Thermoregulation Flashcards
Are humans homeotherms
Yes - they employ physiological as well as behavioural means to keep core body temperature within a narrow range
Are muscles 100% efficient
No - a high proportion of energy is lost as heat
How do humans utilise evaporation to lose heat
Panting
Sweating
How good is sweat at removing heat
Every gram of sweat evaporated removed 2.45kJ of heat
What kind of system is thermo regulation
Homoeostatic negative feedback system with feedforward control
Name 3 key areas involved in thermoregulation
Median preoptic sub nucleus
Median preoptic area
Lateral parabrachial nucleus
What is temperature change
The difference between heat production, heat gain and heat loss (measured in watts) divided by the heat capacity of the system
Show rate of temperature change as an equation
Production+gain-loss
———————————-
Thermal mass
What are the units used for the following:
a) Rate of change in temperature
b) Heat gain
c) heat loss
d) Thermal mass
a) Degrees C per second
b) Watts
c) Watts
d) J/ Degrees C
What is endothermy
Internal generation of heat
Give two advantages and disadvantages of being an endotherm
Advantages: gives organism speed and independence from the environment
Disadvantage: requires considerable energy input
When the heat production is greater than required to stay warm a great deal of water can be needed
What is the source of heat in endotherms
Metabolism – the moment metabolic clique active a tissue, the more heat is generated
Using 1 L of oxygen to break down carbohydrates yields how much energy?
20 kJ or 4.8 kcal
How much heat does the average sized human produce
How much energy does this amount to
80 W at resT
70 kcal/h or 1600 kcal per day
80 W is sufficient power to raise how much water by 1° per hour
70 kg
What happens if you fully insulate a person who is not moving
What does this mean for warming up a cold person
The temperature will rise by 1° C per hour
Simply insulating them and waiting for them to warm up is not a good strategy
it could take longer than 10 hours to raise their temperature by 10°
At rest, how much of the heat generated by the body is generated by the brain
What about muscles and skin at rest
General viscera?
1/6
1/6
1/2
1 L of oxygen consumed releases how much energy
20 kJ
How does temperature affect memory
Cooling of just 1° C can impairment memory
What happens if the brain is warmed by a few degrees C
Confusion and nausea arise
Which is the most temperature sensitive organ
The brain
When is brain cooling used
When brain damage has occurred or blood circulation is compromised
How does fetal oxygen deficiency and life expectancy relate to temperature
Survival improves if hypothermia is induced
What is being used by adults during chemotherapy to prevent hair loss
How does this work
Cool caps
The cooling lowers the metabolic rate of the cells that produce hair and this lowers the toxicity of the drugs
What is the optimal temperature for the heart
It will continue to pump until which lower Boundry
38° C
25° C
What temperature a heart for heart transplants start in
Just a few degrees C
Contraction stop until the solution is replaced by blood and the heart is rewarmed
What will happen if the heart goes below 25° C
The heart will stop beating and the subsequent lack of blood flow to the brain will kill you
Describe the experiments by Starling to investigate the temperature sensitivity of the heart
He used hearts isolated from dogs and cats to share that cardiac output varies a temperature
At about 44° C the heart stops pumping. Above the temperature of the heart undergoes irreversible damage which is colloquially known as cooking
Below 25° say the heart stops. Whilst the damage caused by heating is irreversible hearts that have been called can often be restarted when rewarmed
Which phrase comes from the fact that a cold heart can restart when rewarmed
No one is dead until they are warm and dead
What is the best temperature for skeletal muscle function
Why is this a good thing
Slightly warmer than normal core body temperature
When they are active they produce a lot of heat
cool skeletal muscle produces less force, moves more slowly and produces less power
How does the power of skeletal muscle change between being at 10° and 20°C
The power increases by 10 times
What temperature it to the testes function best at
35 or 34° C
How do you temperatures of 40° affect fertility
Can lead to temporary infertility
Does heat stress only affect male reproductive organs?
