Capillary comes from the Latin capillus which means "hair of the head" (caput, head, plus pilus, a hair). In spite of its etymology, an average capillary is a hollow tube about 8 microns in diameter, about 1/10 that of a typical human hair.

Capillaries were first described in 1661 by the renowned Italian anatomist Marcello Malpighi while observing frog lungs under a microscope; Malphghi was one of the first to use the microscope, invented in the Netherlands a generation earlier, for biological observations.

A human adult has over 60,000 miles of capillaries with a total surface area of 600-1000 square meters, an area equal to 2-3 tennis courts

The total volume of the capillaries is around 4-5 liters, the same as the blood volume; obviously, at any given time, a significant number are empty.

Malpighi's original drawing of frog lung capillaries, modified from Fishman and Richards (1964)