Thermoregulation Flashcards
endotherm
an organism that generates heat to maintain its body temperature, typically above the temperature of its surroundings
ectotherm
an organism that regulates its body temperature largely by exchanging heat with its surroundings (poikilotherm species)
temporal heterothermy
observed in some endotherms: may be short term (torpor) or long term (hibernation) - can reduce metabolism such that body temperature drops to approximately the ambient temperature
regional heterothermy
certain endotherms and ectothermic animals are able to maintain different temperature “zones” in different regions of the body
fluctuations in body temperature occurs when:
the rate of heat gain is not balanced by the rate of heat loss
metabolic rate
rate of energy expenditure (usually per hour), can be measured directly by direct calorimetric methods (using chamber) or indirectly by measuring oxygen consumption
basal metabolic rate (BMR)
rate of energy expenditure in a post absorptive condition (has not eating for 12 hours), following a rest period at room temperature
what is the relationship between body weight and mass-specific metabolic rate?
there is an inverse relationship
what are some factors influencing BMR?
- surface area/mass ratio
- age
- gender
- muscle vs. fat content
- stress and hormones
total metabolic rate
total rate of energy expenditure during ongoing involuntary and voluntary activities
what mediates non-shivering thermogenesis?
hormones that mediate increases in total metabolic rate, this is particularly significant in brown fat
chemical thermogenesis
increased sympathetic activity (short-term) via epinephrine and norepinephrine increases heat production
mechanical thermogenesis
increased muscle activity such as shivering to increase heat production
what is responsible for conductance of core temperature to the skin?
flow of blood to the skin (regulation of blood-flow to skin is an effective way of controlling body temperature)
aldosterone
reduces secretion of sodium and chloride ions in the sweat (increases reabsorption)
control of sweating
cholinergic sympathetic pathway in the subdermal region of sweat gland
-circulating epinephrine/adrenaline can also increase sweating (not related to cooling mechanisms)
panting
an effective mechanism of heat loss, shallow breath, does not increase alveolar ventilation
where is the temperature-regulatory centre located?
in the posterior hypothalamus, which receives nervous feedback signals from sensory thermoreceptors located peripherally and in the anterior hypothalamus
what are the 4 types of sensory receptors?
1) cold pain receptors
2) cold receptors
3) warm receptors
4) heat pain receptors
adaptation
peripheral temperature receptors are strongly stimulated when subjected to abrupt changes in temperature (i.e. extreme cold to extreme heat). the response fades away gradually, adaptation is not 100%
spatial summation
thermal sensation is proportional to the number of thermoreceptors stimulated (small temp change over a large area has a greater effect than large temperature change over a small area)
anterior hypothalamus
i.e. the preoptic area of the hypothalamus contains thermoreceptors and is a temperature sensitive area (contains more heat sensitive receptors than cold-sensitive receptors)
critical temperature set-point
determined by the degree of activity of temperature receptors in the hypothalamus
-core temperatures above the set point initiates response to high temperature, and core temperatures below the set point initiates response to cold temperatures
control of body temperature set-point
feedback from peripheral temperature receptors provides a physiological mechanism for altering the hypothalamic set point (increase in skin temperature results in a decrease in set point in anticipation of the warming trend; decrease in skin temperature results in an increase in set point in anticipation of cooling trend)
specific response to heat:
- increased body temperature inhibits thermoregulatory centres in the posterior hypothalamus
- increased sweating
- vasodilation
- decreased body heat production (decrease shivering and metabolism)
specific response to cold
- decreased body temperature stimulates thermoregulatory centres in the posterior hypothalamus
- vasoconstriction
- piloerection (cholinergic sympathetic stimulation)
- increased shivering
- increased chemical thermogenesis (as a result of sympathetic stimulation by NE and E for short-term results or increased TH for long-term response)
pyrexia
fever; a state of increased body temperature beyond the normal range, could result from infection/allergic reaction/CNS injuries/cancer
what causes pyrexia?
