Why Capillaries Allow Diffusion While Arteries & Veins Do Not

Capillaries are the exchange vessels; arteries and veins are conductance vessels. Structural differences dictate function.

Capillary Features for Diffusion

• Wall: Single endothelial layer + basement membrane (~0.5 µm thick).
• Diameter: 5–10 µm (RBCs deform to pass).
• Surface area: ~500–700 m² total (vast network).
• Flow velocity: ~0.3 mm/sec (allows time for diffusion).
• Permeability: Fenestrations, clefts, or transcellular transport.

Arteries & Veins: Designed for Transport

• Thick walls: Tunica media (smooth muscle, elastin) withstands pressure (arteries) or provides capacitance (veins).
• No exchange function: Endothelium sealed by tight junctions (except postcapillary venules).
• High flow velocity: Arteries ~400 mm/sec; diffusion distance too great.
• Low surface area-to-volume ratio.

Pressure Gradient Role

Arterial pressure drives filtration at arteriolar end; osmotic pressure drives reabsorption at venular end (Starling forces). Capillaries are the only site where net exchange occurs.