Thermoregulation

Humans use metabolic heat in order to maintain a constant body temperature; since human body temperature doesn’t change with the environment or have to be derived from the environment like poikilotherms or ectotherms, respectively, we are considered homeothermic as well as endothermic. Being endothermic, when the temperature drops, additional energy is released from where it is stored to generate larger amounts of heat. Besides the body releasing extra heat, there are other methods in order to produce heat (thermogensis); shivering and non-shivering are the two primary mechanisms which convert chemical energy into heat.
When a person shivers (other endothermic vertebrates and some insects also have this characteristic, but this essay is discussing humans), their muscles contract which produces heat, especially around the vital organs. In order to do this the nervous system triggers antagonistic skeletal muscles, which allows for the body to have the added shivering action in itself and not increasing the net muscle movement by too much. In order for this activated contraction, energy needs to be provided; hence ATP is hydrolyzed for this matter. The chemical energy released materializes into heat because there is no useful physical work for inefficiently timed and mutually opposed muscle contractions.
Non-shivering thermogenesis has to do with the activation of breaking down fats and then oxidizing these molecules in order to produce heat. Since most fats release very little energy in the form of synthesized ATP, a few mammals have were able to evolve an adipose type tissue into a fat-fueled deposit, know as brown fat. There are two types of adipose tissue, brown and white, and brown is primarily used to generate body heat. Brown fat is composed of mainly mitochondria and with such a wide spread vascularization, the utilization of fuel to produce energy in the form of ATP is high in this tissue. Humans have brown fat as infants, but as we grow up, the mitochondria disappears and this tissue becomes similar to white adipose tissue (there may still be functional brown tissue in certain areas of the adult body in many people). Despite whether or not brown tissue is present, white tissue can still be used in the non-shivering mechanism. The adipose tissue is activated by the sympathetic nervous system when norepinephrine attaches to the tissue’s receptors. This stimulates one of two different pathways; either an increase in heat production is caused by a cyclic ion pumping process that goes across the plasma membrane to stimulate hydrolysis of ATP or the ETC is maneuvered in a way that the energy that is usually oxidized and captured in ATP is degenerated as heat instead.