Work on Dairy cattle shows that heat stress affects reproductive performance of both sexes
This may not be a direct effect of heat but the effect of the thermoregulatory stress that the heat causes
How temperature sensitive is the skin
Why is this
It can function over a very wide range of temperatures
At the lower end sensation becomes lost and at the higher end there may be pain
The skin form is one of the thermal layers that sits between the heat generation mechanisms within the organism and the external environment. It is responsible for a lot of thermoregulation and so often suffers from large swings in temperature
How can you think of the distribution of the temperature in an organism
Like an onion but less uniform
What is the core temperature of most endotherm is represented by
The temperature of the arterial blood
Why is the organism mean temperature important
It governs the potential swings and temperature that are possible as bloodflow changes
What is the warmest part of the body
The core temperature except when doing physical exercise
It would then be skeletal muscle
When is the thermal gradient between skin and core temperatures highest and lowest?
Highest: in cold conditions
Lowest: in warm conditions
How would a patient hand appear if they had Raynaud’s syndrome
Very cold fingers (below room temperature)
This arises from poor bloodflow and sensation can be compromised
In a thermal vision image a calf will have patches of red and yellow around the body. What is this?
Underlying brown fat
How does our core temperature vary over a 24 hour cycle
It is typically coolest around 4 am and highest in the early afternoon
How much can exercise a warm an individuals core
3° C
Name three cycles which causes temperature variation
Diurnal variation
Menstrual cycle
Seasonal cycle
What is the worst way to obtain a core body temperature
Why
Oral/sublingual
Airflow through the mouth affects temperature reading
How accurate is axillary temperature measurement
Not very
What is a new popular form of testing core body temperature
Tympanic measurements with an infrared thermometer
They are quick and easy but hard to perform reliably and not very accurate
What are the only two really good ways to test core temperature
Oesophageal/intestinal with either a probe or temperature broadcasting pill
Or
Rectal measurement
What is the Boltzman constant
What does this mean
1x10^-23 J/K/ molecule
One mole of molecules has an energy content of 8.3 J per Calvin
How much energy does each mole have at 37° C
2.6 kJ per mole
Why is it important that at 37° C it molecules have 2.6 kJ per mole?
This is not enough energy to break carbon carbon bonds so proteins and other organic compounds are stable
However it is enough to break Van de Waal’s interactions to enzymes can work
37° C is therefore a good temperature because it renders a molecule sufficiently but not too energetic
What does it mean to say a biological reaction has a Q10 of two
They increase their rate by two times for a 10° C rise
What is the Q10 of most biological reactions
Two or three
What happens if the body temperature gets too low
Not enough energy is provided
Proteins and lipids are too inflexible
Organs can be slowed to a stop
What happens if the body temperature gets too high
Proteins denature
Membranes too flexible
Metabolic rate too high
How high are the body temperatures of birds
42 in the light
Compare the temperature fluctuations of camels when hydrated and dehydrated
Why is this
Hydrated: fluctuation of 3° C throughout the day
Dehydrated: fluctuation of 6° C throughout the day
The camel is trying to reduce the need for a relative cooling with water by starting with a lower court temperature in the morning and allowing its core temperature to rise more during the day. It can then radiate out heat without losing water at night.
What is hibernation
A strategy to reduce the energy required to stay warm
It is a period of reduced metabolism and body temperature during the winter.
How low can cause temperatures fall during hibernation
How long can hibernation last
To only 1° C
For months
What is a shorter term response that can be compared to hibernation
Torpor
When animals such as hummingbirds are in active but not actually asleep
Do bears hibernate?
They have a period of winter lethargy (Denning) where there core temperature drops from which they can awake nearly instantly
This is not hibernation however as the temperature drops for less than true hibernators (<10 degree drop)
What is the survival rate if humans call body temperature reaches 41.5° C
50%
This dropped even lower if you reach 42° C
What is the main cause of death during the heat wave
Haemoconcentration
This causes thrombosis and heart attacks
Heat stress doesn’t usually result in death through overheating but rather the effects of Thermoregulation – sweating
How many XS winter deaths are there each year on average
What does this mean
26,000
Cold weather is indirectly responsible for about 5% of deaths
Why does cold weather in directly lead to increased death
Impaired mobility
Circular Tory Phalia
Diseases e.g. flu
Increase viscosity of blood is seen in the cold
What is perniosis?
