Thermoregulation

• Hypothermia


  Abnormally low temperature, slows metabolic process, eventually function stops
• Hyperthermia


  Abnormally high temperature, can denature proteins and -> death
• Poikilotherms

  • Fish, reptiles and amphibians (odd animals)
  • Body temperature varies with environment
  • Behaviour important in maintaining preferred body temperature (PBT)
  • Maintenance of PBT prevents lactic acidosis and maximises drug absorption, metabolism and excretion
  • Environment critical in captive animals
• Homeotherms


  • Mammals and birds
  • Maintain constant body temperature in variety of environmental temperatures
  • High metabolic rate for heat production
  • High energy intake
• Heat production


  • Metabolism
  • Food intake
  • Muscular activity
• Heat loss

  • Radiation
  • Convection
  • Conduction
  • Evaporation
  • Excretion of urine and faeces
• Homeostasis of Thermoregulation


  • Sensed by thermoreceptors (hypothalamus and skin)
  • Control centre = hypothalamus (always)
  • Heat losing centre = pre-optic area
  • Heat promoting centre = posterior nucleus
• Monitoring body temperature


  1. Use a thermometer

  2. Core vs peripheral temperature measurement

  3. Assess hydration status and cardiovascular function
• Why don't penguins feet freeze?
• Other countercurrent mechanisms


  • Carotid rete – sheep and other ungulates (regulates brain temperature)
  • Cold venous blood draining nose cools brain blood supply
  • Not found in horses
• Responses to increased and decreased core and peripheral temperature


  • Metabolic responses – below thermoneutral zone
  • Vasomotor responses – within thermoneutral zone
  • Evaporative responses – above thermoneutral zone
• Decreased core and skin temperature: physiological responses

  1. Peripheral vasoconstriction – reduces blood flow to skin

  2. Counter-current heat exchange – cools blood heading to periphery to prevent heat loss

  3. Other mechanisms to prevent heat loss: insulin/piloerection/behavioural responses

  4. Mechanisms of heat production: metabolism/shivering and non-shivering thermogenesis
• Mechanisms of heat production


  • Metabolism: most of the chemical energy is lost as heat
  • Shivering thermogenesis: rapid involuntary cycle of contraction and relaxation of muscles
  • Non-shivery thermogenesis: stimulation of metabolism (brown adipose tissue)
  • Thermoregulation is really important in neonatal care
• Increased core and skin temperature: physiological responses


  1. Sweating

  2. Increased evaporation from skin and respiratory tract

  3. Peripheral vasodilation

  4. Decrease in heat production

  5. Decrease muscle tone, reduced adrenaline secretion

  6. Behavioural responses
• How do low environmental temperatures affect animal management?


  1. Care re hypothermia in newborns

  2. Use of heat lamps in lambing sheds

  3. Insulation of animals in cold weather

  4. Calf jackets

  5. Amount of powder in milk replacer needs to be increased to account for higher metabolic requirements and ensure growth
• Thermoregulation in the Neonate


  • High SA:volume
  • Less or even no insulation
  • Increased skin permeability
  • Lower energy supplies – limited amount of brown fat
  • Inability to display behavioural thermoregulation
  • Immature nervous system
  • Require ambient temperature much closer to set point temperature than adults
• Hypothermia


  • Common in neonates and under anaesthesia
  • High SA:volume
  • Low metabolic rate under anaesthesia
  • Can lead to increased morbidity and mortality especially in sick patients
  • Animals can't regulate heat during anaesthesia – affect blood pressure
• Effects of hypothermia

  • Cardiac arrhythmias – AF 30 degrees C VF 24-28 degrees C
  • Bradycardia
  • Impaired coagulation and wound healing
  • Decreased enzyme action
  • Prolonged duration of action of drugs
  • Decreased renal plasma flow
  • Decreased oxygen delivery to tissues
  • Lower anaesthetic requirement in anaesthetized animals
  • Shivering increases oxygen requirement
  • Can be life threatening
• Prevention of hypothermia


  1. Warm environmental temperature

  2. External heat sources

  3. Insulation

  4. Warm fluids

  5. HME breathing system

  6. Clipping and scrub solution

  7. Prevention better than cure
• Severe hypothermia


  1. Warm water enemas

  2. Bladder lavage

  3. Gastric lavage

  4. Peritoneal lavage

  5. Vasodilation may worsen hypotension
• Other causes of hypothermia

  • Sepsis
  • Shock
  • Endocrine disease (hypothyroidism, hypopituitarism, Addisons)
  • Intracranial disease (lesion in hypothalamus)
• Hyperthermia


  • Hyperthermia – general term used to describe an increase in body temperature above normal for that species regardless of aetiology (cause)
  • Pyrexia – specific term that refers to an increase in core body temperature due to an increase in the set point of the hypothalamus
• Effects of hyperthermia


  • Increased basal metabolic rate – 13% for every degrees C above normal
  • Increased oxygen and substrate requirement
  • Parenchymal cell damage
  • T > 41 degrees C – irreversible brain damage
  • T > 43 degrees C – death
• Causes of hyperthermia

  • Locked in a car
  • Brachycephalic animals – compromise of panting
  • Seizure – risk of hyperthermia
• Causes of pyrexia


  • Infection
  • Immune mediated disease
  • Neoplasia
  • Tissue trauma
• Other causes of hyperthermia

  • Heat stroke
  • Hyperpyrexic syndrome
  • Exercise hyperthermia
  • Malignant hyperthermia
• Malignant hyperthermia


  • Pigs and people
  • Autosomal dominant chromosomal defect
  • Abnormality in Ca channel in the sarcoplasmic reticulum -> increase in cytoplasmic Ca
  • Dantrolene sodium