Thermoregulation Flashcards
uses of body temperature measurements?
aid diagnosis and tracking of infections, timing of ovulation, abnormal core temperature, as well as visualising blood flow (e.g. inflammation)
what region of the brain plays key role in thermoregulation?
the pre-optic area of the hypothalamus
what are the inputs integrated by the pre-optic area of the hypothalamus?
cool and warm sensitive neurons, thermal sensors from skin, state of other physiological systems, and learnt responses
how is heat lost from the body? (4)
radiation, conduction, convection, evaporation of water
how much of the total energy transferred in biological chemical reactions is released as heat?
around 75%
what is the purpose of thermoregulation?
controlling energy released by enzymatic reactions, distributing it around the body and transferring it to the environment, to enable normal physiological function
what do most studies of physiological heat production use?
indirect calorimetry involving measurement of oxygen consumption and CO2 production
equation relating temperature, heat energy, SHC, mass?
temperature= heat energy/SHC/mass
what is 1 calorie equal to in J?
around 4.2J
what is endothermy?
the process of internal heat generation
what are homeothermic endoderms?
animals that have a stable temperature due to internal heat production
what is a homeotherm?
animal with stable/controlled body temperature
what is a heterotherm?
animal with different body temperatures at different times
what is a poikilotherm?
animal which conforms to external temperatures
what is a warm-blooded animal?
animal with warmer blood than the surroundings
what is 0K?
the point at which atoms stop vibrating
what is thermal energy?
measure of the amount of energy in system by virtue of its temperature
what organs are important sites for BMR?
liver and brain
BMR relationship to adipose tissue?
adipose tissue isn’t very metabolically active, doesn’t really contribute to BMR
what is the contribution of skeletal muscle to BMR?
highly variable
which organ is particularly temperature sensitive?
the brain- changes of around 1°C produces measurable declines in function
thermal sensitivity of muscle?
generally performs better when warmer than core body temperature, force of skeletal muscle drops dramatically in cold, as does cardiac muscle
what temperature does the heart stop beating at and why?
stops beating below around 25°C due to failure of electrical activity
temperature sensitivity of testes?
sperm production requires cooler temperatures in humans- not same requirement in
e.g. elephants which have internal testes
temperature sensitivity of skin?
can tolerate very wide range but sensory function changes (is worse in cold)
general effect of cooling on body?
reduced MR in turn reducing oxygen and blood flow requirements
why can cooling be useful during birth?
hypometabolism combined with anapyrexia can reduce damage caused by neonatal asphyxia
what are ‘shells’ in reference to endotherms?
temperature gradients within endotherms which change with environmental temperature and exercise, caused by heat flux
most to least reliable methods of measuring core temperature?
oral, forehead, axillary, tympanic, oesophageal, rectal
temperature fluctuation that isn’t cause for concern in humans?
36-38°C
what can core temperature rise to during intense exercise without causing issues?
around 39.5°C
what core temperature is life threatening in absence of exercise?
> 40°C
survival rate at around 41.5°C?
around 50%
what core temperature does survival rate become very low at?
around 45°C
problems caused by excessively low core temperatures?
don’t provide molecules with enough energy, protein flexibility decreases, lipid viscosity increases, blood viscosity increases which increases circulatory load, capillary perfusion decreases, some enzymes slow more than others, whole organs can fail (heart), timings in CNS go wrong due to altered ion channel kinetics
problems associated with excessively high core temperature?
protein flexibility decreases (potential irreversible denature), lipid fluidity too high, enzymes accelerate, timings in CNS can go wrong dure to altered ion channel kinetics, high rates of food and water needed
what does the stable temperature provided by endothermy allow for?
complexity, specialisations, steady-state modulation of temperature to organs (e.g. cooler brain than core), hibernation, high speed and strength
downsides of endothermy?
expensive energy cost, limits behaviour
average human temperature?
