Capillary
From Memory Alpha, the free Star Trek reference
A capillary is the smallest type of blood vessel and a part of the circulatory system in humanoid lifeforms.
In 2365, during Captain Jean-Luc Picard's cardiac implant replacement surgery, the surgeon noted that there had been some capillary reaction and the surgical team needed to proceed carefully. (TNG: "Samaritan Snare")
When Species 8472 imitated humans and recreated Starfleet Command, their simulations were indiscernable from the real thing. The Doctor scanned the David Gentry-impersonator, finding no difference down to the capillaries. (VOY: "In the Flesh")
