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Capillary Exchange: Filtration, Reabsorption, Nutrient/Waste Transfer, and Lymphatic Drainage

Capillaries are the primary exchange vessels (surface area ~500–700 m²). Four integrated processes:

  1. Filtration (Arterial End) – Hydrostatic pressure (Pc ~35 mmHg) > oncotic pressure (πc ~25 mmHg) → net outward flux of plasma water, electrolytes, nutrients, hormones.
  2. Reabsorption (Venous End) – Pc drops (~15 mmHg) < πc → net inward pull of water + waste (CO₂, lactate, urea) via oncotic gradient.
  3. Transcellular/Paracellular Diffusion – O₂, glucose, amino acids down concentration gradients; CO₂, wastes opposite.
  4. Lymphatic Drainage – ~10–20% of filtered fluid (2–4 L/day) returns via lymphatics; carries immune cells, proteins, chylomicrons.

Starling Forces Balance

Net filtration = Kf × [(Pc – Pi) – σ(πc – πi)]. Disruption (↑Pc in heart failure, ↓πc in nephrotic syndrome, ↑Kf in inflammation) → edema.

Intact capillary function is essential for tissue homeostasis; failure underlies shock, ARDS, compartment syndrome, and chronic edema.

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