Capillaries: The Smallest Blood Vessels and Their Vital Role

Structure and Dimensions

• Diameter: 5–10 micrometers (just wide enough for a single red blood cell to pass in single file).
• Wall: Single layer of endothelial cells (simple squamous epithelium) with a basement membrane—extremely thin and porous.
• Network: Vast branching beds connecting arterioles to venules; total length ~60,000 miles in an adult.

Key Functions

• Exchange: Oxygen, carbon dioxide, nutrients, waste, hormones diffuse between blood and tissues driven by concentration gradients.
• Filtration: Hydrostatic pressure forces fluid out at arterial end; osmotic pressure pulls it back at venous end (Starling forces).
• Regulation: Precapillary sphincters and local metabolites control blood flow to match tissue demand.

Clinical Significance

Capillary damage underlies diabetic retinopathy, nephropathy, and neuropathy. Microvascular health is a marker of systemic disease.