For many years — I don’t want to think about how many — there was some microscopic misinformation in my article How Many Muscles Are In the Human Body?
WRONG: “Smooth muscle blends with other smooth muscle, and exists at every scale from microscopic. You have single cells of smooth muscle wrapped around capillaries…”
CORRECTED: “You have single cells of smooth muscle wrapped around all but the tiniest of blood vessels…”
Blood vessels are wrapped in smooth muscle down to about 50 micrometres, 5% of a millimetre, which happens to fit the cliché of being about as thick of a human hair. But below that size, the smallest arterioles, venules, and all the capillaries (5–10μm) lack smooth muscle.
So it’s true in spirit that microscopic blood vessels have even more microscopic muscles wrapped around them — which is amazing. But it’s only seen at the larger end of the “microscopic” range.
Hat tip to reader M.C. for reminding me that capillaries are just too microscopic to have muscles.