Transcript Thermoregulation

Thermoregulation
Why Regulate Heat
• With the exception of some prokaryotes,
life can only exist between temperatures of
about -2ºC and about 50ºC.
• There are several reasons for this:
– Enzymes are temperature sensitive
– Many membrane proteins can only
function properly if they float freely in the
lipid bilayer, which is only possible if it is
liquid; at low temperature the bilayer
freezes.
Why Regulate Heat
– At around 85ºC, the hydrogen bonds
holding the two strands of DNA together
break, causing it to become single
stranded.
Thermoregulation
• This is the regulation of body temperature.
• Homeothermy is the ability to regulate body
temperature.
• Includes birds and mammals
• Remember poikilothermic and
homeothermic.
• Homeothermic is hard to define as there are
animals that can partially regulate their
temperature by their behaviour.
Thermoregulation
• The most useful distinction is based on
how temperature is regulated.
• Ectotherms – regulate their body
temperature by their behaviour; they can
only maintain their body temperatures
above the ambient temperature by
absorbing radiant heat.
• Ecto means outside.
Thermoregulation
• Endotherms keep warm using heat
generated inside the body; they regulate
their heat loss by physiological
mechanisms in the skin.
• Endo means inside.
Body Temperature in
Endotherms
• Most mammals have body temperatures
between 37-39ºC
• Birds have slightly higher temperatures
between 40-42ºC
• Our body temperatures varies slightly for a
number of reasons.
– In healthy humans the average is about
35.8ºC in the early morning and about 37.3ºC
in the evening. (for precise comparisons temp
should be taken at the same time each day)
Body Temperature in
Endotherms
• Temperature varies with level of activity,
and may rise to 40ºC in vigorous exercise.
• Women in the second phase of their
menstrual cycle have temperatures about
0.3ºC higher than the first phase.
• Body temperature varies from person to
person suggesting that there may be
genetic factors involved.
Body Temperature in Endotherms
• Many endotherms hibernate in winter,
body temperature falling to a degree or
two above the ambient temperature.
• Temperature also varies from one part of
the body to another
– Though the core temperature in deeper parts
of the body does not fluctuate much , in the
outer shell (especially the limbs) it varies
considerably.
– Different organs have slight differences,
reflecting variation in heat production.
How We Lose and Gain Heat
• Heat can be gained or lost from any place
where the body is in contact with the
environment (skin and lungs)
• Heat can be gained or lost by:
– Conduction
– Convection
– Radiation
– Evaporation
Direct and
reflected
solar
radiation
Evaporation
from lungs
Evaporation
from Skin
Conduction
and
Convection
Radiation
Conduction
from ground
Conduction
to ground
Conduction
• When your body is in direct contact with
a cooler object you lose heat by
conduction. Heat is also lost by
conduction through the gut wall and the
lungs.
• The rate at which heat is lost by
conduction is affected by 2 factors:
– The thermal conductivity of the material next
to the skin. A naked person is quite
comfortable in air at 20ºC, but quickly feels
cold in water at the same temperature. Water
conducts heat faster than air.
Conduction
– The temperature gradient, the steeper the
thermal gradient, the faster the conduction.
Convection
• This is the transfer of heat by currents in
gases or liquids.
• This is why cold air feels much colder
when there is a wind blowing.
• i.e. wind chill and how a fan makes you
feel cooler.
40°F = 4.4°C
10°F = -12.2°C
Wind Chill
Radiation
• Absorption of heat from the sun or a heat
source such as a fire.
Evaporation
• The change from liquid to gas absorbs
latent heat, which produces a cooling
effect.
• To keep cooler than the surroundings
animals have to lose water – sweating.
– In very hot weather a person may produce as
much as 1.5L of sweat per hour.
• Water is also lost from the respiratory
system.
The Skin
• This is the largest organ in the human
body.
• The skin plays a vital role in
thermoregulation in two ways:
– It contains receptors that detect changes in
the environmental temperature
– It contains effectors that can vary the rate of
heat loss from the body.
