What does capillary mean?
A capillary is one of the smallest blood vessels in the human body — a microscopic tube with walls just one cell thick that connects the arteries to the veins. It is here, across capillary walls, that the vital exchange of life happens: oxygen and nutrients pass out to nourish tissues, while carbon dioxide and other waste products pass back into the bloodstream for removal. The word also has a broader scientific meaning, describing any phenomenon involving the movement of liquids through extremely narrow spaces, known as capillary action — the force that lets water climb up a plant stem or seep into a paper towel. Derived from the Latin 'capillaris', meaning 'of the hair', the term captures just how fine these structures are: a single capillary may be narrower than a human hair. Though chiefly a word of medicine and biology, its figurative sense of extreme fineness gives it occasional use in describing anything threadlike or delicate.
Any of the tiny, thin-walled blood vessels forming a network throughout bodily tissues, through which oxygen, nutrients, and waste products are exchanged between the blood and surrounding cells.
"The surgeon carefully avoided damaging the fragile capillaries near the surface of the skin."
Usually encountered in plural when discussing circulation generally; the singular often appears in specific anatomical or physiological contexts.
Relating to, resembling, or involving capillaries or capillary action; hairlike in fineness.
"The dye spread through the filter paper by capillary attraction."
Standard plural used whenever referring to more than one of these vessels; no irregular forms exist.
"Tiny capillaries deliver oxygen to every cell in the body."
Laid end to end, the capillaries in your body would stretch roughly 100,000 kilometres — enough to wrap around the Earth more than twice.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of capillary
The word capillary comes from the Latin 'capillaris', meaning 'relating to the hair', itself derived from 'capillus', the Latin word for hair. Adopted into English in the mid-17th century, it was first applied in anatomical contexts to describe the fine, hair-thin blood vessels observed under early microscopes. Its use later extended into physics, where 'capillary action' described the way liquids behave in narrow tubes. Cognate terms include 'capillarity' (the physical property) and 'capillary tube' (a laboratory instrument), both built from the same Latin root.
How capillary is actually used
In everyday speech, 'capillary' almost always appears in medical or scientific contexts. The plural 'capillaries' is standard when referring to the vessels themselves; the singular usually appears in technical descriptions of anatomy or physics.
Easily confused with capillary
Capital refers to wealth, resources, or a principal city, whereas capillary is a minute blood vessel or something relating to narrow-tube liquid movement.