Thermoregulation Flashcards
What is convection?
Transfer of her from solid to fluid
Fluid flow required- wind/ water
For a given temp difference, heat transfer is faster by convection than conduction
How does eat transfer between humans and the environment occur?
Conduction
Convection
Evaporation
Thermal radiation
Mechanisms operate simultaneously and in both directions
If heat gain= heat loss body temp remains constant
Why is temperature important?
As temp increases so does rate of reactions
Enzyme catalysed have optimum temperature
Denaturalising proceeds rapidly above 42deg
Core temperature above 41deg will provoke convulsion in most adults
43deg about the absolute limit for life
What neural components control thermoregulation?
Hypothalamus critical
Lies in the diencephalon and surrounds the third ventricle
Important regions on the neural basis of thermoregulation
Pre optic (hypothalamus)
Dorsomedial lateral nucleus (hypothalamus)
Periaquiductal grey matter ( mid brain)
Raphe pallidus (medulla)- may be important in generating fever
Usually inhibited by pre optic area
What is the role of the hypothalamus?
Receives input from peripheral and central thermoreceptors
Contains Neurons who respond to an increased core temp while others respond to decrease
Output is from anterior hypothalamic regions (sympathetic/ motor Neurons)
Responses to increased temperature
Cutaneous vasodilation
Sweating
Shedding clothes
Drinking fluids
Seeking shade
What is cutaneous vasodilation?
Skin blood vessels dilate and cutaneous blood flow increase mediated by ANS- reduction in vasomotor tone
Heat loss by radiation, conduction and convection
What is sweating?
Evaporation of sweat results in cooling
2 types of sweat gland-
Apocrine- axilia, areola, anal regions
Contribution to odour but not much else
Eccrine - involved in thermoregulation
What are eccrine sweat glands?
2.5m, 50% on back and chest
On a hot day may produce 6l per hour
As fluid moves up reabsorption of some components occurs, mostly NA as CL
How are eccrine sweat glands controlled?
Glands innervated by sympathetic cholinergic fibres
Activation results in increased secretion
What is pyrexia?
Resetting set point- fever
Elevation of body temp due to restring of thermostat
Most common cause of fever is infection
Onset is gradual but the form of chill signals arrival
Heat production/ conservation machanisms are engaged which then drives up set point to a new value where it is stabalised.
Mechanism of fever
Initiated by pyrogens
May be by bacteria/ immune cells
Important compounds: interleukin1 and 6
Promotes production of PGE2 In hypothalamus which is the cause of resetting
Anti-pyrrhic drugs
Hypothalamic response may include the release of endogenous cryogens to offset fever
What is heat exhaustion?
Fainting due to hypovolaemia as a consequence of excessive sweating
What is heat stroke?
Complete breakdown of thermoregulation Positive feedback takes over Particularly in elderly Occurs after heat exhaustion usually May occur due to reduced circulation in brain due to dehydration
