Final answer:
The two main mechanisms for increasing heat production in the human body are shivering and enhanced metabolic activity due to the action of thyroid hormones and epinephrine.
Step-by-step explanation:
The two most prominent mechanisms of increasing heat production are shivering and enhanced metabolic activity induced by hormonal action. When the body is exposed to cold, the brain reduces blood flow to the skin, and the blood returning from the limbs is diverted into a network of deep veins to conserve heat. If heat loss becomes severe, the brain triggers shivering, where skeletal muscles contract rapidly to generate heat through ATP consumption.
Simultaneously, the brain stimulates the thyroid gland to release thyroid hormone, and the adrenal glands to release epinephrine (adrenaline), both of which significantly increase the metabolic rate and heat production throughout the body. The thyroid hormone accelerates metabolic activity, while epinephrine causes the breakdown of glycogen into glucose, which then fuels increased cellular metabolism, leading to additional heat production.