- Mammals and birds maintain a constant body temperature
- They are known as homeotherms (= warm-blooded)
- Body temperatures of homeotherms are usually above the environmental
temperature
- Warm blooded animals have many advantages but do need a higher
metabolic rate
- Body temperature results from a balance between production and heat
loss
- In a balanced state production and loss of heat will be equal and
the temperature will be constant
- Tight control of temperature is essential for normal physiological
functions
- Core temperature is invariably higher then skin temperature
Control of body temperature
- Temperature is controlled from sites in the hypothalamus
- Control requires sensors, a control centre and effectors
- Temperature sensor are found throughout the body in the skin, brain
and other organs
- There are two types- respond to hot and cold
- Control centre is in the hypothalamus of the brain
- The hypothalamus acts as a thermostat- has a temperature set point
- Effectors have two mechanisms
- Produce more heat (increased metabolic rate, shivering, brown fat
metabolism)
- Change heat loss (blood vessel dilation or constriction, erection
of hair, curling up, sweating)
The skin
- Skin is the primary organ for removal of metabolic heat
- About 90% of body heat is lost through the skin
- The remaining 10% is lost in urine and exhaled air
- If body temperature is too high the skin can dilate blood vessels
and increase blood flow by 150 times to loose excess heat
- In cold weather skin will constrict blood vessels and reduce heat
loss
- Heat loss is by:
- Radiation
- Conduction
- Convection
- Sweating
- Newton's law of cooling governs heat loss by radiation and
conduction
- Heat loss = heat conductance x temperature difference
- Temperature difference = body temperature - ambient temperature
- Sweating can be used to lose enormous amounts of heat
- Heat of vaporisation of water is about 580 Calories/litre
- If the ambient temperature is higher than the body temperature,
sweating is the only way we can lose heat
- Sweat glands are activated by nerves from the sympathetic nervous
system
Mechanisms of pyrexia
- Fevers are caused by in increase in the temperature set point- the
thermostat has been set higher
- Often caused by bacterial toxins or inflammatory mediators
- Act directly on hypothalamus
- Fevers can be caused by increased metabolism, reduced heat
conduction, or both
- The benefits of a fever are uncertain
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