Homeotherms - are at the other end of the spectrum, these animals have a variety of structural and physiological adaptations which maintain their body temperature at a very stable value in spite of wide fluctuations in their en vironmental temperature.
Humans are homeotherms, except under some pathological conditions.
In this chaotic world, the temperature is always changing, so there are two basic issues in temperature regulation, 1. Staying Warm 2. Heat transfer occurs with the same basic priciples as diffusion, e.g. heat moves from regions of high temperature to low temperature. In the more primitive reptiles which do not have complex physiologic mechanisms for thermal regulation, behaviour patterns permit a crude regulation of body temperature. Thus if air temperature is low, the lizard or Iguana will search out sunny areas to heat their body to optimum temperature and will engage in vigourous muscle activity to assist in elevation of core temperature.