Raynaud phenomenon is a phenomenon where there’s vasoconstriction of arteries near the skin that make a body part, often the fingers, turn white, then blue, and then red in response to a trigger like cold weather.
The phenomenon is named after Auguste Gabriel Maurice Raynaud, a French physician, who first described it.
To clear up some potentially confusing terminology, it’s called Raynaud disease or primary Raynaud phenomenon where the condition occurs alone and is not associated with any other disease, and it’s called Raynaud syndrome or secondary Raynaud phenomenon when it is associated with diseases like systemic lupus erythematosus or scleroderma.
Normally blood flows from large arteries into medium-sized or muscular arteries, and then into small arterioles which carry the blood to capillary beds.
All arterial vessels have three layers: from inside moving out, there’s the endothelium, then the media layer which contains smooth muscle, and finally the adventitia layer which has loose connective tissue and nerves.
Some nerve fibers in the skin function as thermoreceptors, which sense changes in temperature.
The hypothalamus is what triggers the thought – “Hey, it’s pretty cold here. Maybe I should find a friendly llama to snuggle with for warmth.”
For example, normally, there’s a lot of heat energy in the blood, that gets lost to the environment.
That shunts blood away from the skin and towards the body’s core or organs where less heat energy is lost.
If the vasoconstriction happens briefly or intermittently it’s called vasospasm.
When the arterioles vasoconstrict it dramatically decreases blood flow to downstream tissue.
For example, if there’s vasospasm in the arterioles in the fingers, it causes the fingers to turn white reflecting ischemia, then blue reflecting hypoxia, which is the low oxygen state that occurs after prolonged ischemia.
Finally, once the vasospasm ends, oxygenated blood rushes into the tissue, causing what’s called reactive hyperemia, which makes the fingers look red.