Ectotherms
– Animals that have a variable body temperature.
– Use behavioural mechanisms (e.g. lying in the sun when cold, moving into shade
when hot). Such mechanisms can be very effective, particularly when coupled with
internal mechanisms to ensure that the temperature of the blood going to vital organs
(brain, heart) is kept constant.
We use both!
Thermoregulation
All mammals generate heat and have ways to retain it within their bodies. They also
have physiological methods to balance heat gain, retention of body heat and heat loss
so that they can maintain a constant body temperature. As a result, they are not
dependent on absorbing heat from their surroundings and can be active at any time of
day or night, whatever the external temperature. Most other animals (except birds) rely
on external sources of heat and are often relatively inactive when it is cold.
The heat that mammals generate is released during respiration. Much of the heat is
produced by liver cells that have a huge requirement for energy. The heat they produce
is absorbed by the blood flowing through the liver and distributed around the rest of the
body.
In humans, body temperature is controlled by the thermoregulatory centre in
the hypothalamus. It receives input from 2 sets of thermoreceptors:
– Receptors in the hypothalamus monitor the temperature of the blood as it passes
through the brain (the core temperature), that remains very close to the set point, which
is 37 °C in humans. This temperature fluctuates a little, but is kept within very narrow
limits by the hypothalamus.
– Receptors in the skin (especially on the trunk) monitor the external temperature.
Both sets of information are needed so that the body can make appropriate adjustments.
Our first response to encountering hotter or colder condition is voluntary:
– if too hot, we may decide to take some clothes off, or to move into the shade;
– if too cold, we put extra clothes on or turn the heating up!
It is only when these responses are not enough that the thermoregulatory centre is
stimulated. This is part of the autonomic nervous system, so the various responses are
all involuntary.
Response to low 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 to
several different effectors to adjust body temperature:
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.
Response to high temperature
When an increase in environmental temperature is detectedby skin receptors or the
central thermoreceptors, thehypothalamus increases the loss of heat from the body
andreduces heat production.
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.
Behavioural responses
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:
– The hypothalamus releases a hormone which activates the anterior pituitary gland 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.
VIDEO
Controlling body temperature
https://www.youtube.com/watch?v=Vlp_NHuC0rw
Homeostasishttps://www.youtube.com/watch?v=e4YbdGBvFAE