Regulation of Body Temperature

Body temperature reflects the balance between heat production and heat loss. All body tissues produce heat, those that are the most active metabolically produce the most heat.

• At Rest: Liver, heart and brain produce most heat. Muscles 20-30% of total.
• During Exercise: Muscles produce 30-40 times rest of body put together.

Body temperature under normal conditions is in narrow range of 36.1-37.8 °C. This rarely varies by more than 1°C throughout the day. This precise temperature homeostasis relates to the effect of temperature on the rate of biochemical reactions. At normal body temperature, conditions are optimal for enzymatic activity.

Core and Shell Temperatures

Different regions of the body have different temperatures at rest. The body's core (that is, organs within the skull and the thoracic and abdominal cavities) has the highest temperature. Its shell, or heat loss surface, essentially the skin, has the lowest temperature.

Blood serves as the major heat transfer or exchange agent between the body core and shell. Whenever the shell is warmer than the external environment, heat is lost from the body. When heat must be dissipated, warm blood is allowed to flush into the skin capillaries. Oppositely, when heat must be conserved, blood largely bypasses the skin, which reduces heat loss and allows the shell temperature to drop to that of the environment. Thus, while the core temperature stays relatively constant, the temperature of the shell may fluctuate substantially.

Mechanisms of Heat Exchange

The body uses four mechanisms of heat transfer:

1. Radiation
2. Conduction
3. Convection
4. Evaporation

Radiation is the loss of heat in the form of infrared waves (thermal energy). Any dense object that is warmer than objects in its environment-for example, a radiator and (usually) the body-will transfer heat to those objects. Under normal conditions, close to half of body heat loss occurs by radiation.
Because the direction of radiant energy flow is always from warmer to cooler, radiation explains why an initially cold room warms up shortly after it is filled with people. The body can also gain heat by radiation, as demonstrated by the warming of the skin during sunbathing.

Conduction is the transfer of heat between objects that are in direct contact with each other. For example, when we hold a badminton racket we transfer heat to the handle by conduction. Unlike radiation, conduction requires molecule-to-molecule contact of objects; as the thermal energy must move through a material medium.