This schematic illustrates the organization of blood vessels and the regulation of blood flow (arrows) in the palm of the hands. Arteriole-venule shunts are small muscular valves that connect directly between an arteriole and a venule to bypass the capillaries. Arrows indicate the direction of blood flow. As shown on the left, in order to radiate heat from our skin when we are hot, activation of the sympathetic nerve fibers close the shunts so that oxygenated blood (red arrows) in the arterioles is forced into the capillaries and deoxygenated (blue arrows) blood returns to the venules. As shown to the right, in order to conserve heat when we are cold, activation of sensory nerve fibers dilate the shunts and the blood bypasses the capillaries. Fibromyalgia patients were found to have an excessive amount of sensory fibers around the shunts.