Thermoregulation

Ectotherms such as reptiles an fish can't control their core temperature internally. They control their temperature by changing their behaviour, (eg basking in the sun).

Their internal temperature depends on the external temperature.

Their activity level depends their external temperature, they're more active at higher temperatures and less active at lower temperatures.

They have a variable metabolic rate and they generate very little heat themselves.

Endotherms control their body temperature internally by homeostasis. They can also control their temperature by behaviour.

Their internal temperature is less affected by the external temperature (within certain limits).

Theit activity level is largely independent of the external temperature, they can be active at any temperature (within certain limits).

They have a constantly high metabolic rate and they can generate a lot of heat from metabolic reactions.

In order to lose heat, mammels can;

Sweat- More sweat is secreted from sweat glands when the bodys too hot. The water in sweat evaporates from the surface of the skin and takes heat from the body. The skin is cooled.

Hairs lie flat- Mammals have a layer of hair that provides insulation by trapping air (air is a poor conducter of heat). When  it's 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.

Vasodilation- When its hot, arterioles near the surface of the skin dilate (this is called vasodilation). More blood flows through the capillaries in the surface layers of the dermis. This menas more heat is lost from the skin by radiation and the temperature is lowered.

In order the produce heat, mammals can;

Shiver- When its cold, muscles contract in spasms. This makes the body shiver and more heat is produced from increased respiration.

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

In order the conserve heat they:

Sweat less- Less sweat is secreted from sweat glands when it's cold, reducing the amount of heat loss.

Their hairs stand up- Erector pili muscles contract when it's cold, which makes the hairs stand up. This traps more air and so prevents heat loss.

Vasoconstriction- When it's cold arterioles near the surface of the skin constrict (this is called vasoconstriction) so less blood flows through the capillaries in the surface layers of the dermis. This reduced heat loss.

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

Information about internal temperature comes from thermoreceptors in the hypothalmus that detect blood temperature.

Information about external temperature comes frm thermorecptors in the skin that detect skin temperature.

Thermorecptors send impulses along sensory neurones to the hypothalmus, which sends impulses along motor neurones to effectors. The neurones are part of the autonomic nervous system, so its all done unconsciously.

Homeostatic systems involve receptors, a communication system and effectors. Recpetors detect when a level is too high or too low and the imformations communicated via the nervous system or the hormonal system to effectors. The effectors respond to counteract the change- bringing the level back to normal.

The mechanism that restores the level to normal is called a negative feedback mechanism. Negative feedback keeps the things around the normal level.

Negative feedback only works within certian limits though, if the change is too big then the effectors may not be able to counteract it.

For example: a huge drop in body temperature caused by prolonged exposure to sold weather may be too large to counteract.