Capillaries: The Microscopic Exchange Vessels of the Circulatory System

Capillaries are the smallest blood vessels (5–10 μm diameter), forming vast networks (capillary beds) that penetrate nearly all tissues. Their structure is exquisitely adapted for exchange:

• Endothelium: Single layer of squamous endothelial cells (basement membrane only).
• Permeability: Tight junctions (continuous), fenestrations (fenestrated), or gaps (sinusoidal) allow selective passage.
• Low flow velocity: ~0.3 mm/sec — maximizes diffusion time.
• Large surface area: ~1,000–3,000 m² total in adults.

Exchange Mechanisms

• Diffusion: O₂, CO₂, nutrients, waste — down concentration gradients.
• Transcytosis: Vesicular transport for larger molecules (albumin, antibodies).
• Bulk flow (filtration/reabsorption): Hydrostatic vs. oncotic pressure (Starling forces) moves fluid.

Capillary Types

Type	Location	Permeability
Continuous	Muscle, skin, lung, BBB	Low (tight junctions)
Fenestrated	Kidney glomeruli, endocrine glands, intestinal villi	High (pores + diaphragm)
Sinusoidal (discontinuous)	Liver, spleen, bone marrow	Highest (gaps, no basement membrane)

Clinical Relevance

• Capillary leak syndrome, edema, ischemia-reperfusion injury, diabetic microangiopathy, tumor angiogenesis.

Capillaries are where the circulatory system fulfills its fundamental purpose: sustaining cellular metabolism.