Temperature homeostasis Flashcards
Define thermoregulation
Balance between heat gain and heat loss. It is controlled by the nervous system
How are small alterations in body temperature achieved?
Altering skin blood flow
How are large adjustments in body temperature achieved?
Shivering and sweating
Where are temperature sensitive receptors found?
Anterior hypothalamus
What does activation of heat-sensitive neurons cause?
Skin vasodilation
Sweating
What does activation of cold-sensitive neurons cause?
Inhibition of heat-sensitive neurons
Vasoconstriction
Shivering
Outside the hypothalamus, where else are thermoreceptors found? How do they have their affect?
Peripheral - connect centrally to cortex via conscious sensation and hypothalamus
Internal surfaces - respiratory and GI tract: Inhalation of cold air –> shivering, consumption of hot food causes sweating and vasodilation
What is the most rapid method of temperature control?
Reflex vasodilation and constriction
Afferent = cutaneous nerve
Centre = hypothalamus and spinal cord
Efferent = increased or decreased sympathetic stimulation
Describe the difference between the core and peripheral temperature
Core = temperature of thoracic, abdominal. contents and brain
Need to be kept at optimal temperature, measured per rectum
0.5 higher than mouth and axilla
Varies during menstrual cycle, 0.5 higher in latter half
Shows diurnal variation - higher in evening than morning
Peripheral temp less than core as heat is lost from surface to environment
How is heat lost from the surface of the body to the surrounding environment?
Conduction and evaporation from skin to air
Convection from ski via air flow
Convection from lungs via tidal air flow
Radiation from naked skin
What is the normal core temperature?
36-37.5
What is the mechanism of pyrexia?
It is thought that there is a set-point temperature around which the core temperature is regulated by the hypothalamus.
If the set-point temperature is raised then the hypothalamus activates mechanisms leading to increasing core temperature and vice versa.
Set-point temperature is increased; therefore, the body feels cold as the core temperature is lower.
As a result the hypothalamus activates responses to increase body temperature:
■ vasoconstriction
■ piloerection (‘goose flesh’)
■ shivering.
Name the causes of pyrexia
- illness
- exercise
- heatstroke
- anterior hypothalamic lesions, e.g. neoplasia, ischaemia, surgery
- hyperthyroidism
- malignant hyperpyrexia (abnormal muscle response to anaesthetics)
- failure of heat-loss mechanism, e.g. dehydration.
Define heatstroke
- Occurs with body temperature above 41°C.
- Unacclimatised individuals undertaking exercise in hot, humid conditions.
- Symptoms:
■ nausea
■ vomiting
■ weakness
■ headache
■ skin feels hot
■ circulatory failure
■ cerebral oedema
■ hepatic failure
■ renal failure.
What is hypothermia?
Core temperature (rectal) below 35°C.
At what temperature is hypothermia fatal? Why?
Usually fatal below 32 degrees
Below 30 degrees temperature regulatory mechanism fails completely
What are the symptoms of hypothermia between 32 and 35 degrees?
shivery and feeling cold
What are the symptoms of hypothermia below 32 degrees?
bradycardia, hypotension, respiratory depression, muscle stiffness, metabolic abnormalities
What is the usual mechanism of death caused by hypothermia?
cardiac arrhythmias, e cially ventricular fibrillation
How do anaesthetics affect thermoregulation?
■ depress hypothalamic function
■ vasodilatation with increased heat loss
■ lack of shivering
■ consequently drop in body temperature.
How does exercise affect thermoregulation?
■ increases body temperature
■ hypothalamus cannot launch responses that result in loss of heat faster than its production from muscle metabolism.
How does circulatory shock influence thermoregulation? Name the exception
■ reduced tissue perfusion
■ reduced cellular metabolism and heat production
■ results in decreased body temperature
■ compensatory mechanisms include vasoconstriction, piloerection and increased secretion of catecholamines
■ skin feels cold
■ exception is septic (endotoxic) shock, where there is vasodilatation and skin feels hot.
How do spinal injuries affect thermoregulation?
■ thermoregulatory mechanisms lost below level of injury
■ vasoconstriction lost; therefore, heat loss increased
■ patient unable to shiver
■ sweating in relation to hyperthermia lost below level of lesion
■ quadriplegics tend to assume temperature of environment.
How does hyperthyroidism affect thermoregulation?
■ increased basal metabolic rate (BMR)
■ increased O 2 consumption
■ patient hyperactive
■ all of above contribute to increased temperature
■ patient intolerant of heat and feels hot.
How does hypothyroidism affect thermoregulation?
■ opposite effects to hyperthyroidism
■ patient feels cold, intolerant of cold weather
■ body temperature low
What is the impact of being a neonate/premature baby on temperature homeostasis?
■ large surface area to body weight ratio
■ inability to shiver
■ less insulating fat
■ temperature regulating mechanisms less developed
■ therefore predisposed to increased heat loss.