How can it be avoided
Chilblains
Cold exposure or rapid warming following cold damages capillary beds in the skin which causes redness, itching, blisters and inflammation
Appropriate insulation
What happens in frostbite
Not just capillaries are damaged but the underlying tissue to and the cells are broken apart by ice crystal formation
Where are the thermoregulatory inputs integrated
In the preoptic area of the hypothalamus
What are the Thermo receptors
How do they work
Both warm and cold sensitive nerve fibres found in the skin
They respond to relative temperature changes
Warm fibres increased discharge above 37° C
Cold fibres increased discharge below 37° C
However noxious heat is capable of activating cold fibres
Which channel type underlies the action potential firing of Thermo receptors
TRP (Transient receptor potential)
Which ion channels are activated by an increase in temperature
Above which temperature are they usually activated
TRP-V1
TRP-V2
TRP-V3
Above 30° C
Which channel is activated by cold
TRP-M8
What can activate TRP – M8
Temperature is below 25° C
menthol
Are the TRPM8 channels only useful for temperature regulation
They also Have a role in regulating eating behaviour and fuel utilisation
Knock out mice will get fat
Does the actual temperature of the hypothalamus affect heat regulation
Yes
If the hypothalamus is heated above a certain temperature, cooling mechanisms become activated and vice versa
Describe brown adipose tissue
Similar to skeletal muscle without the contractile proteins
It is rich in blood vessels and mitochondria
Its activation underlies nonshivering thermogenesis
What activates brown adipose tissue
Noradrenaline released from sympathetic nerves under hypothalamic control
How much of the body weight of a newborn calf is estimated to be brown adipose tissue
2%
How much energy can brown adipose tissue produce
300 W per kilogram
What makes mice a poor model for humans
How might we overcome this
The dominant role of brown adipose tissue as an energy consumer
Keep the mice in warm conditions (30° C)
What protein gives brown adipose tissue the ability to generate heat
UPC1 in mitochondria
What does UPC1 do
Allows H+ to leak out of mitochondria
This allows the electron transport chain to operate unhindered, making the mitochondria inefficient and generating lots of heat
Briefly describe the molecular mechanism behind UCP1
There is a binding of the long chain fatty acids to UCP1 producing a complex capable of transporting proteins
Which receptor activates brown adipose tissue
Why might these receptors be activated
β-3 adrenergic receptors
In low nutrient Content diets were brown adipose tissue may help burn off excess calories
How fast does skeletal muscle contract when shivering
200Hz Isolation
True or false:
Shivering is more powerful in individuals who are chronically exposed to cold
False
Shivering tends to be lost as a response in these patients as other production mechanisms are up regulated
What is malignant hyperthermia
A single point mutation of the ryanodine receptor
The receptor becomes spontaneously active causing XS muscle contractions, resulting in large muscles
How can malignant hyperthermia be treated
Active cooling can delay death
Dantrolene is the only effective treatment
What are the issues with dantrolene?
It is hard to prepare and not very water-soluble
It is pH 9.5
Which hormone is up regulated in response to chronic cold
Thyroxine
This is a powerful way of regulating general heat production
Name three physiological controls of heat loss in humans
Bloodflow
Piloerection
Sweating (panting in other mammals)
What are our three ways of losing heat
Radiation
Conduction
Evaporation
In a cool room how do we lose most of our heat
Radiation
What happens if the walls of a room are above body temperature
Radiation only results in heat gain
Discuss the specific heat capacity of air, stone, and water
What does this mean
Air: Low
Stone: high
water: high
Solid floors and water are very good ways of losing heat via conduction
Heat is also lost through water via convection
True or false
Even in what feels like Warm water we can lose the equivalent amount of heat as our BMR
True
What is glabrous skin
Where is it found
Describe its role in Thermoregulation
Hairless skin
On the palms of the hands and face
It does not play a major role in some regulation But they are rich and arteriovenous anastomoses which can bypass the high resistance capillary bed
What is the innovation of apical skin
Sympathetic vasoconstrictor fibres
What are the two sympathetic innervations of hairy skin
Vasoconstrictor
Sympathetic vasodilator (cholinergic)
What is the normal thermoregulatory pathway in hairy skin
What about in hairy skin during high core temperatures
The vasoconstrictor nerve supply
The vasodilator pathway
Normal resting bloodflow to the skin is ___?
