What effect does ambient temperature have on the cardiovascular system?
In relation to the cardio vascular system a combination of adrenaline from exercise and the bodies natural response to the cold causes constriction of blood vessels in the skin and extremities. This helps to reduce heat loss.

The movement from running increases the amount of cold air that runs across your body and into your lungs, which would offset the effect of heat generated from running. According to the article about Thermoregulation in cold conditions on Wikipedia, the offset is great enough to make shivering while standing still a better way to keep warm.