36.8°C
how much can human core body temperature vary by during the day, when is it lowest?
about 1°C during day, lowest at night
what individuals tend to have lower core temperatures?
aerobically fitter individuals, older individuals
what is the most common cause of hyperthermia?
infection
effect of haemoconcentration?
increased risk of thrombosis, increased cardiac workload
risk to cold extremities?
chilblains (inflammatory process associated with capillary bed damage) and frostbite (potentially irreversible damaged due to water crystals and osmolarity changes)
difference between hibernation and denning?
hibernation involves prolonged fall in core temp to only a few °C, denning involves less major drop by about 10°C
what is the feedback loop of thermoregulation?
core temperature
what is the feedforward control of thermoregulation?
skin temperature
what is the output from the pre-optic area?
via ANS to brown adipose tissue, blood flow and heart rate, via somatic nerves to produce shivering
is there a set point for core body temperature?
no single fixed temperature set point, heat loss mechanisms activated above certain temperature, heat generation mechanisms activated below certain temperature, both mechanisms become greater further from activation temperature- which can be modified
how can both shivering and sweating be activated at once?
if heat generation activated at temperatures above the point of activation of cooling
what do activation temperatures of heat generation/heat loss mechanisms depend on?
temperature of the skin, what the organism is doing, time of day, presence of pyrogens and more
how is temperature sensed by the skin?
skin has multiple classes of warm and cold sensitive nerve fibres (thermoreceptors)- receptors where temperature is the stimulus.
how can cold sensitive nerve fibres be wrongly activated?
by noxious stimuli, therefore giving rise to false sensation of cold. because they have broad response range
what non-temperature sensitive recepors can be activated by noxious cold and heat?
nociceptors (pain receptors)
what are the key thermoreceptor channels?
the transient receptor potential (TRP) ion channels- activation depolarises the thermoreceptive neurons leading to generation of APs
what are the heat production mechanisms?
activation of brown adipose tissue, shivering, exercise, change in BMR
what is BAT similar to?
skeletal muscle, but without motor proteins
where is BAT located?
specific regions, especially the thorax in small mammals
what underlies the process of non-shivering thermogenesis?
BAT
what is BAT activated by?
noradrenaline (sympathetic nervous system), under hypothalamic control
what does BAT play a key role in?
nocturnal cold survival, hibernation, birth cold stress
how much of the energy expenditure can BAT account for in small mammals in non-thermoneutral environment?
50%
what is BAT activated by?
LCFAs produced within brown adipocytes upon adrenergic stimulation, which act on mitochondrial protein UCP1 which is preferentially expressed in BAT, allowing H+ to enter mitochondria, removing brake from the ETC increasing metabolism, generating lots of heat (but little ATP)
how much can shivering increase heat production?
by 4x, for short periods
why is shivering sub-optimal for small mammals?
involves movement, makes them vulnerable to predators
what happens to shivering after repeated cold exposure?
can become blunted
what limits heat generation by exercise?
the rate of taking up and consuming O2- referred to as VO2max, subject to training, blood supply to active tissue, mitochondrial function, rise in core body temperature leading to motivational inhibitions
what is malignant hyperthermia?
condition where uncontrolled skeletal muscle activation can cause hyperthermia
what causes malignant hyperthermia?
mutation of the ryanodine receptor allowing it to become spontaneously active
what is malignant hyperthermia associated with?
high skeletal muscle mass
most notable trigger of malignant hyperthermia in humans?
general anaesthetic administration meaning excess calcium release leads to skeletal muscle activation
treatment/cure for malignant hyperthermia?
active cooling can delay death, only cure is administration of dantrolene
what does long term metabolic upregulation to chronic cold environment involve?
thyroid hormones- thyroxine production upregulated, increases BMR to provide energy
when does conduction dominate as a mechanism of heat loss?
when there is good contact to substrates of high thermal mass
when is radiation a large contributor to heat loss?
when high skin surface areas are exposed to the environment at a very different temperature
when does evaporation dominate as a mechanism of heat loss?
when surfaces are wet and cooler or less humid air is present
how is most metabolic heat production lost at rest in a cool room?
through radiation with evaporation and conduction accounting for the rest
how is heat lost in a room where the walls are above core body temperature?
only way to lose heat is evaporation
when is there no radiant heat loss?
if skin is wet from sweat so is cooler than the surroundings
what can make fur more conductive?
lying on hard surfaces, compressing the fur
what is rate of heat loss by conduction dependent on?
blood flow
anatomical structure in birds that utilises counter-current exchange to reduce heat loss via conduction?
blood flowing down to feet warms blood returning from feet so steep temperature drop as blood moves down- therefore feet in contact with ice are cold but well perfused with little energy loss
how is skin blood flow controlled?
under autonomic control as result of sympathetic vasoconstrictor tone (noradrenaline)
why can increased skin blood flow be a stressor for old people?
