Capillary Exchange: How Diffusion, Filtration, and Reabsorption Keep You in Balance

Capillaries are the body’s exchange highways—microscopic vessels where oxygen, nutrients, waste, and fluid move between blood and tissues. Three physical processes orchestrate this continuous traffic, each driven by distinct forces.

1. Diffusion: The Primary Engine

Mechanism: Passive movement down concentration gradients across the ultra-thin (0.5–1 µm) endothelial wall.

• Outward: O₂, glucose, amino acids, hormones.
• Inward: CO₂, lactate, metabolic waste.

No energy required—just surface area (vast capillary networks) and gradient maintenance.

2. Filtration: Pressure-Driven Fluid Exit

Mechanism: Hydrostatic pressure (blood pressure) forces water and small solutes (<30 kDa) through endothelial clefts and fenestrations.

• Dominates at the arterial end (≈35 mmHg capillary pressure vs. ≈15 mmHg interstitial).
• Net filtration pressure ≈10–15 mmHg outward.

Delivers fluid for interstitial bathing and lymph formation.

3. Reabsorption: The Osmotic Pull Back

Mechanism: Plasma proteins (mainly albumin) create oncotic pressure (~25–28 mmHg) that draws fluid back in.

• Dominates at the venular end (hydrostatic pressure drops to ≈15 mmHg).
• Net reabsorption pressure ≈10–13 mmHg inward.

Recovers ~90% of filtered fluid; the remaining 10% enters lymphatics.

Structural Adaptations for Efficiency

• Thin walls: Single endothelial cell layer.
• Large surface area: ~1,000 mi² in adults.
• Tight junctions / fenestrations: Organ-specific permeability (brain: tight; kidney/liver: fenestrated).
• Pericytes: Regulate capillary tone and angiogenesis.

Clinical Relevance

Imbalance → edema. Elevated hydrostatic pressure (heart failure), low oncotic pressure (nephrotic syndrome, liver failure), or increased permeability (sepsis, burns) disrupts the Starling equilibrium. Understanding these forces guides fluid resuscitation, diuretic use, and critical care management.