Metabolism produces waste that must be removed from the body through excretion. The main excretory organs are the skin, lungs, kidneys and liver. Sweat and urine remove water, salts and urea through the skin and kidneys. The lungs remove carbon dioxide through respiration. The kidneys are the primary organ that filters urea and other waste from the blood and removes it from the body in urine. Dialysis is needed when the kidneys fail to effectively remove waste from the blood.

1. 1) Metabolism includes all chemical reactions that
take place in the body.
(a) During metabolism activities, cells in the body
take in water, oxygen and digested food, and
produce waste products such as carbon dioxide,
water and urea.
(b) Metabolism waste products are not needed
by the body cells and must be removed from our
body.
2) Excretion is the removal of metabolic waste
products from the body.

3. 1. The skin excretes water, mineral salts and
some urea when a person perspires or sweats.
(a) Sweat is salty.
(b) From the sweat glands just below the skin,
water, mineral salts and urea travel along the
sweat ducts (Figure 3.2).
(c) They are removed through the sweat pores
on the surface of the skin.
2) The body sweats more on a hot day. The
evaporation of sweat cools the skin and the body.

5. 3) During respiration, waste products such as
carbon dioxide and water are produced.
4) These are removed from the lungs during
gaseous exchange. The lungs are also excretory
organs.
5) The kidneys, the main human excretory organ,
removes urea. Urea is a waste product formed in
the liver from excess amino acids in the body.
Urea is removed in urine. Besides urea, urine also
contains water and mineral salts.

7. 1. Why must waste products be excreted? Wastes
are by-products of the chemical processes in the
body.
2. (a) If these wastes accumulate in the body,
they become toxic.
(b) We will be ill. Some organs may be
damaged.
(c) If an illness persists, death can take place.
(d) Waste products must be eliminated through
excretion in order to maintain a healthy body.
3) Carbon dioxide from respiration must be quickly
removed. A high level of this gas will increase the
breathing rate.

8. 4) Urea, if left in excess in the body, is toxic to the
body.
5) By removing excess water and mineral salts,
the kidneys help maintain the chemical
composition, water and pH balance of the body.
6) The kidneys are located in the back of the
abdominal cavity.
7) The liver removes excess amino acids from the
blood by converting them into urea which is excreted
by the kidneys.
8) The urea is sent to the urinary system. The flow
of the urea into urinary system starts from the
kidneys through the ureters to the bladder, and lastly
to the urethra. From urethra, it removed as urine
through the penis or opening.

13. The functions of kidney

14. 1. The blood vessel that brings blood containing
waste products to the kidneys is the renal
artery.
2. The blood vessel that carries 'clean' blood
away from the kidneys is the renal vein.
3. Substances that pass from the blood into the
kidneys are urea, glucose； amino acids,
mineral salts and water.
4. Protein cannot pass from the blood into the
kidneys. The protein molecules are too large.

15. Substances which are reabsorbed into the
blood from the kidneys are glucose and
amino acids. Both these substances are
not found in the urine. These substances
are required by the body. Glucose is an
energy source and amino acids are
needed for body building.

18. A high intake of salt will affect the water
balance. It will also cause hypertension. A
high concentration of salt in the blood may
lead to the formation of kidney stones.
Kidney stones can result in kidney failure.
Too much sugar in our blood will ultimately
cause diabetes.
We should avoid taking all kinds of drugs
and unprescribed medicines and food that
can damage the kidneys.

20.  Dialysis is a technique of separating particles of
different sizes. A liquid mixture is passed through
a partially permeable membrane.
 During dialysis, the blood is filtered through an
artificial membrane in a dialysis machine.

21.  This is how a dialysis machine works.
(a) The dialysis machine works on the principle of
diffusion. Substances move from an area of high
concentration to an area of low concentration
across a partially permeable membrane. This is
similar to a Visking tube.
(b) A tube is connected to an artery in the arm of
a patient. This allows the blood to flow along the
tube and enter the dialysis machine (Figure 3.7).
(c) Blood from the artery flows through the semi-
permeable membrane of the tube in a dialyser.
(d) These tubes are immersed in a dialysis
solution called dialysate.

22. (e) Waste products such as excess water,
urea, mineral salts and other impurities diffuse
out from the blood through the pores of the
semi-permeable membrane into the dialysate.
(f) They are removed by the flowing dialysate
in the dialyser.
(g) Proteins and blood cells which are too big
to pass through the membrane remain in the
blood.
(h) The blood with the waste products
removed goes back into the vein of the patient.

23. 1) (a) A patient's blood must pass through the
dialysis machine many times to make sure
that all wastes are removed.
(b) As such, the patient must be connected
to the machine for about four hours. Dialysis
treatments are usually needed two or three
times a week.
2) Dialysis treatments are not cheap. In
addition, the limbs are painful from the
repeated use of needles.
3) Dialysis does not cure damaged kidneys. It
is only an artificial substitute for the kidneys.