pyrogens, which are released by macrophages and neutrophils
- bacterial lipopolysaccharides toxins (LPS)
- viral polyribonucleotide pyrogen (Poly I:Poly C)
- cytokines released from injured cells
what is the mechanism by which pyrogens act?
binds and activates Toll family of membrane receptors which initiates cellular innate immune responses
- ascyotkines (interleukins, TNF) act through specific hypothalamic membrane receptors resulting in activation of COX-II enzyme which results in production of prostaglandins, prostaglandins increase the hypothalamic set-point which increases body temperature (activates responses to cold)
how does aspirin reduce the degree of fever?
blocks prostaglandin production from arachidonic acid (inhibits COX-II enzyme)
antipyresis
response to reduce body temperature when the factor causing fever is removed, which causes set-point to be reduced to lower level
hyperthermia
beyond the upper limit, loss of ability to regulate temperature
-further increases body temperature and causes development of symptoms of heat stroke
counter-current heat exchanger
- anatomical adaptation to prevent overheating of the brain
- head carotid artery passes through a venous blood sinus (sinus cavernosus) which contains cool blood returning from nasal cavity
hypothermia
sustained exposure to extreme cold will result in decreased body temperature, may reach a lower limit if heating mechanisms are not sufficient.
- leads to loss of ability to regulate temperature
- results in further decline in body temperature and development of initial symptoms of hypothermia
frostbite
ice crystal formation (if ice crystals come into contact with cell membranes, cell will die)
regional heterothermy
- part of the body such as extremities are maintained at lower temperatures (physiological adaptation)
- cold-acclimatization results in a change in the lipids by becoming less saturated in the extremities - more oily and viscous to allow adequate functioning of cell membranes (biochemical adaptation)
- changes in fur thickness (anatomical adaptation)
deep sleep
temporal heterothermy; adaptation to cope with extreme cold
-body temperature decreases by several degrees
torpor
temporal heterothermy; adaptation to cope with extreme cold (e.g. bats, small birds/hummingbirds)
- short-term changes in body temperature for a few hours to help animals survive in cold climate
- reduction in core temperature
- decrease in metabolic rate, heart rate, breathing rate, and oxygen consumption
- low blood supply to limbs, blood is mainly restricted to the vital organs
hibernation
temporal heterothermy; adaptation to cope with extreme cold
- a period of deep torpor with a significant drop in body temperature
- hypothalamic thermostat is reset to very low level (but thermoregulation is not suspended)
- change in hibernating metabolic rate is smaller in animals with larger masses
which animals can go through hibernation
animals with medium masses/metabolic rates
is hibernation and temporal heterothermy only observed in endotherms?
no, a number of ectotherm vertebrates also remain dormant and inactive in winter (e.g. snakes and frogs)
poikilotherms
- does not need to maintain a constant body temperature (energy efficient)
- requires thermoregulation and physiological/anatomical adaptations to deal with extreme temperatures
- regional heterothermy is present in some species (e.g. bumblebees, female python, tuna)
rate of metabolism in poikilotherms depends on:
environmental temperature, developmental stages, and species
Q10 effect
rate of enzyme activity doubles for every 10 degree increase
all ectotherms have an:
upper critical temperature (UCT) beyond which there is a breakdown in physiological processes
how do poikilotherms survive cold temperatures?
1) freeze avoidance (super cooling - body fluids are cooled below their freezing temperatures but prevent the formation of ice nucleating centres, production of antifreeze-proteins that lower the freezing point)
2) partial freeze tolerance (freezing of extracellular fluid compartment-increases ECF osmotic pressure and results in dehydration of cells which increases intracellular concentration and lowers freezing point, production of cryoprotectants)
examples of animals that produce antifreeze
1) flounder and arctic cod contain specific glycoprotein antifreeze protein that lowers freezing point by several degrees
2) certain insects increase the concentration of sorbitol and glycerol in their body fluid, this raises the osmotic pressure and lowers the freezing point
cryoprotectants
trehalose (sugar) and proline in insects forms a gel phase as cells dehydrate, this prevents disruption of internal cell organelles by stabilizing the membrane
