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Excretion
Kabilan Mahesan
Define Excretion
• Excretion is any process that removes the
body of toxic substances, metabolic waste
products and excess ions and water.
• The main excretory organs are the liver,
kidneys, skin and lungs
Describe the functions of the Kidneys
1. Excretion: the kidneys rid the body of excess
urea which is toxic to the body
2. Osmoregulation: the kidneys control water
and salt levels in the blood by the hormone
ADH which acts on the kidneys preventing
dehydration
Label a Diagram of the Urinary System
Renal Vein
Renal Artery
Label a Diagram of a kidney
Nephron
Label a Diagram of a Nephron
Explain how a Nephron Filters
1. Ultrafiltration: An intense pressure is created
in the glomerulus when blood travels from a
wide artery into a narrow vein. This pressure
causes the blood plasma to be filtered out of the
blood. The filtrate is collected in the Bowman’s
Capsule. It contains WUGI- water, urea, glucose
and ions.
Explain how a Nephron Filter (Part II)
2. Selective Reabsorption: Useful molecules like
glucose and amino acids are reabsorbed into the
blood in the first coiled tube of the nephron. Also,
50% of the urea is also reabsorbed due to there
being a concentration gradient between the blood
plasma in the nephron and the blood in the
capillaries, so urea will diffuse out. Note that
glucose and amino acids do not diffuse but are
instead actively transported, so that all of the useful
glucose and amino acids are reabsorbed, not just
50% of them. Some ions are also reabsorbed.
Osmoregulation
The processes explained previously were the
excretory functions of the kidneys. However,
they also perform another function, which is
osmoregulation
Osmoregulation is the way in which the kidney
controls water and salt levels in the body.
Describe the process of
Osmoregulation in the Kidneys
• ADH is a vital hormone released by the pituitary
gland, and osmoregulation depends on it.
• ADH stands for anti-diuretic hormone. Basically,
this means it anti-peeing (diuretic means peeing)
• So, ADH stops you from needing to go to the
toilet
• It works by making the collecting duct more
permeable to water( in high concentrations), and
vice-versa.
More on ADH
• Higher levels of ADH means more water is
reabsorbed in the collecting duct, leading to a
small amount of concentrated urine.
• Lower levels of ADH means less water is
reabsorbed in the collecting duct, leading to a
large amount of dilute urine.
ADH and Negative Feedback (a bit of
homeostasis)
Water content too
high- blood plasma
less concentrated
Kidney reabsorbs more water
Pituitary Gland releases
more ADH.
Thirst centre in hypothalamus
less stimulated
Optimum water
level in blood
Thirst centre in
hypothalamus less
stimulated
Pituitary Gland releases less ADH
Kidney reabsorbs
less water
Water content too
low- blood plasma
more concentrated
Outline the Functions of the Liver
The liver is also an excretory organ. Its functions are:
1. Manufacture of bile, which is important for the digestion of fats.
2. Storage of excess glucose and glycogen.
3. Interconversion of glucose and glycogen, which keeps the glucose
concentration constant for the working tissues of the body.
4. Interconversion of amino acids- the liver can convert some amino
acids into others that the body might require in a process called
transamination.
5. Excretion of excess amino acids- the amino part of the amino acid
is removed in a process called deamination and excreted in the
urine as urea.
6. Removal of old red blood cells from the circulation and storage of
the iron they contained.
Diagram of the Liver
Deamination
occurs here
Describe the process of Deamination
This is the structure of an amino acid:
H2 N
R
C
COOH
H
R varies depending on each different types of amino acid (there
are 22 different types).
Deamination is when the NH2 part of the structure is cut off.
Define Deamination
• Deamination is done by enzymes in the liver.
The nitrogen part (NH2) of the amino acids is
removed. It is combined with CO2 to form
ammonia, which is then broken down to urea.
The rest of the amino acid can be respired for
energy or stored as fat.
• Urea dissolves in blood plasma and travels to
the kidneys where it is excreted as urine.
Describe and explain how a dialysis
machine filters the blood of wastes
Sometimes, a person’s kidney may stop working properly. Urea and
other toxic substances will build up, which could lead to death. To treat
this, a dialysis machine can be used as an artificial kidney. Blood is
taken from a vein and passed through a roller pump to push blood
through the tubes of the dialysis box (dialyser). The tubes are partially
permeable and allow waste products like urea, toxins, excess water
and ions to diffuse out of the blood down a concentration gradient
into dialysis fluid. The dialysis fluid has a similar composition to blood
plasma. The fluid is changed regularly to keep the concentration
gradient steep, so urea and other wastes will diffuse. Note that the
dialysis fluid can be cleaned and recycled. Before the blood goes back
into the body, it passes through a bubble trap to remove air bubbles.
Note that useful substances like glucose and amino acids cannot
diffuse through the partially permeable tubes because their molecules
are too large to fit.
Kidney Transplants
Kidney transplants are basically when a damaged or
dead kidney is replaced by another person’s kidney.
Kidney transplants need a donor, and the donor’s
kidney must be compatible with the recipient’s
body. Family members are the most likely to be
compatible (especially brothers and sisters).
Patients must also use immuno-suppressant drugs.
List the advantages of dialysis and
kidney transplants.
Dialysis
Kidney Transplant
There is an almost 100% chance that it
will treat the patient; there is no risk of
incompatibility, as there is with kidney
transplants.
It is a permanent solution if the operation
is successful.
It is cheaper than kidney transplants at
first.
It is cheaper than dialysis in the long run.
It is much easier to find a dialysis machine The patient can have a normal diet.
quickly (for example, if the patient needs
it immediately or else they will die).
Outline the disadvantages of dialysis
and kidney transplants
Dialysis
Kidney Transplant
The patient has to spend two to 3 nights a It is difficult to find a donor, and even
week connected to the machine at 8
more so to find one whose kidney is
hours per session- the treatment disrupts compatible.
the patient’s life.
The patient must eat a special diet that is
low in protein so that less urea is made.
They can’t take alcohol or drugs.
The body often rejects the donated organ.
It is not a cure and only gives temporary
results.
To prevent rejection, the patient is given
immuno-suppressant drugs. These make
your immune system weaker, so you are
more susceptible to diseases e.g.
pneumonia, tuberculosis.