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Biology for Secondary Schools
Control centre
Brain signals release
of oxytocin
FOR ONLINE READING ONLY
Receptor Effector
Stretch receptors Oxytocin causes uterine
activated in uterus muscles to contract
Stimulus Response
Figure 5.2: Positive feedback mechanism of oxytocin hormone during delivery to facilitate labour
Baby’s growth in the Contraction of muscles
uterus completed continues until the baby
is delivered
Figure 5.2: Positive feedback mechanism of oxytocin hormone during delivery of the baby
Negative feedback mechanism vasoconstriction generate and
Negative feedback is a self-regulating conserve heat to restore normal
system in which a stimulus triggers a temperature.
response that counteracts the initial change, (c) Blood sugar regulation: The
maintaining equilibrium. It is the most pancreas plays a key role in regulating
common feedback mechanism in biological glucose levels. When glucose levels
systems. Examples include: rise, insulin is released to promote
(a) Blood pressure regulation: glucose uptake by cells which inturn
Baroreceptors in blood vessels detect reduces the blood sugar. When
blood pressure changes and send glucose levels drop, glucagon signals
signals to the hypothalamus. If blood the liver to release stored glucose,
pressure is too high, vasodilation and thus restoring the balance.
reduced heart rate lowers it. If blood (d) Osmoregulation: The hormone
pressure is too low, vasoconstriction antidiuretic hormone (ADH)
and increased water retention raises it. regulates water retention in the
(See Figure 5.3 for an illustration of kidneys. If water levels are too low,
negative feedback in blood pressure ADH promotes water reabsorption
regulation.) to prevent dehydration.
(b) Thermoregulation: When body
temperature rises, sweating and
vasodilation helps dissipate heat.
When it drops, shivering and
108 Student’s Book - Form Three
BIOLOGY FORM 3 FINAL EDITED 16.09.2025.indd 108 23/10/2025 11:52:47 KAMISHNA
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BIOLOGY FORM 3 FINAL EDITED 16.09.2025.indd 108
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