urea, formula O=C(NH2)2.
The urea is soluble and transported to the kidneys, where it can be
filtered out and excreted from your body in urine.
Summary of the three primary kidney functions in terms of cleaning the blood of waste products and maintaining
the correct concentrations of various substances.
(i) The kidney removes urea, formed
from the waste of excess amino acids.
The body can't store proteins,
so excess amino acids are converted into storable fats and carbohydrates and
the waste product of the process is urea.
Urea is a poisonous substance so
on its release from the liver into the bloodstream it is filtered out by the
kidneys before storage in the bladder prior to excretion in urine.
(ii) The kidney adjusts the water content of
the blood e.g. to avoid you becoming dehydrated.
(iii) The kidney also adjusts the concentration of
ions like sodium (Na+), potassium (K+) and chloride
(Cl–) in the blood.
A healthy kidney
produces urine by:
first filtering the blood,
excess ions such as sodium are removed, though some ions are lost in sweat,
reabsorbing all the sugar,
reabsorbing the dissolved ions needed by the body,
reabsorbing as much
water as the body needs,
releasing urea, excess ions and water as urine.
Obviously, if any of the above functions of the kidney
are not working correctly
there will be problems! So, some notes on malfunctioning kidneys
and kidney failure
Urea, the waste product from
excess amino acids, is released by the liver and filtered out by the kidneys
and excreted via the bladder in urine.
People whose kidneys do not
function properly may die because toxic substances accumulate in their
blood.
If the kidneys are not working
properly-efficiently then potentially toxic substances including
ammonia can build up
in your blood stream and your body is unable to deal with the
problem.
If you lose your ability to
control the levels of ions and water in your body it can have fatal
consequences.
If kidney function deteriorates to
the point of complete kidney failure your life is in danger.
Their lives can be saved by
using dialysis machines or having a healthy kidney transplanted.
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(g) Dialysis - an artificial means of
purifying the blood
People who suffer from
kidney failure can't filter their blood properly.
Kidney patients may be treated either using a
kidney dialysis machine
or having a healthy kidney transplanted.
If the kidneys don't function
properly then waste products build up in the body's bloodstream and the
body's ability to control the water content and ion concentrations is lost,
a situation that can be fatal - kidney failure is a bad situation - a major
lack of this aspect of homeostasis control.
Though uncommon, kidney failure
can be caused by infection, severe poisoning, an injury with severe loss of
blood or very high blood pressure
Treatment by haemodialysis restores the
concentrations of dissolved substances in the blood to normal levels and has
to be carried out on a regular intervals to keep them at the right 'normal'
concentrations and remove waste that that the malfunctioning kidney cannot
deal with.
In particular, keeping the ion
concentrations at the right level and removing waste products is essential
and a dialysis machine can perform some of the functions of a real kidney.
In a
dialysis machine a
person’s blood flows between partially permeable membranes (selectively
permeable membranes).
Haemodialysis
Diagram of kidney dialysis
machine procedure - the dialyser connected to a patient's arm - blood is
pumped from an artery to the dialysis machine and the 'cleaned' blood pumped
back into a vein.
In a dialysis machine there is a
selectively permeable membrane (partially permeable) where the patient's blood flows on one side and
the dialysis fluid on the other.
The membrane allows substances like
excess ions
and urea waste to pass through the membrane from the blood to the dialysing fluid,
but, does not allow larger molecules like proteins through or white or red
blood cells.
This is a
similar action to the membranes in the kidney - the membranes are
indicated by the blue lines in the dialysis machine diagram
above.
The dialysis fluid contains the
same
concentration of useful substances e.g. of ions and glucose as healthy blood to ensure that glucose and useful mineral ions are
not lost from the blood
in the dialysis process - minimises the formation of diffusion gradients, so
minimises unwanted ion/molecule transfers.
Only waste substances such as urea
and excess ions pass
out from the blood through the membrane into the dialysis fluid.
This will
naturally happen by diffusion because the dialysis fluid contains no urea,
so, the diffusion gradient is from blood to dialysis solution.
However, large molecules
like proteins cannot pass through the dialysis membrane
and neither can any blood cells.
