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1. Excretion
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Biology

2. Excretion
●The process by which metabolic waste products and
toxic materials are removed from the body of an
organism.
Metabolism
●The sum total of all the chemical reactions taking
place in living cells in order to keep an organism
alive.
●Metabolism = Catabolism + Anabolism
Catabolism
●Chemical reactions in which complex substances are
broken down into simpler ones.
●Eg. Tissue respiration:
C6H12O6 + 6O2 --> 6H2O +6CO2 + 2898 kJ
●Eg. Deamination of proteins and amino acids

3. Anabolism
●Chemical reactions in which simpler molecules are built up
into complex substances.
●Eg. Photosynthesis
●Eg. Formation of new proteins from amino acids
●Eg. Conversion of glucose into glycogen in the liver and
muscles
Importance of Excretion
Metabolic reactions produce waste products which are
harmful if accumulated in the body.

4. Excretory product Excretory
organ
Remarks
Carbon dioxide Lungs Gas in expired air
Mineral salts Kidneys Constituents of urine
Nitrogenous waste products
- Mainly urea
●(from deamination of proteins)
- Creatinine
●(from muscle tissue breakdown)
- Uric acid
●(from breakdown of nuclear
materials)
Skin
Constituents of sweat, only
in small quantities for
nitrogenous waste products
Excess water Kidney
Skin
Lungs
Main constituent of urine
Main constituent of sweat
Water vapour in expired air
Bile pigments Liver Via the intestine

6. Kidneys
●Contains a lot of nephrons which remove urea and excess water and
heat from the blood to form urine
●Responsible for osmoregulation
Ureter
●Narrow tube through which urine flows from each kidney to the
urinary bladder (by peristalsis)
Urinary bladder
●A hollow, distensible muscular bag which stores urine temporarily
Renal pelvis
●The enlarged portion of the ureter inside the kidney
Sphincter muscle
●Contracts or relaxes to control the flow of urine into the urethra

7. Kidneys
●A pair of dark red, bean-shaped organs embedded in a mass of
fat in the abdominal cavity
●They are attached to the dorsal body wall, one on each side of
the vertebral column.
●The left kidney is slightly higher than the right one.
Each kidney is covered by an
outer layer of protective cells
called the fibrous capsule.
It has two main regions:
a. Cortex: the outer dark red
region.
b. Medulla: the inner thicker
pale red region. In man, it
consists of 12-16 conical
structures, the pyramids.

8. Structure of a Kidney Tubule –
The Nephron
●The functional unit of
the kidney.
●About 1 million of them
in each kidney.
●Each is about 3 cm
long

9. Kidney Tubule - The Nephron
●Each kidney tubule
(nephron) begins in the
cortex as a cup-like
structure called the renal
(Bowman’s) capsule.
●The capsule leads into the
first (proximal) convoluted
tubule.
●It then straightens out as it
passes into the medulla,
make a U-turn and passes
back into the cortex where it
becomes coiled again

10. Kidney Tubule - The Nephron
●It then opens into a
collecting tubule/duct and
eventually opens into the
renal pelvis.
●The U-shaped portion of
the tubule in the medulla
is known as the loop of
Henle.

11. Kidney Tubule - The Nephron
●Blood enters the kidney by the renal
artery which gives off numerous
branches or arterioles.
●Each branch breaks up into a mass of
blood capillaries in the renal capsule.
●This mass of blood capillaries is called
the glomerulus.
●The renal capsule with its glomerulus
is called the renal / Malpighian
corpuscle.
●Blood leaving the glomerulus enters
blood capillaries surrounding the
tubule.
●These blood capillaries then unite to
form venules, which in turn join to form

12. Associated blood vessels
Renal Arteries
Bring oxygenated blood
containing urea, excess
water and heat to kidneys
Renal Veins
Take away
deoxygenated blood
(from which urea, excess
water and heat have
been removed) from the
kidneys.

13. Blood exits
the kidney
via the renal
vein
1
2
branch
of
renal
vein
3
4
5
6
7
7 June 2011
Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
Renal artery
branches into
many arterioles
Blood enters
the kidney via
the renal
artery
1
2
Blood
capillaries
unite to form
venules
Blood leaves
Malpighian
corpuscle
and enters
tubule
Arterioles
further divide
into a mass of
blood
capillaries
(glomerulus)
3
4
5
Venules join to
form renal vein
6
7
How blood enters the kidney tubule

14. Urine Formation
Two main processes are involved:
●A. Ultrafiltration of small molecules from
the blood.
●B. Selective reabsorption of useful
materials.

