Thermoregulation, Pyrexia and Hypothermia

• Poikilotherm - an animal who's temperature fluctuates with the environment.  
• Homeotherm - an animal who can maintain a steady body temperature by increasing or decreasing their body temperature.

• a) poikilotherm
• b) homeotherm

• a) narrow
• b) wide

Organs in the cranium, thoracic cavity and abdominal cavity

Diurnal animals have a higher temperature during the middle of the day whereas nocturnal animals have a higher temperature in the middle of the night

• a) increased temperature in summer/decreased temperature in winter
• b) a hot environment will cause a slight increase in body temperature 
• c) as a result of muscular contraction
• d) 'specific dynamic action' associated with the packaging of nutrient derived energy into stores

Body temperature in females is influences by steroids and thus varies with ovarian/menstrual cycles

• Radiation - energy given off or absorbed by an object
• Conduction - energy transferred between an object and the material next to the object by direct passage.  Conduction is increased by convection.
• Evaporation - loss of water from an organism in the form of water vapour

The zone of thermoneutrality is the range of ambient temperature over which little work has to be done to maintain body temperature

The lower critical temperature and upper critical temperature

Peripheral and central temperature sensors send information to the pre optic area of the hypothalamus --> POA acts as an integrating centre for the 'set point' --> POA sends information about the 'error' to either the a) heat promoting centre to promote shivering, peripheral vasoconstriction, etc b) heat losing centre - to promote sweating, panting, behavioural changes, etc

Increase in heat production/promotion

• Curled up posture
• Huddled together

• Cutaneous vasoconstriction
• Countercurrent exchange systems
• Piloerection/horripilation 
• Shivering thermogenesis
• Non-shivering thermogenesis

heat loss

• Open posture
• Expose areas with low insulation
• Minimal touching
• Cool location

• Altering conductance
• Evaporative heat loss - sweating, panting or spreading saliva on fur

• Increase thermal gradient
• Increase metabolic rate
• Decrease core temperature

• Decrease thermal gradient
• Decrease metabolic rate
• Increase core temperature

An alteration from the set point i.e. an increase in core body temperature

As it promotes interferon activity, increases metabolic rate and accelerates tissue repair and decreases bacterial and viral replication

• Neutrophils and macrophages secrete pyrogen IL1.  IL1 stimulates the anterior hypothalamus to secrete PGE.
• PGE raises the set point for body temperature.
• Shivering and peripheral vasoconstriction increases body temperature.  Liver and spleen hoards zinc and iron depriving bacteria of the minerals needed to reproduce.
• When infection is gone pyrogen secretion decreases and the set point is reset.
• Heat loss mechanisms are activated.

• Heat cramps
• Heat exhaustion
• Heat stroke

Hypothermia