## 3 Results and Discussion
### Discussion  
In humans, in response to thermal stress, changes in the skin blood circulation represent a vital aspect of normal thermoregulation. As part of such regulation, vasodilation and increased skin blood flow occur in order to dissipate heat during heat exposure and exercise. In contrast, during cold exposure, vasoconstriction in the skin decreases heat loss from the body, protecting the body against hypothermia. Therefore, the control of skin blood flow has significant clinical implications and can substantially impair the ability to maintain normal body temperatures [15,16]. There are a number of ways to visualize skin structures during such external changes of heating and cooling, such as evaluating the cutaneous microvasculature of the vascular organization, and several noninvasive in vivo optical techniques exist. Cutaneous blood flow is measured by confocal microscopy, venous occlusion plethysmography, Doppler ultrasound, and laser Doppler flowmetry [8]. All of these techniques have limitations such as limited depth range, no direct visualization, and limitations with respect to spatial localization. Furthermore, because the nature of observation involves measurement from the surface, the localization of signals for depth is almost impossible. In contrast, optical coherence tomography (OCT) is an optical imaging technique where light from a low coherent source is focused onto the tissue and reflectivity of the internal microstructures at different depths is measured by an interferometer [9]. In relation to heat stress, [PERSON] et al. employed optical coherence tomography (OCT) to assess acute changes in the density and size of cutaneous microvessels in male subjects only [11]. In this study, our aim was to use bispeckle optical coherence tomography developed by us [13,14] to investigate the dynamic changes within the skin structures under a heat stress environment. Biospeckle OCT uses a conventional OCT to obtain a sequence of optical reflectivity images from different depths to construct structural images and then investigates the temporal fluctuation within the structural images. These fluctuations correspond to the interfering scattered light from the scattering structures within the coherence volume of the tissue. Heat stress (40 \({}^{\circ}\)C for 5 min) was applied to the ventral part of forearm and OCT scans were obtained from a region in the immediate vicinity. A total of 20 subjects, 10 males and 10 females, participated in the study. The subject group was made up of two different age groups: those in their 20s and in their 30s. The average biospeckle contrast, a method used to measure the fluctuation results, was compared for statistically significant differences (\(t\)-test, \(p<0.05\)) under the three different conditions of before heating, soon after heating, and after 5 mins of rest following heating for different depths, ages, and genders. In relation to the use of OCT in heat stress, [PERSON] et al. 's (2016) [11] study involved the passive heating of legs and measurements taken from the skin on the forearm of male subjects only. In that study, lower limb heating in 40 \({}^{\circ}\)C water successfully elevated the  
Figure 11: Averaged biospeckle contrast for all male and female participants from the shallow region, ROII, under different conditions (error bar represents standard error). * indicates that there is a significant difference between the males and females for both immediately after heating and after 5 min of rest, but no significance before the start of the heat stress experiment. participants' core temperature and stimulated systemic thermoregulatory reflexes, resulting in increased forearm skin blood flow. OCT images of the ventral surface of the forearm skin were obtained to a depth of 500 \(\upmu\)m both prior to and during heating. The study investigated the vascular density based on the speckle properties over a plane or B-scan at a fixed depth of 500 \(\upmu\)m and found acute changes in the density of the cutaneous microvasculature in response to heating.