Heat production is only part of the equation; controlling heat loss is also very important.
High levels of physical fitness
allow for more rapid heat production, as energy can be produced more rapidly. Young individuals can produce heat more rapidly than elderly people. Levels of thyroid hormone
govern metabolic rate, and overly high or low levels of this hormone may increase or decrease heat production. Shivering is a natural response to cold temperatures, and allows for rapid heat production in the muscles.
Control of heat loss probably contributes at least as much to body temperature regulation as heat production. In general, larger people will lose heat more slowly than smaller ones, so children
are more prone to hypothermia
than adults. Fat tissue insulates against heat loss, and a rounder body shape has less surface area relative to its volume. Therefore, obese people tend to lose less heat, but may be prone to overheating. Blood vessels near the surface of the body will expand or contract to control heat loss, and under cold conditions, blood is shunted away from areas that lose heat easily such as fingers and toes.
Body temperature is carefully regulated by control centers in the brain, and damage or disease of these regions can cause serious abnormalities of body temperature. People with paraplegia
tend to have more difficulty regulating body temperature due to impairments in control of the muscles and blood vessels. Certain medications can also affect temperature control, particularly many medications used in treatment of schizophrenia
. Therefore, people with serious mental illnesses may also have difficulty tolerating extreme temperature conditions.