As you know, mammals have to thermo regulate or maintain a steady-state body temperature within a narrow range. Otherwise their body temperature would result from simply a balance between their...

As you know, mammals have to thermo regulate or maintain a steady-state body temperature within a narrow range. Otherwise their body temperature would result from simply a balance between their metabolic heat generation and convective heat loss at the body surface, as for the fish problem above. Explain how the blood vessels under the skin are actively used to thermo regulate when a) the environment is much colder than the body (cold stress); b) the environment is much warmer than the body (heat stress).