Body Control Temperatures

Body temperatures are primarily sensed by temperature sensors in the hypo­thalamus near the center of the brain. Arterial blood flowing over and near the hypothalamus gives it information about the average thermal condition of

The body. In addition, there is evidence that temperature sensors in the spinal cord and gut also give the hypothalamus core temperature information. s The skin has abundant numbers of warm and cold sensors that also communicate to the hypothalamus (Fig. 5.2).

Thermal Sensation

The temperatures monitored in Fig. 5.2 are used by the brain to regulate shivering, blood flow to the skin, and sweating. The sensed temperatures also contribute to our overall feelings of warmth and other thermal sensations. Thermal sensation (TS) can be predicted over a wide range of activities (0.8 to 4 met) from simple deviations in the mean body temperature (Tmb) from the mean body temperature when the person feels neither warm or cool but neu-

Tral (Tmbn) (Fig. 5.2).

The mean body temperature is a weighted average of core and skin tem­peratures, with core temperature being much more important. That is as it should be as the purpose of the regulation system’s operations is to maintain core temperature for the brain and other vital organs. The mean body temper­ature for a neutral thermal sensation is about 36.2 °C. At temperatures above or below that, one feels progressively warmer or cooler, which further protects the individual by stimulating conscious behavioral actions to reduce physio­logical strain and restore neutral sensations.

During transients the rate of change of mean body temperature can have a strong effect on thermal sensation.

(5.7)

Feeling: Thermal Sensation TS +3 — hot +2 • • warm + 1 ■■ slightly warm 0 — neutral -1 — slightly cool -2 — cool -3 •• cold

TS = K, s ■ <Tmh — Tmbn) + K, ■ d(Tmb)/dt Ft Important during transients

Where 7mb = 0.9 Tc + 0.1 Tsk ffs$ =4.6 K, = 0.5 not yet well defined

Tc Hypothalamus — center for temperature control (surrounded by flowing blood)

Body temperature sensors

Brain

H

T

■Q*

"□*

38 ;~X’ 37 <L> *3 .3.5 34 Ch Ј 33 H 32 31

♦ T, □ TA

B

Met

FIGURE 5.3 Schematic of skin and core temperatures for a neutral thermal sensation.

In most transient environmental situations, it is rapid changes in skin temper­ature that affect our feelings of warmth; rapid changes in core temperature only occur during rapid changes in metabolism and possibly during transient radar or other microwave exposures. Diving into cold water after a hot sauna is pleasant rather than cold because core temperature remains high and dT reduces the hot thermal sensation.

In summary, core temperature is much more important than skin temper­ature in determining how warm we feel. Core temperature is affected by meta­bolic activity and heat storage. It is relatively isolated from the environment except through whole-body heat balance and resulting heat storage. Feet and hands have little metabolic heat generation themselves and depend on warm blood from the core for their temperature. The feeling of cold feet then means that the whole body heat balance has caused the core to lose temperature and the hypothalamus is restricting heat flow to the feet in order to stabilize the core temperature.

The consequence of the relationships of Table 5.3 and Fig. 5.2 is that for a neutral thermal sensation, at steady state, the core temperature increases while the skin temperature decreases with increased metabolic activity (Fig. 5.3). The increase in metabolism causes sweating which decreases skin tem­perature.

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