Covers topics such as the control of blood glucose, heart rate and ventilation, and outside the body in an ecological perspective of populations. Colour coded for the use of PEEL (point, explain, evidence, link).
Negative feedback is a crucial principle in living organisms. It is a regulatory mechanism that
maintains constant conditions where a positive or negative change can cause departure from the
fixed normal state. This is involved primarily in homeostasis, the maintenance of a constant internal
environment, these changes are detected by receptors and then corrected by effectors.
One homeostatic system that relies on negative feedback is the control of blood glucose. In the
pancreas receptors detect when the blood glucose level goes below or above the correct amount.
The cells in the pancreas are stimulated to release the hormone glucagon from alpha-cells, if blood
glucose levels are too low, and insulin from beta-cells, if it is too high. Glucagon stimulates
glycogenolysis, in which glycogen stored in cells is broken down to make glucose, also
gluconeogenis in which glucose is formed from other substances such as glycerol. This stimulates
cells to release glucose into the blood, therefore increasing blood glucose concentration
back to the normal. The insulin stimulates cells to take up glucose from the blood and undergo
glycogenesis, decreasing the blood glucose levels back to normal level again. These are therefore
both negative feedback mechanisms.
Negative feedback is also involved in heart rate and ventilation. Heart rate can be altered to meet
varying demands for oxygen, for example during exercise the heart rate will increase to more than
double. Chemoreceptors in the blood are sensitive to changes in the pH, when carbon dioxide
produced by tissues increases due to increased respiration, the blood pH is lowered. Within the
wall of the right atrium of the heart are a district group of cells known as SAN, it is here that the
initial stimulus for each contraction comes from. Changes to heart rate are controlled by a section
in the brain called the medulla, this has two centres; one to increase and one to decrease heart
rate. The SAN increases the heart rate when these carbon dioxide levels are high, this then
removes the carbon dioxide faster as the heart is pumping the blood at a faster rate and it’s
concentration returns to normal.
Another homeostatic system is thermoregulation, the maintenance of constant internal
temperature. This is necessary for enzyme action, as enzymes have an optimum temperature of
roughly 37degreesC in humans, if they go above this they will denature and therefore will not work.
Thermoreceptors in the hypothalamus monitor internal temperatures by detecting changes in the
blood temperature while thermoreceptors on the skin monitor the external temperature.
Vasodilation and sweating are both examples in reduction of blood temperature, veins dilate to
allow the blood nearer to the skin surface to cool the blood inside down. This blood then passes
over the hypothalamus, signalling it to send fewer impulses to the heat loss temperature centre
and therefore has brought the body temperature back to the normal, but not below it.
Negative feedback can also been seen outside the body in an ecological perspective, as
populations tend to remain roughly constant. Predation has this effect, when removing other
factors, an increase in a predator’s population size will cause a decrease in it’s prey’s, which
eventually will loop round and cause a decrease in the predator’s population, because there is not
enough food (prey) for them all. This keeps the population size stable and maintains the right
balance of predator to prey ratio. Another example of this is intraspecific competition, when it is low
the population size tends to increase, as there is an excess of resources such as food. However
when it increases too much it is brought back to the normal as competition has increased and
resources are scarce. This is therefore another example of negative feedback, as when the change
increases or decreases, it is brought back to the norm.
Point
Explain
Evidence
Link
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