Low
What must occur with sweating for heat loss to continue
Increased skin blood flow
How does piloerection work
Small muscles at the base of hair follicles change the angle of the hair to the skin to trap air or provide defence advantages
What is the most powerful source of heat loss from the skin
How much energy can this lose
Evaporation of water
2500 kJ/L of water
What are the two major sites of evaporation
Skin or respiratory surfaces
Why is panting better than sweating
Starting from the skin result in salt loss and requires airflow – panting does not
What is a disadvantage of panting
How can this be overcome
It produces muscular heat and can alter arterial pCO2
Shallow panting can minimise alkalosis by ensuring that most of the ventilation is in the upper respiratory dead space.
Muscular work can be minimised by panting at the natural resonant frequency of the respiratory system
What is the natural resident frequency of the respiratory system of a dog
5Hz
Do birds pant or sweat
They have a form of panting called gular flutter
What does panting provide that sweating does not
The ability to cool the brain selectively through a structure known as the rete mirabile
What is the rete mirabilia?
A structure where veins returning cooled blood from nasal passages are associated with the arterial vessels supplying the brain
This network allows heat transfer and a cooler brain than the core
Rete mirabilia also occur in the blood supplying the testes and feet
What are the malodorous sweat glands
Apocrine
What do eccrine sweat glands produce
A weakly salty solution used in Thermoregulation
What are the dimensions of an eccrine sweat gland
40μm in diameter
Several mm long (longer in people who sweat more)
They are coiled tubes formed from a polarised epithelium with a tubular lumen
What triggers sweating
Sympathetic cholinergic innervation – the sudomotor nerves
Which Ion transporter plays an important role in sweat production
How
The Na-K-Cl2
The secretion produced at the base of a sweat gland is modified by this transporter as it flows to the surface and salt is removed
How complete is the removal of sodium from the initial secretion in eccrine sweat glands
At a low flow rate it is almost complete (5mM NaCl)
At higher rate, this fact is saltier – still hypotonic but the salt loss is significant (60mM)
How can you train your sweat glands to produce more sweat
What can do the opposite
Heat acclimation and aerobic training
Bed rest
What kind of people produce more sweat
Fitter/trained people are able to produce more sweat earlier during exercise
What makes elderly people particularly susceptible to both overheating and haemoconcentration in heatwaves?
The detraining of sweat glands by failure to exercise
What is the difference in sweat responses between men and women
The only difference is fitter individuals sweat more because they can produce more muscular power
However women are still at more risk than men of the heat simply because they are typically smaller and less powerful so their sweat glands have been trained less by normal heat stress
What is pyrexia
Fever
What causes fever
Infection
Pyrogens
Prostaglandin E2
What stimulates the production of prostaglandin E2
Where is it secreted from
Pyrogens
endothelial cells in small venules of the hypothalamus
What does prostaglandin E2 do
Raises the point at which cooling mechanisms are activated
What is the symptom of a fever
A feeling of cold in a Thermoneutral environment
Name an anti fever drug
NSAID drugs block PGE2 and are thus antipyrexic
Why do infections cause fever
They are exogenous pyrogens that activate white blood cells to produce endogenous pyrogens and activate the hypothalamic endothelium to produce PGE2 and raise the set point
Why are fevers clinically useful
Diagnostically – useful sign of presence of infection and effectiveness of treatment
Also indicate the need for rest and reduce desire to perform intense exercise that might be dangerous e.g. endocarditis
How was Fever used to cure syphilis
Malaria could be controlled with quinine
Malaria caused fever
Infecting a patient who had syphilis with malaria caused a fever that killed the bacteria causing syphilis
Give the different definitions for organisms based on how they thermoregulate
Homeotherm- body temperature above environment (best avoided)
Poikilotherm - changeable temperature/ “cold blooded” (again best avoided)
Endotherm- Able to maintain a high body temperature by internal heat production
Heterotherm – mostly have a high regulated body temperature but can cool down
How would you define a hibernating mammal?
Heterothermic endotherm
