AQA A2 BIOLOGY UNIT 5: Feedback Mechanisms

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  • Created on: 18-04-14 14:24
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The control of any self-regulating system involves a series of stages that feature:
The set point which is the desired level at which the system operates which is
monitored by the...
Receptor which detects any deviation from the set point and informs
Controller which coordinates information from various receptors and
sends instructions to an appropriate...
Effector which brings about the changes needed to return the system to
the set point ­ this return to normality creates a...
Feedback loop which informs the receptor of the changes to the system
brought about by the effector
Negative Feedback
Negative feedback occurs when the feedback causes the corrective measures to be turned off.
In doing so, it returns the system to its original level.
If the temperature of the blood increases, thermoreceptors in the hypothalamus send nerve impulses to
the heat loss centre, which is also in the hypothalamus.
This is turn sends impulses to the skin.
Vasodilation, sweating and lowering of the body hairs all lead to a reduction in blood temperature.
If the fact that blood temperature has returned to normal is not fed back to the hypothalamus, it will
continue to stimulate the skin to lose body heat.
Blood temperature will then fall below normal and may continue to do so cause hypothermia.
What happens in practice is that the cooler blood returning from the skin passes through the
The thermoreceptors detect that the blood temperature is at its normal set point again and so they
cease to send impulses to the heat loss centre.
This in turn stops sending impulses to the skin and so vasodilation and sweating cease and blood
temperature remains at its normal level rather than continuing to fall.
The blood, having been cooled to its normal temperature, has turned off the effector that was correcting
the rise in temperature.
This is therefore negative feedback.
Blood glucose concentration
If there is a fall in the concentration of glucose in the blood, the cells in the islets of Langerhans in the
pancreas produce the hormone glucagon.
Glucagon causes the conversion of glycogen to glucose and gluconeogenesis in the liver.
As a result, the blood glucose concentration rises to normal.
As this blood circulates back to the pancreas, the cells detect the change and stop producing glucagon
= negative feedback.
In the same way, if the blood glucose level rises, rather than falls, insulin will be produced from the
cells in the pancreas.
Insulin increases the uptake of glucose by cells and its conversion to glycogen and fat.
The fall in blood glucose levels return to normal = negative feedback.
Having separate negative feedback mechanisms that control departures from the norm in either
direction gives a greater degree of homeostatic control.

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Positive Feedback
Positive feedback occurs when the feedback causes the corrective measures to remain turned on.
In doing so, it causes the system to deviate even more from the original level.
Examples are rare, but one occurs in neurones when a stimulus causes a small influx of sodium ions .
This influx increases the permeability of the neurone to sodium ions so more ions enter, causing a
further increase in permeability and even more rapid entry of ions.…read more

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The oestrous cycle is the regular pattern of changes that takes place in the reproductive system of
female mammals.
It is controlled through the interaction of a number of hormones.
These hormones circulate in the blood plasma and so reach all parts of the body.
However, only cells with the appropriate receptors can respond to a particular hormone.
In humans, and other primates, the lining of the uterus is shed along with some blood between each
cycle.…read more

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The menstrual cycle begins when the uterus lining is shed, along with some blood (days 1-5)
2. From day 1, the pituitary gland releases FSH into the blood which stimulates follicles in the
ovary to grow and mature
Each follicle contains an egg
3. The growing follicles secrete small amounts of oestrogen into the blood
This low level of oestrogen cause the uterus lining to build up again and also inhibits the
release of FSH and LH from the pituitary gland = negative feedback
4.…read more


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