Thermoregulation 2, Heat gain mechanisms

Heat is gained by the body as a result of metabolic processes. All energy chains, i.e. sequences of energy conversions, end as heat so whenever fuels are used heat will be generated. Metabolic activity in an organ will therefore result in heat being produced. For example, the liver is a large metabolically active organ and so produces a lot of heat. Heat is also generated by the use of energy in organs such as the brain, heart, intestines and kidneys. Muscles produce a lot of heat when actively contracting. When muscles are contracting only about 20% of the energy they use is converted into movement, the remainder is converted into heat.

Rate of heat generation is determined by the metabolic rate of the body. This does vary between individuals and may partly explain why some people are more tolerant of cold or heat than others. Increased levels of thyroid hormone in the blood will stimulate metabolic rate whereas reduced thyroxine levels will allow metabolic rate to fall. After a meal the metabolic rate of the body is increased for a few hours increasing total heat production. Only a small part of this effect is due to the energy required to physically and chemically digest the food. Most of the additional energy used after eating is as a result of the metabolic processing of food after it has been absorbed. When people die they are no longer carrying out metabolic processes so body temperature drops until it is the same as the surrounding environment.