Direct noninvasive, real-time observation of thermoregulation physiology: periodic fluctuations in hematocrit and vascular volume in the peripheral circulation

Thermoregulation is a mammalian physiological function fulfilled in large part by autonomic control of blood flow. We demonstrate the variation in hematocrit (Hct) and intravascular volume (VV) in the peripheral circulation when the external means of maintaining the initial thermal disequilibrium is removed using a PV[O]H device capable of noninvasively measuring both Hct and VV with unprecedented sensitivity, accuracy and precision on a 3 second timescale. Calibrated using an FDA approved device now in standard use for monitoring Hct during dialysis, the PV[O]H detection limit for measuring Hct variation is ±0.03 where 45% is normal. Observing the return to thermal equilibrium at 2 separate anatomic locations, we observe the return to normal homeostasis in a matter of a few minutes. Heat induced vasodilation results in an antecedent increase in plasma volume in greater proportion than for red blood cells into the dilated capillaries. At equilibrium homeostasis i.e. when there is no externally maintained thermal gradient we observe periodic fluctuations in the peripheral Hct and VV on a roughly 15 second to 1.5 minute timescale.