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Biology for Advanced Level Secondary Schools

For instance, an increase in the level of measure allows blood glucose level to
glucose in the body triggers a sequence remain constant. This type of system in
of events that tend to remove excess which change in the level of an internal
glucose from the blood by converting it factor causes a corrective mechanism is
into glycogen. In contrast, a decrease in referred to as a self-adjusting system. The
the level of glucose in the body causes science of a self-regulating control system,
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the liver to break the stored glycogen to which operates via feedback mechanisms
glucose in order to supply more glucose in organisms is known as cybernetics.
to the cells of the body. This corrective

Low blood sugar High blood sugar

Pancreas

Low level of sugar in the blood High level of sugar in the
promotes glucagon release blood stimulates beta cells of
from the pancreatic alpha cells pancreases to secrete insulin

Glucagon Insulin Tissue cells
Glucagon stimulates glycogen Insulin stimulates
breakdown into glucose in the tissue cells to take
liver cells up glucose from
the blood
Liver
Glycogen Glucose

Lowers blood sugar level
Raises blood sugar level

Figure 7.2: Glucose and insulin negative feedback loop

Positive feedback feedback occurs during labour, when the
Positive feedback is the self-regulatory hormone oxytocin stimulates muscular
mechanism that operates when the system contraction of the uterus; which in turn
is deviated from a set point which initiates stimulates the release of more oxytocin.
a sequence of events that tends to deviate Positive feedback mechanism may also
further the system. The positive feedback occur in the nerves where a small stimulus
mechanism makes the system to be can bring about a large response to the
unstable; that is why it is not common in effectors (Figure 7.3).
living organisms. An example of positive

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