Capillary

A capillary was 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. 

In 2370, Reginald Barclay anxiously informed Doctor Beverly Crusher that he thought his capillaries were shrinking. 

In 2372, the capillaries in Jake Sisko's cerebral cortex dilated 20 percent, due to Onaya's stimulation of his brain. 

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.