The difference between core (Tc) and skin temperature (Tsk), interacts physically with total skin blood flow. The greater the (Tc - Tsk) difference, the greater the amount of heat transferred to the body surface. To a first approximation, the heat transfer is proportional to the product of the temperature difference and the blood flow.

A change in either Tc or Tsk affects heat transfer through the effect on the (Tc - Tsk) difference and also skin blood flow, through the thermoregulatory reflex influence of both Tc and Tsk. A Tsk change that reduced the temperature difference (a change in the direction of reducing heat transfer) would simultaneously induce an increase in skin blood flow (a change in the direction of increasing heat transfer. Arrows in the diagram show effects of an increase in skin temperature.

The first line is the physical effect. Increased skin temperature decreases the difference between core and skin temperatures. Consequently, less heat is delivered to the skin surface by a given amount of skin blood flow.

The bottom line is the reflex effect. Increased skin temperature increases skin blood flow with the consequence of increased heat transfer.

The two effects could balance. If the gradient was reduced by, say, 30% percent and the increase in blood flow was also 30%, the net effect of the skin temperature change on heat transfer could be zero. Core temperature would remain steady despite the changes in skin temperature and skin blood flow.

If the physical and reflex effects did balance one another, skin temperature could be driven from one extreme of the neutral zone to the other with no effect on core temperature. The whole job of regulation of thermal balance would be accomplished through this reflex control from skin temperature that perfectly balanced the physical effect of the altered gradient.

If the effects do not balance, then core temperature will seek a new equilibrium after a skin temperature change. What you expect intuitively is that core temperature should follow skin temperature. Don't you expect that, if skin temperature fell from the high side of the neutral range to the low extreme, core temperature would fall at least a little? It would not if the reflex reduction of skin blood flow were proportionately greater than the increase in core:skin gradient. Core temperature would change the wrong direction -- it would increase in response to a decrease in skin temperature.

As a matter of fact, that happens all the time. Afterdrop is a familiar phenomenon in which core temperature falls when skin temperature increases. You hear less often of afterrise, but many have observed that a sudden fall in skin temperature causes a rise in core temperature. These observations are usually at skin temperatures far outside the neutral zone, but the same phenomenon can be seen within that narrow range.

Click for next page, which begins the sequence covering the design for an experimental investigation of skin blood flow control in humans in the neutral zone. followed by experimental methods and brief discussion of the results.