Physical training and the control of skin blood flow.

The process of physical training places frequent significant demands for increased blood flow to cardiac and skeletal muscle tissues and sets into action adaptive responses to better enable the circulatory system to meet those demands. These adaptive changes and the associated mechanisms are dealt with elegantly in other portions of this symposium. The repeated bouts of dynamic exercise with training also expose the temperature regulatory system to increased body temperatures and attendant demands for increased heat loss. These frequent demands for increased heat loss lead to adaptations in the control of the cutaneous circulation. There are consistent results among the limited number of studies conducted to test this question directly. The primary result is that skin blood flow in the trained state is higher at a given level of internal temperature than in the sedentary or less trained state. This result is seen in both cross-sectional and longitudinal comparisons, in older and younger subjects, in responses to heat at rest and during exercise, and in the changes with detraining as well as those attending training. In some studies this adjustment is made by a shift in the threshold internal temperature at which skin blood flow begins to rise, whereas in others it is accomplished by an increase in the sensitivity of the skin blood flow-internal temperature relationship. Reasons for this variation are not clear. The cutaneous circulation is controlled by vasoconstrictor and separate vasodilator nerves, but it is not clear how much of the training effect is manifest through one or the other neural system. However, indirect data suggest that vasoconstrictor activity is generally reduced and that active vasodilator activity is initiated at lower internal temperatures. It is also not clear to what extent the mechanism for the training effect is through the acclimatization process, as opposed to the influence of training, itself. In any case, the adjustments in control of the cutaneous circulation with physical training increase the capacity of the circulation to transport and eliminate heat as that training process increases the capacity of the active tissues to produce that heat.