The Skin
The Skin
• Consists of two distinct layers:
– An outer epidermis
• Has to withstand wear and tear.
• The innermost cells are in constant mitotic
division and form the Malpighian layer
• As they differentiate the epidermal cells
produce large amounts of the tough, fibrous
protein, Keratin becoming strongly bonded
together.
• Cells near the surface make up the dead
cornified layer.
The Skin
– An inner dermis
• This is much thicker than the epidermis and
contains a network of fibrous proteins,
collagen and elastin.
• It contains several structures that are
important in thermoregulation.
The Inner Dermis
• Hair Follicles
– These are pits from which hairs grow.
– The angle of the hair can be adjusted by
contraction of a small erector muscle attached
to each follicle.
– In most mammals this allows the thickness of
the fur to be increased.
– In humans this thermoregulatory role has
been lost, though the muscles still contract to
produce “goose pimples” when the body is
cold.
The Inner Dermis
• Sweat Glands
– Deep in the dermis these are supplied by
sympathetic nerve fibres.
– In humans most sweat glands are eccrine
glands and secrete a dilute salt solution
which cools the skin when it evaporates.
– In most mammals most sweat glands open
into hair follicles and secrete pheromones
and play no part in thermoregulation.
The inner Dermis
• Blood Vessels
– Besides nourishing the skin, these bring heat
to the body surface.
– Stimulation of the smooth muscle in the
arterioles causes them to constrict and thus
reduce the delivery of heat to the skin.
Inner Dermis
• Afferent nerve endings
– Some of these are sensitive to changes in
temperature
• Beneath the dermis is a layer of adipose
tissue which contains fat that acts as an
energy store and a thermal insulator.
Heat Production
• Most energy taken into the body is
eventually lost as heat produced in
metabolism, this is the main source of heat
in endotherms.
• Respiration or ATP production occurs in
the mitochondria.
• There are three main steps.
Step 1
• A series of enzymatic reactions known as
the TCA Cycle or Kreb’s Cycle where
carbon from glucose and lipids is
converted into CO2, while electrons are
transferred to the inner membrane of the
mitochondria.
• This CO2 is exhaled.
Step 1
Step 2
• In the inner mitochondrial membrane a
series of oxidation/reduction reactions
take place (electron transport or
oxidative phosphorylation).
• Electrons are transferred through a series
of specialized molecules in the membrane.
• This results in the movement of H+ ions to
the space between the inner and outer
membranes – these will build up unless a
channel opens up allowing them back
across the membrane.
Step 2
• Meanwhile the electrons are transferred to
O2 to make H2O.
• Importantly this chemical reaction
releases heat and is a major site of heat
production in the body.
Step 2
Step 3
• A by-product of this process is that it
creates an ion gradient across the
membrane.
• This difference in H+ ions drives a protein
ATP Synthase that is embedded in the
membrane, which adds phosphate to ADP
to make ATP.
• In the process H+ ions are transported
back across the membrane so the electron
transport in step 2 can only keep going if
ATP synthase is active.
Step 3
Exercise and Heat Production
• When a person exercises, muscles have
increased demand for ATP.
• Feedback signals are sent to increase the
rate of oxidative phosphorylation. This
means there is greater demand for O2 and
increased production of CO2.
• As a by-product of this heat is produced.
• This is why you get hot when you
exercise.
Shivering
• This is a feedback mechanism that occurs
when a mammal is faced with cold
temperatures.
• Shivering is a result of involuntary
muscle contractions, causing a rise in
oxidative phosphorylation and thus heat
production.
• This is known as shivering-induced
thermogenesis.
Non-Shivering Thermogenesis
• Babies are not able to shiver.
• They use non-shivering thermogenesis
to keep warm.
• This happens in a special tissue called
brown fat.
• In brown fat, there are a large number of
mitochondria that express a special
protein called uncoupling protein (UCP).
Non-Shivering Thermogenesis
• UCP is a channel that allows H+ ions to
travel back across the membrane without
having to produce ATP.
