**4. Effects of biological aging on thermoregulation in warm and hot conditions**

The susceptibility to heat related-illnesses in the elderly [6] is caused by a deterioration in thermoregulatory responses with aging [47, 48]. Previous studies have indicated that even healthy elderly individuals have an impaired thermal perception [49] and impaired autonomic [50, 51] and behavioral [52, 53] thermoregulatory responses. A recent study indicated that skin warmth detection thresholds in the extremities and the whole-body thermal sensation deteriorated with normal aging under both normothermic conditions and under passive heat-induced mild hyperthermic conditions [54]. Decreased VO2max and cardiovascular capacity associate with the deteriorated thermoregulation with aging [51]. Nevertheless, elderly individuals with a similar level of VO2max to young individuals are known to show an attenuated response of SkBF both during passive heat stress in whole-body or local-body parts and during exercise under a hot environment compared to young individuals [55, 56]. Specifically, Kenney et al. [56] used bretylium tosylate to block the local release of norepinephrine on the forearm skin. They suggested that the attenuated SkBF response to hyperthermia during exercise in a hot condition was caused not by an enhanced vasoconstrictor system but mainly by a decreased sensitivity of the active vasodilator system to increased Tes. In addition, the whole-body and local sweat rate in response to passive heat stress or exercise are attenuated in the elderly compared to young adults [57].

Several physiological changes with advancing age, for example, such as decreased PV and increased Posm at baseline [58], diminished thirst sensation [58], and responses in antidiuretic hormone and aldosterone after thermal dehydration [38], are suggested to be associated with the decreased thermoregulatory responses in the elderly. Decreased renal concentrating ability [59] and lower reabsorptive ability of sweat gland ducts [59] with advancing age are also suggested to be associated with the deteriorated thermoregulatory responses with aging. Vasoconstriction of splanchnic organs during exercise, which enhances the redistribution of cardiac output to the skin vasculature, is associated with increased cutaneous vasodilation in youth, which is also attenuated with aging [60]. Furthermore, elderly individuals commonly take a variety of prescription drugs that may affect thermoregulatory responses and body fluid regulation [1]. Exercise training and heat acclimation can improve the blunted body fluid regulation and thermoregulation with aging, although generally the improvement of these is lower or limited relative to their younger counterparts.
