Control of Body Temperature

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Temp control in ectotherms and endotherms

Animals are classed as either ectotherms or endotherms, depending on how they control their body temperature. 

Ectotherms

Ectotherms can't control their body temperatures internally- they control their temperature by changing their behaviour. This means the internal temperature of ectotherms depends on the external temperature. They have a variable metabolic rate becuase they can't keep their internal temperature constant. They generate very little heat themselves. This means the activity level of ectotherms depends on the external temperature- They are more active at higher temperatures. 

Endotherms 

Ectotherms control their body temperature internally by homeostasis, as wellas by altering their behaviour. This means that the internal temperature is less affected by the external temperature. Endotherms have a constantly high metabolic rate because they can keep their internal temperature constant. They generate a lot of heat from metabolic reactions. This means the activity level of endotherms is largely independent of the external temperature- they can be active at any temperature. 

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Mechanism to reduce body temperature

Mammals use differnet mechanisms to reduce or increase their body temperature.

Mechanism to reduce body temperature

1) Sweating- more sweat is scereted from sweat glands when the body is too hot. The water in sweat evaporates from the surface of the skin and takes heat from the body. The skin is cooled.

2) Hairs lie flat- mammals have a layer of hair that provides insulation by trapping air. When it is hot, erector pili muscles relax so the hairs lie flat. Less air is trapped, so the skin is less insulated and heat can be lost more easily.

3) Vasodilation- when it is hot, arterioles near the surface of the skin dilate. More blood flows through the capillaries in the surface layers of the dermis. This means more heat is lost from the skin by radiation and the temperature is lowered. 

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Mechanism to increase body temperature

Mechanism to increase body temperature

1) Shivering- when it is cold, muscles contract in spasms. This makes the body shiver and more heat is produced from increased respiration. 

2) Hormones- the body releases adrenaline and thyroxine. These increase metabolism and so more heat is produced.

3) Much less sweat- less sweat is secreted from sweat glands when it is cold, reducing the amount of heat loss.

4) Hairs stand up- erector pili muscles contract when it is cold, which makes the hairs stand up. This traps more air and prevents heat loss. 

5) Vasoconstriction- when it is cold, arterioles near the surface of the skin constrict so less blood flows through the capillaries in the surface layers of the dermis. This  reduces heat loss. 

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Control of body temp by the hypothalamus

Control of body temperature by the hypothalamus

Body temperature in mammals is maintained at a constant level by a part of the brain called the hypothalamus. The hypothalamus receives information about both internal and external temperature from thermoreceptors. 

  • Thermoreceptors in the hypothalamus dectes internal temperature, that of the blood.
  • Thermoreceptors in the skin detect external temperatures, that of the skin.

Thermoreceptors send impulses along sensory neurones to the hypothalamus, which sends impulses along motor neurones to effectors. The effectors respond to restore the body temperature back to normal.

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Rise in body temperature

When thermoreceptors detect body temperature is too high, they send impulses to the hypothalamus, which sends impulses to effectors. Effectors respond to increase heat loss from the body, e.g. sweat glands produce more sweat, and to reduce the amount of heat that is produced by the body. Body temperature then returns to normal. 

Example

  • Body temperature rises above 37 degrees C
  • Thermoreceptors detect change
  • Hypothalamus sends impulses to effectors
  • Effectors respond by vasodilation, sweating, no shivering, no adrenaline or thyroxine released.
  • Body temperature lowers to 37 degrees C.
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Fall in body temperature

Fall in body temperature

When thermoreceptors detect body temperature is too low, they send impulses to the hypothalamus, which sends impulses to effectors. Effectors respond to produce more heat and to conserve it. Body temperature then returns to normal.

Example

  • Fall in body temperature
  • Thermoreceptors detect change
  • Hypothalamus sends impulses to effectors
  • Effectors respond by: Vasoconstriction, less sweating, shivering and adrenaline and thyroxine released.
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