15. A. Ultrafiltration
●Blood passes from the renal artery into the
glomeruli in the renal capsules.
●Mechanical filtration occurs in each
glomerulus.
●Blood plasma is forced out of the glomerular
blood capillaries into the renal capsule.
●The afferent arteriole that brings blood into the
glomerulus is wider than the efferent arteriole
that carries blood away.
●A high pressure is built up in the glomerulus,
squeezing the blood through two living
membranes: the capillary wall of the
glomerulus (partially permeable membrane)
and the inner wall of the renal capsule.
●It is this blood pressure that provides the main

16. Why is it called Ultrafiltration?
●It is caused by very high blood pressure.
●The membrane around the glomerular
blood capillaries is like a very fine filter.
●Only very small molecules are filtered off
the blood. These include water, mineral
salts, glucose and urea.
●Most proteins and fats are too big, so they
stay in the blood, together with the blood
cells.

17. Ultrafiltration
●In human, the kidneys filter about 60
litres of blood an hour, and it takes
only 5 minutes to filter an amount
which is equal to the body’s entire
blood supply (5 litres).
●The filtration process produces about
7.5 litres of glomerulus filtrate an
hour, and this liquid contains not only
urea, but many useful substances
(e.g. glucose).
●If all of this is excreted, the body
would lose most of its water and
soluble food supplies in a few hours.
●However, this does not happen
because 99% of the filtrate is

18. B. Selective
Reabsorption
●This takes
place in the
kidney tubules /
nephrons.
●The useful
materials are
taken back into
the blood
stream through
the capillaries
which surround
the tubules.

19. Selective
reabsorption at
Description
Proximal Convoluted
Tubule
●These include all the glucose, amino acids and most
mineral salts.
●by diffusion and active transport.
●Most of the water is reabsorbed by osmosis back into
the blood stream (through surrounding blood capillaries).
the Loop of Henle and
Distal Convoluted
Tubule
●Some of the water is reabsorbed back into the blood
stream (through surrounding blood capillaries).
the Distal Convoluted
Tubule
●Some salts are reabsorbed.
the
Conducting Duct
●Some water is reabsorbed.
Selective Reabsorption

20. Selective Reabsorption
●Excess water, mineral salts, and
nitrogenous wastes (e.g. urea, uric acid
and creatinine) pass out of the collecting
duct into the renal pelvis as urine.

21. Per 100 cm3
Water 96.0g
Mineral salts (mainly sodium chloride) 1.8g
Urea 2.0g
Other nitrogenous substances 0.2g
Total 100.0g
Normal constituents of urine

22. Composition of urine
Varies depending on several factors:
●Rich protein diet higher content of urea
●Heavy sugary food in the diet some sugar in urine
●Larger intake of fluid urine lighter in colour ; more
urine
●Cold weather (less sweat) more urine
●Salty food in the diet excess salt excreted in urine
Abnormal situation
●Sugar diabetes (diabetes mellitus) – large amounts
of glucose in urine

23. Kidneys as Osmoregulators
●The water potential of the blood has to be kept relatively
constant.
●If the blood plasma is too dilute, water will enter the
blood cells by osmosis and the blood cells will swell and
burst.
●If the blood plasma is too concentrated, water will leave
the blood cells by osmosis and the blood cells will
become dehydrated and shrink (may be fatal).
●The water potential of the blood depends on the amount
of water and salts in the plasma.
●Kidneys help to regulate the water or salt concentration
in the blood, thus maintaining a constant water potential

24. Osmoregulation by kidneys –
an example of Homeostasis
●If the water content of the blood is lower than usual (eg. through
heavy perspiration), the additional quantities of water needed are
taken in through the tubule. How?
○Hypothalamus of the brain produces vasopressin or antidiuretic hormone
(ADH).
○Dehydration of the body is detected by the hypothalamus.
○The pituitary gland is stimulated to release ADH into bloodstream to increase
the water reabsorption by the kidney tubules.
○Water content of the blood is thus restored.
○Urine production drops and urine becomes more concentrated.
●If the blood becomes too diluted (eg. large intake of water), the
secretion of ADH is inhibited.
○The tubules will reabsorb less water.
○Urine production increases and urine becomes less concentrated.

