Page 341 - Biology_F5

Biology for Advanced Level Secondary Schools

in the skin containing heat and cold This causes a person to go into a state called
receptors, which are responsible for hyperthermia. The patient experiences a
detecting increase and decrease in the very weak pulse rate, becomes excessively
skin temperatures respectively. Third, irrational, and sluggish. If the situation is
the comparator is the cortex of the brain not controlled, the patient may go into a
which is responsible for controlling coma and death may occur.
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conscious thoughts and feelings. Thus,
if the affected individual feels too cold Maintenance of a constant body
or too hot can decide to take appropriate temperature in warm environments
action, either to move to a cooler or
warmer place, remove or add more Task 7.3
clothes or take other appropriate voluntary Search for information from the
actions that may help to bring the skin library and internet sources on the
temperature back to favourable state. The mechanisms of maintaining constant
error signals are sent to voluntary skeletal body temperature in warm and cold
muscles through nerve impulses. Fourth, environment conditions. Write short
corrective mechanisms are initiated notes on the searched information.
through behavioural responses.
When the environment is overheated,
the animals use the following adaptive
In regulating the body’s core temperature,
actions are involuntary, controlled by mechanisms to overcome the effects of
internal physiological responses. The overheating.
set point of the homeostatic control (a) Vasodilation
mechanism is the mean body temperature This is an increase in the diameter of
(36.7 °C or 37 °C) which is genetically superficial blood vessels near the body

determined. In this system, the sensor and surface caused by nerve signals, resulting
control centre for body temperature are
located in a small part of the brain called into the relaxation of the vessel’s walls.
the hypothalamus which is sensitive to The blood in the capillaries of the skin
temperature fluctuations. It possesses two may take three alternative routes; through
thermoreceptors, namely the heat loss and capillaries close to the skin surface, in
heat gain centres. The heat loss centre is the dermis, and beneath the layer of
located in the anterior hypothalamus and is subcutaneous fat. In warm climates,
activated by increase in blood temperature, superficial arterioles dilate in order to
whereas heat gain centre is located in the allow blood flow close to the skin surface.
posterior hypothalamus and is activated Heat from the blood is rapidly conducted
by decrease in blood temperature. through the epidermis to the skin surface

from where it is radiated away from the
On the other hand, if the body core body. Rise in blood pressure within the
temperature rises to above 40 °C, the capillaries cause them to dilate, and

homeostatic control mechanism breaks that facilitate heat loss due to radiation,
down and positive feedback takes place.

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