Temperature compartments

The human body can essentially be seen as having two separate temperature compartments:

1) The core compartment, in which the temperature varies little from 37°C.

2) The peripheral compartment, which acts as a heat sink, absorbing or releasing thermal energy to maintain core normothermia. The temperature here varies widely between 36-28°C.

The adult core comprises 66% of body mass and the periphery 34% of body mass, in a thermoneutral environment.

In neonates and infants, the core extends almost to the body surface and they have a very small peripheral compartment. This causes an accelerated heat loss because of an increased temperature gradient between the skin surface and the environment.

Thermoregulation has three components:

• Afferent thermal sensing
• Central integration
• Efferent response

Afferent thermal sensing

80% of input originates from core tissues, with the remainder coming from peripheral tissue. The following five anatomical areas each contribute 20% to afferent input.

• Brain
• Hypothalamus
• Spinal cord
• Deep thoracic and abdominal tissue
• Skin

The hypothalamus is said to produce an integrated body temperature in response to information from sensory receptors (primarily cold receptors).

Temperature-sensitive neurones

Cold receptors fire continuously, the rate of which increases with a decrease in temperature. Transmition occurs through Ad (delta) myelinated fibres.

Warm receptors are quiescent at normothermia and increase their firing rate in response to an increase in temperature. Transmission is through unmyelinated C fibres.

Most of the central integration takes place in the hypothalamus, with only limited afferent integration occurring in the spinal cord.

95% of all young adults would be expected to have a morning oral temperature of 36.3-37.1°C. The hypothalamus acts to maintain core temperature to within ~0.2°C of 37.0°C. Thermoregulatory responses are quiescent within this range, but any deviation outside of this will invoke thermoregulatory responses to conserve or release heat as appropriate.

Effectors

• Neuronal
• Hormonal: catacholamines and thyroid hormones