Overview

Capillaries are the smallest blood vessels in the circulatory system, forming extensive networks that connect arterioles and venules. Their principal role is to permit the exchange of oxygen, carbon dioxide, water, nutrients and metabolic waste between blood and surrounding tissues. Because they lack the thick muscular and elastic layers of larger vessels, capillaries are specialised for permeability rather than for conducting blood under high pressure.

Structure and characteristics

A typical capillary wall is formed primarily by a single layer of endothelial cells resting on a thin basement membrane, often supported by pericytes. The thinness of this barrier allows diffusion and selective transport. Capillary diameters are commonly on the order of a few micrometres (often reported around 5–10 μm), just wide enough for red blood cells to pass in single file.

Types of capillaries

  • Continuous capillaries: Have uninterrupted endothelial cells and tight basal lamina; found in muscle, skin and the central nervous system.
  • Fenestrated capillaries: Contain pores (fenestrations) that increase permeability; typical in endocrine glands, intestines and kidneys.
  • Sinusoidal (discontinuous) capillaries: Larger gaps between endothelial cells and an incomplete basement membrane; occur in liver, spleen and bone marrow where freer exchange is needed.

Function and microcirculatory control

Capillaries enable gas exchange by diffusion, support nutrient delivery and remove metabolic waste. Fluid movement across capillary walls is governed by hydrostatic and oncotic pressures (Starling forces), and local blood flow into capillary beds is regulated by precapillary sphincters and arteriolar tone. These mechanisms allow tissues to match perfusion to metabolic demand and participate in thermoregulation and immune responses.

Development, measurement and clinical importance

Capillaries emerge during embryonic vasculogenesis and later expand by angiogenesis in growth, repair and in certain diseases. Clinically, capillary function is central to conditions such as edema, inflammation and diabetic microangiopathy; impaired capillary exchange can lead to tissue hypoxia and organ dysfunction. Simple bedside checks (capillary refill) and specialised imaging (capillaroscopy, intravital microscopy) assess capillary health in research and medicine.

For concise definitions and further reading, see blood vessel, the role of the endothelial cell, and general exchange across membranes in organisms. Typical size and comparisons with arteries and venules place capillaries in context. Key transported elements include water, oxygen and carbon dioxide, together with other nutrients and metabolic waste, all carried by blood.