The levels of ions and glucose
in the dialysis fluid are set at approximately those required in the
patient's blood so that diffusion may occur in either direction to maintain
the correct concentrations in the blood.
For people with poorly
functioning kidneys, dialysis might be required several times a week with
each session lasting several hours (3-4 hours).
Regular dialysis will keep the
concentrations of dissolved substances in the blood at the normal levels
and remove potentially toxic waste substances.
BUT, dialysis is not without risks:
Dialysis may cause blood clots.
There is an increased risk of
blood infection.
It is also an expensive
unpleasant experience, even if its free on the NHS.
These risks and
unpleasantness must be balanced against giving the patient with
kidney failure some time, and therefore a chance, getting a
life-saving transplant of a donor organ.
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and sub-index
(h) Kidney transplants
and problems - dialysis or transplant?
In kidney transplants a diseased
kidney is replaced with a healthy one from a donor.
You can function well, with care, with
only one kidney.
This the case if e.g. a living family
member donates one of their kidneys to another family member.
This situation gives the best chance of
tissue matching and minimises the danger of rejection by the kidney
recipient.
Source of kidney organs
Kidney transplants are one of
the most common organ transplant operations done in our modern health
service (eg the NHS of the UK).
The only cure for serious kidney
disease is to have a kidney transplant, hopefully from a suitable matching
donor.
The donor can be a relative
donating one of their kidney's or a fatal accident victim e.g. somebody who
has died suddenly.
The person who has died must be
on the donor organ register or carrying a donor card (providing
relatives agree to the organ donation) - this is the current
situation in UK Law - but it may change.
Any donated organs must be
stored in cold conditions, for as shorter time as possible, to
minimise tissue deterioration.
Since we have two kidneys, one
can be transplanted from a living person to the kidney failure
recipient, but with some residual risk to the donor.
In the surgical operation the
recipient's abdomen is opened below the navel and the new kidney
connected to an artery and vein so it can function with full blood
supply.
The kidney is further
connected to the bladder for urine collection.
The kidney should, at least
partially, work immediately, but it may take several weeks to be
fully working.
It is a complex operation
taking 2-3 hours.
Problems with kidney transplants
However, the donor kidney may be
rejected by the immune system unless precautions are taken.
The immune system treats the
transplanted tissue as 'foreign' and its cells are attacked by
antibodies.
Transplant
rejection
1.
The white blood cells of the organ recipient sense the antigens of the
cells of the transplanted kidney are foreign and not part of the body of
the transplant patient.
2. The
recipient's white blood cells respond to the perceived 'foreign'
antigens by producing matching antibodies
3.
The antibodies attack the 'donated' kidney cells and destroy them and so
the kidney transplant is rejected.
Therefore, precautions must therefore be
taken to minimise the risk of kidney rejection.
(i) You can test how well the
donor's kidney tissue type closely matches the recipient's kidney
tissue.
The tissue-type is based on
the cell's antigens.
The more similar the antigens
of the donor and recipient's kidney cells, the greater the
chance of organ acceptance i.e. the less chance patient's white
blood cells identify the donor's antigens as 'foreign' and
produce antibodies to attack the donated organ.
(ii) The patient needs to be treating
with immunosuppressant drugs to prevent organ rejection, but it may still happen.
These drugs are designed to
suppress the patient's immune system to stop it attacking the
cells of the transplanted kidney tissue.
Unfortunately, these drugs
reduce the body's white cells ability to produce antibodies, so
they won't attack the kidney cells of the donated organ.
So, these drugs make the
patient more susceptible disease that would normally be
counteracted by their immune system - there is a risk of both
infection and cancer.
There are also problems with
availability of kidney organs, e.g. a long waiting list and the cheaper dialysis is
often used as an interim treatment.
Antigens are proteins
on the surface of cells., and a recipient’s antibodies
(from white blood cells of the immune system) may attack the antigens
on the donor organ as they do not recognise them as part of the recipient’s
body and so the transplanted kidney is rejected.
So, summarising - in order to
minimise rejection of
the transplanted kidney:
A donor kidney with a ‘tissue-type’ similar to
that of the recipient is used - which is where a generous relative can mean
the difference between life and death.
The recipient is treated with
drugs that suppress their immune system response to reject the
kidney transplant.
