Final answer:
To defend against cold environments, humans primarily use shivering and vasoconstriction, which generate heat and conserve it within the core of the body.
Step-by-step explanation:
The human body has developed several mechanisms to maintain its core temperature and defend against a cold environment. The two primary physiological responses used for thermoregulation in the cold are shivering and vasoconstriction. Shivering is an involuntary reflex where the hypothalamus detects a drop in body temperature and triggers the skeletal muscles to rapidly contract and relax, producing heat to warm the body. Vasoconstriction is the narrowing of blood vessels, particularly in the skin and extremities, which reduces blood flow to these areas to conserve heat and maintain the temperature of vital organs.