Cambridge International A Level Biology Revision Guide

Chapter 14: Homeostasis
The hypothalamus receives information about
temperature from other sources as well. The skin contains
receptors that monitor changes in skin temperature.
The skin temperature is the first to change if there is a
change in the temperature of the surroundings. These skin
receptors give an ‘early warning’ about a possible change
in core temperature. If the core temperature decreases,
or if the temperature receptors in the skin detect a
decrease in the temperature of the surroundings, the
hypothalamus sends impulses that activate the following
physiological responses.
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Vasoconstriction – muscles in the walls of arterioles
that supply blood to capillaries near the skin surface
contract. This narrows the lumens of the arterioles and
reduces the supply of blood to the capillaries so that
less heat is lost from the blood.
Shivering – the involuntary contraction of skeletal
muscles generates heat which is absorbed by the blood
and carried around the rest of the body.
Raising body hairs – muscles at the base of hairs in the
skin contract to increase the depth of fur so trapping
air close to the skin. Air is a poor conductor of heat
and therefore a good insulator. This is not much use in
humans, but is highly effective for most mammals.
Decreasing the production of sweat – this reduces the
loss of heat by evaporation from the skin surface.
Increasing the secretion of adrenaline – this hormone
from the adrenal gland increases the rate of heat
production in the liver.
The hypothalamus also stimulates higher centres in the
brain to bring about some behavioural responses. Some
animals respond by curling up to reduce the surface area
exposed to the air and by huddling together. We respond
by finding a source of warmth and putting on
warm clothing.
When an increase in environmental temperature is
detected by skin receptors or the central thermoreceptors,
the hypothalamus increases the loss of heat from the body
and reduces heat production.
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Vasodilation – the muscles in the arterioles in the
skin relax, allowing more blood to flow through the
capillaries so that heat is lost to the surroundings.
Lowering body hairs – muscles attached to the hairs
relax so they lie flat, reducing the depth of fur and the
layer of insulation.
Increasing sweat production – sweat glands increase
the production of sweat which evaporates on the
surface of the skin so removing heat from the body.
The behavioural responses of animals to heat include
resting or lying down with the limbs spread out to
increase the body surface exposed to the air. We respond
by wearing loose fitting clothing, turning on fans or air
conditioning and taking cold drinks.
When the environmental temperature decreases
gradually, as it does with the approach of winter in
temperate climates, the hypothalamus releases a hormone
which activates the anterior pituitary gland (page 312)
to release thyroid stimulating hormone (TSH). TSH
stimulates the thyroid gland to secrete the hormone
thyroxine into the blood. Thyroxine increases metabolic
rate, which increases heat production especially in the
liver. When temperatures start to increase again, the
hypothalamus responds by reducing the release of TSH by
the anterior pituitary gland so less thyroxine is released
from the thyroid gland.
There are two other examples of the role of
negative feedback in homeostasis later in this chapter:
osmoregulation and blood glucose control. Sometimes
control mechanisms do not respond in the way described
so far. If a person breathes air that has very high carbon
dioxide content, this produces a high concentration of
carbon dioxide in the blood. This is sensed by carbon
dioxide receptors, which cause the breathing rate to
increase. So the person breathes faster, taking in even
more carbon dioxide, which stimulates the receptors even
more, so the person breathes faster and faster. This is an
example of a positive feedback. You can see that positive
feedback cannot play any role in keeping conditions in
the body constant! However, this method of control is
involved in several biological processes including the
transmission of nerve impulses (page 335)
QUESTION
14.2 Use Figure 14.2 on page 301 to make a flow diagram
to show the negative feedback loop that keeps
temperature constant in a mammal. Your diagram
should include the names of the receptors and
effectors, and the actions that the effectors take.
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