Tests for blood grouping and
compatibility tables are part of the procedure to give the
transplant the best chance of not being rejected.
Dialysis or transplant?
Advantages and Disadvantages - the 'Pros and Cons'
In the long run, transplants are
cheaper than dialysis, but there is a shortage of donated kidney organs for
transplant operations.
Organ donation involves
ethical issues for us and other animals.
Some people may think it is
wrong to use parts of other peoples bodies.
Relatives may override the
wishes of the deceased even though they may have carried a donor
card.
Some poor people are prepared
to sell one of their kidneys for money - should this be allowed
to increase the availability of organs?
Should we transplant organs
from animals? but there are concerns over cross-species
infections.
What animal rights involving
breeding animals specifically for a particular organ like the
kidney?
Will biotechnology enable us
to grow complex organs like the kidney in the future?
There is a long waiting list for
surgery, but
at least dialysis keeps patients with kidney failure alive, and
increases the chance of undergoing kidney transplant surgery!
For elderly patients, dialysis
can reduce life expectancy due to complications - maybe due to other
underlying health problems.
Dialysis patients can suffer from
sudden falls in blood pressure, and they must always be on the
lookout for signs of infection.
Without a kidney transplant,
dialysis patients must continue with the treatment for the rest of
heir lives.
A kidney transplant gives the
patient a much better freedom of lifestyle - greater independence
and general quality of life - no need to go to a medical centre for
dialysis or receive dialysis in the home.
Patients with other underlying
health issues like heart disease may not be able to withstand the
stress of transplant surgery, so dialysis is the only option to stay
alive.
Dialysis patients must take great
care in lifestyle e.g. no smoking, minimum alcohol and avoid excess
weight.
Many of the points raised here
apply to the advantages and disadvantages of other transplant
procedures.
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and sub-index
Practical work to
help develop your skills and understanding may have included the following:
dissecting and making observations of a kidney
designing a model kidney dialysis machine using Visking tubing as the filter
testing urine from diabetic and non-diabetic people using Clinistix.
Be able to evaluate the advantages and disadvantages of treating
kidney failure by dialysis or kidney transplant,
Homeostasis notes index:
Homeostasis - introduction to how it functions (negative
feedback systems explained)
Homeostasis - control of blood sugar level
- insulin and diabetes
Homeostasis - osmoregulation, ADH, water control, urea and ion
concentrations and kidney function, dialysis
Homeostasis - thermoregulation, control of temperature
TOP OF PAGE
and sub-index
Other general HUMAN BIOLOGY revision notes
Introduction to the organisation of cells =>
tissues => organs => organ systems (e.g. in humans)
Examples of surfaces for the exchange of substances in
animal organisms gcse biology revision notes
See also
Enzymes - section on digestion and synthesis gcse
biology revision notes
The human circulatory system - heart, lungs, blood,
blood vessels, causes/treatment of cardiovascular disease
The brain - what the different parts do and the dangers
if damaged gcse biology revision notes
An introduction
to the nervous system including the reflex arc
gcse biology revision notes
Hormone systems - Introduction to the endocrine
system - adrenaline & thyroxine hormones
gcse biology revision
Hormone systems - menstrual cycle, contraception,
fertility treatments
gcse biology revision notes
Respiration - aerobic and anaerobic in plants and animals. gcse
biology revision notes
Keeping healthy - communicable diseases -
pathogen infections gcse
biology revision notes
Keeping healthy - non-communicable diseases
- risk factors for e.g. cancers gcse
biology revision notes
Keeping healthy - diet and exercise
gcse biology revision notes
Keeping healthy - defence against
pathogens, infectious diseases, vaccination, drugs, monoclonal antibodies
See also
Culturing microorganisms like bacteria - testing
antibiotics/antiseptics gcse
biology revision
Food tests for reducing sugars, starch, proteins and
lipids gcse
biology revision notes
The eye - structure and function - correction of vision
defects gcse
biology revision notes
Optics - lens types (convex, concave, uses),
experiments, ray
diagrams, correction of eye defects (gcse physics)
Enter specific biology words or courses e.g. topic, module, exam board,
structure, concept, animal or plant feature, 'phrase', homework question! anything
of biological interest!