Hair-thin vessels where blood actually trades
Capillaries are the body’s smallest blood vessels—only five to ten micrometres across—built from a single-cell intima wall. Arteries narrow to arterioles, then to these hairlike tubes where nutrients and wastes swap, before venules gather the flow home. The Latin root capillaris literally means hair-like.
English borrowed the word in the mid-1600s for that tiny diameter; as an adjective it also names capillary action in physics. True capillaries and thoroughfare channels stitch into beds that blanket organs. The hungrier a tissue’s metabolism, the denser the mesh must be to deliver fuel and haul away leftovers.
Lymphatic capillaries run a parallel game: slightly wider, closed at the tip, one-way doors for interstitial fluid that cannot easily leak back out. Their walls are more permeable than blood capillaries, suited to scavenging fluid and particles the closed arterial–venous loop leaves behind.
Blood capillaries come in three designs. Continuous ones have an unbroken lining that lets only water, ions and other small molecules squeeze between cells; the version with few vesicles forms part of the blood–brain barrier. Fenestrated capillaries carry pores of 60 to 80 nanometres bridged by a fibrous diaphragm, and in the kidney's glomerulus they are wrapped by podocyte foot processes. Sinusoidal capillaries have far wider openings of 30 to 40 micrometres with no diaphragm at all. Traffic across the wall splits by size: particles under about 3 nanometres, such as water and gases, pass between or through cells directly, while larger ones like albumin ride inside vesicles. New capillaries form in the embryo by vasculogenesis and later by angiogenesis, budding from existing vessels. Capillary beds can also hold their flow steady when central pressure changes, because stretched arterioles clamp down, a reflex called the Bayliss effect, while the kidney adds tubuloglomerular feedback.
Source: Capillary
