Normal body temperature, the neutral zone, the body temperature set point, and the inverse relationship between core and skin temperature

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The neutral zone for humans is defined as the range of skin temperatures over which adjustments of skin blood flow suffice to maintain core temperature constant in a resting person. This range probably varies among individuals, but as a first approximation we can take 33 to 35 degrees as the skin temperature range, based on calorimetric data dating from the World War II era.

It is this range in which we expect to find what we call normal body temperature. It follows that changes in normal body temperature must occur under the control of the regulatory system that operates in the neutral zone and, if there is such a thing as a "set point" in thermoregulation, it must be manifest in the control of skin blood flow. Assuming a normal resting metabolism, if you see changes in core temperature, they must be the consequence of changes in skin blood flow.

If you want to understand these changes, you need to understand how skin blood flow is controlled in the neutral zone. The fact is that we know little or nothing about this control scheme.

Nonetheless, in everyday language as well as in scientific discussions, thermoregulation is described in terms a "set point" and this notion may or may not imply a specific concept of how the control system works.

This series of pages is intended to cover:

  1. An introduction to the "neutral zone" concept
  2. Discussion of the "set point" concept in relation to simple types of control systems and what we know about how temperature regulation works
  3. The inverse relationship between skin temperature and core temperature:
    1. What we might expect as a consequence of the dual involvement of skin temperature in temperature regulation in the neutral zone: as one side of the gradient for heat transfer from the body interior to the surface, and as an independent variable in the reflex drive for control of skin blood flow.
    2. Experimental findings


Normal body temperature and thermal neutrality

The context in which we expect normal body temperature

Experimental origins of the neutral zone concept

Brief mention of the calorimetric experiments that first defined the neutral zone, and an illustration of the concept of what makes it possible to achieve a thermal steady state over a 2:1 range of core:shell temperature gradients.

Thermostatic temperature controllers

The everyday temperature controllers with unequivocal set points, plus discussion of what kind of experimental approach would reveal the existence of a thermostatic controller

Four pages on the simplest kind of proportional controller

Proportional temperature controllers (1)

More advanced than thermostatic controllers, more similar to biological systems. The page describes the basic components of a propotional control system and what can be regarded as the set point in this type of system. Note that the equilibrium temperature is not, in general, equal to the set point temperature.

Proportional temperature controllers (2).

Analysis of equilibrium temperatures regulated by a proportional controller. What is meant by "gain" and "load error" (or "error signal").

Proportional temperature controllers (3).

How equilibrium temperature changes, in fixed environmental conditions, when set point is altered.

Proportional temperature controllers (4).

How equilibrium temperature changes, in fixed environmental conditions, when gain is altered. Note that it takes more than the evidence of altered equilibrium temperature to justify the interpretation that set point has changed.

The control scheme of human thermoregulatory control in the defense against hyperthermia approximated by characteristics of a dual-input proportional control system.

The very simplest view of the mechanism of body temperature regulation is that it responds to not one, but at least two inputs that affect the state of the effectors. Control properties are described graphically and in an equation with four parameters that is worth studying if you are interested in how to define a "set point" in this simplified version of thermoregulation. On a separate page is an example of how the numbers work out for two different core temperature:skin temperature combinations.

The control of skin blood flow in the neutral zone approximated as a dual-input proportional control system

Focus on thermal balance in the neutral zone, how the skin blood flow at thermal balance relates to the four parameters of control, and the fallacy of interpreting altered equilibrium temperatures as the consequence of changes in "set point." Introduces the possibility that core temperature in the neutral zone might remain constant over a range of skin temperature, or even rise when skin temperature falls, or vice versa.

Analysis of equilibrium core temperature versus skin temperature in the neutral zone.

The question is how the reflex effects of skin temperature changes interact with the associated change in the core:skin temperature gradient important in heat transfer. If the parameters of control from skin temperature are just right, the physical and reflex effects of a skin temperature change could combine for no net effect on core temperature. Implications for the phenomenon of afterdrop.

A series of pages on experimental findings from a study on control of skin blood flow in the neutral zone.

For details of the study, thesis work of Margaret Savage, see Savage, M. V. and G. L. Brengelmann. Control of skin blood flow in the neutral zone of human body temperature regulation. J. Appl. Physiol. 80: 1249-1257, 1996.

The rationale behind the experimental design

Why skin temperature was driven in a square-wave patttern.

Summary of experimental results.

from 8 subjects' response to skin temperature square waves

Methods for

control of skin temperature, following skin blood flow through measurement of forearm blood flow and recording esophageal temperature

Some interpretations of the results

Significance of the temperature changes recorded. Two degree changes in skin temperature may seem like a trivial thermal stress, but consider that, without the regulatory response, core temperature would eventually change as much as skin temperature. Yet core temperature rose slightly.

Significance of the flow changes recorded.A crude estimate of the overall change in skin blood flow that accomplishes the adjustment of heat transfer in response to skin temperature changes

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