IGCSE Biology
1.
Chapter 8
ANSWERS
a) homeostasis = keeping the internal environment constant or stable.
b) excretion = removal of metabolic waste (i.e. material that has been processed by the
body)
c) ultrafiltration = the removal of dissolved material from the blood by forcing it through the
capillary wall at high pressure in the glomerulus.
d) selective reabsorption = taking back certain solutes from the filtrate into the blood as the
filtrate passes along the nephron. It involves the active transport of glucose and some
important ions.
e) endotherm = an organism that generates heat from the metabolic activities within its
body. The body of an endotherm may be warmer than its surroundings (‘warm blooded’
organisms are endotherms, but they can also maintain a constant body temperature, so they
are endothermic homeotherms).
2. a) X = glomerulus
Y = Bowman’s capsule
Z = Loop of Henle
b)
A
B
C
D
water, urea, protein, glucose, salt
water, urea, glucose, salt
water, urea, salt
urea, salt
3. ADH stands for ‘anti-diuretic hormone’. Diuresis refers to the production of urine. Antidiuresis therefore means the inhibition of urine production. To produce less urine means to
make more concentrated urine as more water is reabsorbed from the glomerular filtrate back
into the body.
ADH effects greater water reabsorption by making the cells of the kidney tubules more
‘leaky’, or permeable to water.
A person who has lost a lot of water by sweating will need to conserve water. His
hypothalamus detects that the water concentration of the blood has decreased and responds
by instructing the pituitary gland to produce more ADH. The ADH is transported in the blood
to the target organ, the collecting ducts in the kidney. The cells of the collecting ducts
become more permeable to water and the volume of urine produced consequently
decreases. The water concentration in the blood is constantly monitored by the
hypothalamus. If the person was to have a drink, then the hypothalamus would sense the
elevated water concentration in the blood and the production of ADH would be inhibited.
This would lead to less water being reabsorbed in the kidneys, and so more dilute urine. This
is an example of negative feedback since the hypothalamus monitors the levels of water in
the blood, then effects hormonal changes that will correct any deviation from the body’s
‘correct’ water levels.
IGCSE Biology
Chapter 8
ANSWERS
4. a) The production of urine increased after drinking, then gradually decreased to a steady rate
of urine production.
b) At 60 minutes more urine was produced because following drinking the water levels in
blood are elevated. The hormonal response is to inhibit the production of ADH until the levels
of water in the blood return to normal. As the rate of urine production decreases from 60 to
90 minutes, this reflects increasing ADH levels as a response to falling water levels in the
blood.
c) A person in a hot room would be losing more water by sweating, so they might be
expected to produce less urine after drinking since blood water levels will already be lower
than those of the person in the first experiment. The ADH levels of the person in the second
experiment will be elevated.
d) Time interval = 120 – 90 = 30 minutes. Rate of filtration = 125cm3/min
The total volume of blood filtered in 30 minutes = 125 x 30 = 3750 cm3, or 3.75 litres
Volume of urine produced = 150cm3.
% of blood converted to urine = 100 x 150/3750 = 4%
% reabsorbed = 100 – 4 = 96%
5. Dual award students are not required to answer questions relating to temperature control.
6. Dual award students are not required to answer questions relating to temperature control.