Subject: How (and mostly why) does temperature affect the heart rate?

I am asking this question to supplement my preliminary research to conduct a lab 
experiment. The research I have done so far has not provided any specifics or 
explanations of how exactly the process occurs. To me it seems like the body 
would increase blood flow to the skin's surface to help keep the body warm, thus 
increasing the heart rate. However, this is not the case. Why do lower or higher 
temperatures have the effect that they do? 

Also, what increase or decrease in temperature would produce a noticeable effect 
in heart rate? 

Re: How (and mostly why) does temperature affect the heart rate?