Stress and anger are associated with the [HPA Axis](https://en.wikipedia.org/wiki/Hypothalamic%E2%80%93pituitary%E2%80%93adrenal_axis). Triggering this system releases adrenaline (edit: and cortisol), which cause widening of capillaries in preparation for needing more blood to muscles. Subjectively, the increased rate of blood moving from inside the body to the outside feels warm, because the blood comes from deeper and warmer body parts.
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