CO is limited so this can take a large proportion of available cardiac output
what 2 major regions can skin be divided into from a thermoregulation standpoint?
hairless (apical/glabrous) and hairy (non-apical, non-glabrous)
describe hairless/apical/glabrous skin
on face and palms of hands, has little role in thermoregulation, has typical sympathetic vasoconstrictor control rich in arteriovenous anastomoses
what are arteriovenous anastomoses?
shunt vessels that allow large amounts of blood flow close to the skin- cause of blushing
describe hairy/non-apical/non-glabrous skin?
over rest of body other than face and palms, has both vasoconstrictor (NA) and vasodilatory (ACh) innervation
nervous output controlling blood flow and heat loss in thermoneutral conditions?
vasoconstrictor tone
effect of rising core temperature on vasoconstrictor tone regulating blood flow and heat loss?
rising core temperatures activate the sympathetic cholinergic vasodilator nerves- the sudomotor nerves- gives rise to sweating also
role of piloerection?
allows skin insulation to be modulated by trapping more air. also plays key role in defence
what is piloerection?
raising of small hairs on skin using arrector pili muscles
why does evaporation of water cause heat loss?
high specific heat capacity and very high latent heat of vaporisation
methods of evaporation for heat loss?
breathing, panting, grooming (licking) and sweating
difference between panting and sweating?
sweating loses salt, panting provides its own airflow, potentially cools special surfaces (rete mirabile), can alter pCO2 and produces some heat which sweating doesn’t do
facts about panting?
can reduce respiratory alkalosis by ventilating the upper respiratory dead space. muscular work can be reduced by panting at natural resonant frequency of the respiratory system
what is gular flutter?
birds have no sweat glands so rely on ventilatory evaporation driven by oscillations of floor of mouth called gular flutter
what is the rete mirabile?
in some animals (e.g. camels, ostriches) there is a special circulation that allows venous blood flow from, for instance, the nasal membranes, to cool arterial blood to the brain. this allows panting/ventilation to preferentially cool the brain while maintaining higher core temperature elsewhere- can allow higher muscle temperature
what are the 2 types of sweat gland?
apocrine and eccrine
role of apocrine glands?
produce relatively protein rich, thick secretion, bacteria makes smelly. in horses they are important, in humans little role in thermoregulation
role of eccrine sweat glands?
produce watery sweat, critical in heat loss in humans. few animals have them.
structure of eccrine sweat glands?
ducts around 40µm diameter and 2-5mm long that exit on surface of the skin
where do apocrine glands exit?
in the hair follicle
how many sweat glands do humans have?
between 2-4 million
innervation of eccrine sweat glands?
sudomotor nerves (cholinergic sympathetic)
how is sweat modified in the duct?
NaCl reabsorbed
sweat tonicity?
always hypotonic with low flow rates associated with lower NaCl than at high flow rates
how is sweating trainable?
activating sweat glands can cause them to hypertrophy allowing them to produce more sweat and reabsorb more NaCl, also activate at lower temperatures
effect/limit of human sweat losses during exercise?
can exceed 1L/hour, total of 2-3L without requiring immediate replacement. some plasma volume decrease and osmosis from intracellular compartment.
causes of pyrexia (fever)?
infections, drugs, environmental stresses, inflammatory disorders
what are pyrogens?
substances that raise body temperature. can be produced by pathogens, self cells or be administered experimentally
effect of pyrogens?
stimulate prostaglandin E2 production from endothelial cells in hypothalamus which raises ‘set-point’- so normal temperatures feel cold causing shivering
effect of anti-pyretics (non-steroidal anti-inflammatory drugs)?
can block PGE2 production
why are fevers sometimes useful?
indicate the course of an infection, modify behaviour, may also help fight certain infections
syphilis treatment with fever?
giving syphilis patients malaria induced fever which killed the syphilis bacteria
