The smallest blood vessels, with a lumen only about as wide as a red blood cell
Capillary — about 5–10 micrometres across.
Anatomy · Heart & blood vessels
Capillary
Smallest blood vessel, about 5–10 micrometres wide
Microcirculation · site of exchange
The smallest and thinnest blood vessels, linking arterioles to venules. A capillary wall is a single layer of endothelial cells on a basement membrane, so oxygen, nutrients, carbon dioxide and wastes can pass between blood and tissue. Capillaries are so narrow, about 5–10 micrometres in diameter, that red blood cells squeeze through in single file.
How to spot them
- ▸A wall one cell thick: endothelium on a basement membrane, with scattered pericytes
- ▸Lumen barely wider than a red blood cell, about 5–10 micrometres
- ▸Three types: continuous, fenestrated (with pores) and sinusoidal (discontinuous, large gaps)
- ▸Organised into beds, fed by arterioles through precapillary sphincters and drained by venules
What to know
Their wall is a single layer of endothelium on a basement membrane, allowing gases and nutrients to cross
Capillary — the site of exchange between blood and tissue.
Marcello Malpighi saw them in a frog's lung in 1661, completing William Harvey's picture of the circulation
Capillary — the missing link Harvey could not see.
Three types are recognised: continuous, fenestrated and sinusoidal
Capillary — fenestrated in the kidney glomerulus and gut; sinusoidal in the liver and marrow.
The blood-brain barrier is formed by a continuous type with tight junctions, surrounded by astrocyte end-feet
Capillary — cerebral capillaries.
Blood flow is slowest here because their combined cross-sectional area is the largest of any part of the circulation
Capillary — allows time for exchange.
Smooth-muscle rings at the entrance of each bed regulate its blood supply
Capillary — precapillary sphincters.
August Krogh won the 1920 Nobel Prize for discovering how the muscles' beds of these vessels are regulated
Capillary — Krogh, capillary motor regulating mechanism.
Key ideas
- Starling forces: hydrostatic pressure pushes fluid out at the arteriolar end, and plasma protein oncotic pressure draws most back at the venular end. Excess is returned by lymphatics.
- Velocity falls as total cross-sectional area rises: aorta about 3 cm2, capillaries about 4,000 cm2 in total.
Quick-fire facts
- •Diameter roughly 5–10 micrometres, similar to a red blood cell (about 7–8 micrometres).
- •Wall: endothelial cells and basal lamina; pericytes wrap around them.
- •Continuous capillaries are found in muscle, skin and brain; fenestrated in kidneys, endocrine glands and intestine; sinusoidal in liver, spleen and bone marrow.
- •Discovered by Marcello Malpighi in 1661 using a microscope on frog lung.
- •William Harvey, in 1628, had predicted such links between arteries and veins but could not see them.
- •The total capillary surface area of the body is often quoted at 600–700 square metres; figures vary widely by source.
- •August Krogh received the 1920 Nobel Prize in Physiology or Medicine for his discovery of the capillary motor regulating mechanism.
- •Capillary refill time, pressing on a fingertip and timing return of colour, is a bedside test of perfusion.
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Sources
- OpenStax — 20.1 Structure and Function of Blood Vessels — Anatomy and Physiology 2e
- TeachMeAnatomy — Blood Vessels — Ultrastructure