26. Summary of the Kidney Functions
1.Excretion of metabolic wastes, especially the nitrogenous
wastes (e.g. urea), & excess water, mineral salts.
2.Regulation of composition of the blood plasma – by the
selective reabsorption of useful substances (salts and water)
along the kidney tubules.
3.Regulating the water content
4.Regulating the salt content.
○If there is too much sodium chloride in the plasma, less of them is
reabsorbed from the glomerulus filtrate.
5.Regulating the pH of the blood to be around pH 7.3 to 7.4 by
the exchange of ions when the acidity or alkalinity of the blood
tends to rise.

27. ●High blood pressure and diabetes are
common causes of kidney failures.
●A person can still lead a normal life if only
one kidney fails to function.
●But if both kidneys fail to work, urea and
other wastes will accumulate in the blood.
The patient can die unless prompt medical
treatment is given.
●Treatment: Dialysis or kidney transplant.
Kidney Failure

28. Dialysis or Kidney Machine
●What the kidney performs by ultrafiltration and selective reabsorption, a
kidney machine performs in one step – diffusion.
●Blood is drawn from an artery in the patient’s arm and allowed to be
pumped through the tubing in the dialysis machine.
●The tubing is bathed in a dialysis fluid which contains the same
concentration of essential substances (e.g. mineral salts) as the blood,
but without metabolic waste products.
●The walls of the tubing are partially permeable.
●Substances pass from a higher concentration to a lower concentration
along a concentration gradient through this membrane
●Since there is no urea, uric acid and creatinine in the dialysis fluid, they
diffuse out of the tubing into the dialysis fluid. Excess water and mineral
salts also diffuse out of the tubing. These waste products are thus
removed from the blood.
●Big molecules like proteins and blood cells remain in the blood.

30. Dialysis or Kidney Machine
Other points to take note:
●The tubing is narrow, long and coiled to increase the surface
area to volume ratio. This speeds up the rate of exchange of
substances between the blood and the dialysis fluid.
●The direction of blood flow is opposite to the flow of the dialysis
fluid. This maintains the concentration/diffusion gradient for the
removal of waste products.
●The filtered blood is then returned to a vein in the patient’s arm.
●The patient needs to be treated about 2-3 times a week. Each
treatment takes several hours.

31. Patient’s blood
enters dialysis
machine
Exchange of Substances between the Blood
and the Dialysis Fluid
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Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
essential
mineral salt
red blood cell
protein
molecul
e
urea
molecul
e
dialysis
fluid
patient’s
blood
partially
permeable
membran
e
dialysi
s fluid
dialysis
machine
2
1
Dialysis fluid contains the same
concentration of essential
substances as blood. No metabolic
wastes present
Concentration
gradient set up
between dialysis
fluid and blood
3
Removal of
metabolic
waste
products from
the blood
dialysis fluid with
waste products
4
Filtered
blood is
returned to
the patient
filtered blood
5
Blood flows in the opposite
direction to flow of dialysis fluid

32. Larger molecules
(e.g. plates and
blood cells)
remain in the
tubing
The filtered
blood is
returned to a
vein in the
patient’s arm
arter
y
dialysis
tubing
pum
p
filtere
d
blood fresh
dialysis
fluid
dialysis
machin
e
dialysi
s
fluid
vei
n
1
2
3
6
4
5
7 June 2011
Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
Blood is drawn
from an artery in
the patient’s arm
1 Blood is pumped
through a tubing
to the dialysis
machine
2
The tubing is
bathed in a special
dialysis fluid and
the tubing is semi-
permeable
3
Small molecules
(e.g. urea) and
metabolic waste
products diffuse
out of the tubing
4
5
6
How Is Blood Cleaned in a Dialysis Machine?

33. Excretion
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Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
is carried out
by
Excrete carbon
dioxide
and water (water
vapour)
Lungs
Excretory Organs
Kidneys
Structure Function
s
Excretio
n
Osmoregulati
on
Maintenance of
composition
of blood plasma
Kidney
dialysis
In the case of kidney failure, blood is drawn out,
cleaned in a dialysis machine and returned to the
patient
Liver
Excrete bile
via the
intestines
Skin
Produces and
removes sweat
containing waste
products, e.g.
creatinine