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Raynaud phenomenon is a peripheral vascular condition characterized by vasoconstriction of arterioles near the skin, most often of the fingers and toes, which makes them turn white, then blue, and then red.
Okay, now let’s quickly review some anatomy and physiology. Normally, blood flows from large arteries into medium and then small arteries called arterioles, which in turn carry the blood to capillary beds. All arterial vessels have three layers: from inside out, there’s the endothelium; then the media layer, which contains smooth muscle; and finally the adventitia layer, which has loose connective tissue, as well as vasa vasorum or vessels that supply the artery, and nervi vasorum or nerves that innervate the artery and help regulate vasoconstriction via contraction of the smooth muscle in the media layer.
Now, some nerves in the skin function as thermoreceptors, which sense changes in temperature and then send signals up through the spinal cord to the hypothalamus, which is at the base of the brain. The hypothalamus serves as the body’s thermostat and coordinates the body’s response to temperature changes through the sympathetic and parasympathetic nervous systems.
Normally, there’s a lot of heat energy in the blood that gets lost from skin to the environment. So, when the hypothalamus gets the signal that we’re cold, it stimulates the sympathetic nervous system, which in turn signals the nervi vasorum to trigger vasoconstriction in certain body areas, such as the skin, resulting in a reduction of blood flow. That shunts blood away from the skin and towards the body’s core or organs, where less heat energy is lost. On the other hand, when we’re hot, the hypothalamus stimulates the parasympathetic nervous system, which ultimately leads to vasodilation in skin to dissipate the heat.
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