Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
312
Flow rate / cm 3 min –1
QUESTION
14.6 a Figure 14.14 shows the relative rate at which fluid
flows through each part of a nephron. If water flows
into an impermeable tube such as a hosepipe, it
will flow out of the far end at the same rate that it
flows in. However, this clearly does not happen in a
nephron. Consider what happens in each region, and
suggest an explanation for the shape of the graph.
b Figure 14.15 shows the relative concentrations of
four substances in each part of a nephron. Explain
the shapes of the curves for: i glucose, ii urea,
iii sodium ions iv potassium ions.
160
80
40
20
10
5
2.5
1.25
proximal
convoluted
tubule
loop of
Henle
distal
convoluted
tubule
collecting
duct
Figure 14.14 Flow rates in different parts of a nephron.
Control of water content
Osmoreceptors, the hypothalamus and
ADH
Osmoregulation is the control of the water potential
of body fluids. This regulation is an important part of
homeostasis and involves the hypothalamus, posterior
pituitary gland and the kidneys.
The water potential of the blood is constantly
monitored by specialised sensory neurones in the
hypothalamus, known as osmoreceptors. When these cells
detect a decrease in the water potential of the blood below
the set point, nerve impulses are sent along the neurones
to where they terminate in the posterior pituitary gland
(Figure 14.16). These impulses stimulate the release of
antidiuretic hormone (ADH), which is a peptide hormone
made of nine amino acids. Molecules of ADH enter the
blood in capillaries and are carried all over the body. The
effect of ADH is to reduce the loss of water in the urine
by making the kidney reabsorb as much water as possible.
The word ‘diuresis’ means the production of dilute urine.
Antidiuretic hormone gets its name because it stops dilute
urine being produced, by stimulating the reabsorption
of water.
hypothalamus
neurone
anterior
pituitary gland
posterior
pituitary gland
hormone
secreted
hormone in blood
Figure 14.15 Relative concentrations of five substances in
different parts of a nephron.
Figure 14.16 ADH is produced by neurones in the
hypothalamus and is released into the blood where the
neurones terminate in the posterior pituitary gland.