• This means that the transfer of electrons
to oxygen and the pumping of H+ ions
outwards can happen without the need for
ATP.
• This means that heat production can
happen without making extra ATP.
Non-Shivering Thermogenesis
• Babies have a significant amount of brown
fat, and they use it until they develop the
ability to shiver.
• Other animals, e.g. bears, rely on brown
fat when they hibernate, as it allows them
to produce heat without having to
exercise.
Non-Shivering Thermogenesis
Feedback Mechanisms to
Restrict Heat Loss
• Skin is an area where animals can lose
heat to the environment.
• Blood is a major way to transfer heat
around the body.
• Animals can rapidly reduce blood flow to
the skin, reducing heat transfer and thus
reducing heat loss.
• This is why your fingers and toes go blue
when they are very cold.
Feedback Mechanisms to
Restrict Heat Loss
• In a cold environment, small muscles in
the skin are activated that make the hair
stand upright.
• This is called piloerection (goosebumps).
• This blocks the flow of air around the skin
slowing heat loss.
How does the Body Cool Down
when it is too Hot?
• Sensors in the skin detect temperature
changes, send signals to the brain and
induce responses that lower temperature.
– Blood Flow to the skin can be increased when
body temperature is high, so increasing the
rate of heat loss to the atmosphere by
radiation.
e.g. large ears in elephants.
How does the Body Cool Down
when it is too Hot?
• Sweating occurs mostly in humans and other
primates.
• Sweating occurs as a feedback mechanism
in response to a rise in external temperature
or due to heat produced during exercise.
• Sweating occurs when signals from the brain
activate sweat glands throughout the skin.
• The sweat is a saline solution.
• It cools the body as it evaporates from the
skin.
How does the Body Cool Down
when it is too Hot?
• Many animals use panting as a means of
losing heat.
• Panting transfers heat to the water in the
lining of the airways leading to the lungs.
• These are normally kept moist so by
panting the animal is able to induce more
water evaporation and thus more cooling.
How is it Controlled?
• The major control centre for regulating
body temperature is the hypothalamus.
• This has its own sensors, which can sense
very small changes in temperature and
send out signals to regulate cooling
mechanisms.
• This is called the central regulatory
mechanism as it is sensing temperature
at the core of the body.
The Hypothalamus
How is it Controlled?
• It takes time for core temperatures to be
affected by the environment so the body
also has temperature sensors in the skin
that tell the brain what is happening to the
environmental temperature, enabling the
body to react more quickly.
• These sensors are a special type of
neuron found only in the skin. They
contain a series of protein channels called
TRPV channels.
How is it Controlled?
• These TRPV channels are found in the
plasma membrane of these nerve cells,
and their structure is highly tuned to a
particular temperature.
• It the temperature changes, the structure
of the TRPV proteins changes., then their
function changes.
• When TRPV proteins are at their optimum
temperature, they make a channel through
the membrane.
How is it Controlled?
• This allows a number of ions to cross the
membrane changing the electrical charge
across the membrane.
• This triggers an electrical pulse that
moves along the nerve cell to the next
nerve cell until it reaches the
hypothalamus, where it adds to
information provided by the central
regulatory sensors.
Fever
• This is associated with infection.
• The immune system produces fever-inducing
chemicals called pyrogens, which travel
through the blood to the hypothalamus,
where they trick it into raising the body
temperature.
• Aspirin and paracetamol lower temperature
by blocking the production of pyrogens.
Frostbite
• The blood vessels in the skin constrict in
very cold conditions to preserve heat.
• If this is prolonged it starves the cells
peripheral tissue of nutrient and heat.
When nerve cells stop working no signals
get sent to the brain, the fingers and toes
go numb.
• If this happens for a short time it can be
reversed. If it happens for longer the cells
die i.e. frostbite
Frostbite
Why do Chillis Burn?
• Capsaicin, a chemical found in chillis
binds directly to the TRPV channels in the
lining of the mouth.
• This activates these receptors in the same
way as if the food were genuinely too hot,
so the brain generates the same type of
signal.