Lymph Formation & Lymphatic Capillary Uptake: Starling Forces & Pump Mechanisms

Lymph originates as interstitial fluid filtered from blood capillaries and is returned to circulation via lymphatic capillaries.

Filtration at Blood Capillary (Starling Forces)

Net filtration = Kf × [(Pc − Pi) − σ(πc − πi)]

• Pc (capillary hydrostatic) ~30 mmHg arterial end, ~15 mmHg venous end.
• Pi (interstitial hydrostatic) ~ −3 to +3 mmHg.
• πc (plasma oncotic) ~25–28 mmHg (albumin).
• πi (interstitial oncotic) ~5–10 mmHg.
• Net outward filtration at arterial end; reabsorption at venous end.
• ~10–20% of filtered fluid (2–4 L/day) enters lymphatics.

Entry into Lymphatic Capillaries

• Primary valves: Overlapping endothelial cells anchored by filaments; open when Pi > Plymph.
• Anchoring filaments: Connect endothelium to collagen; transmit tissue stretch → open flaps.
• Osmotic gradient: Not the primary driver; lymph capillaries are highly permeable, protein‑rich lymph enters passively.

Lymph Propulsion

• Intrinsic pump: Lymphangions (segments with smooth muscle) contract rhythmically (1–10/min).
• Extrinsic pumps: Skeletal muscle contraction, respiratory diaphragm, arterial pulsation, passive tissue compression.
• Valves: Ensure unidirectional flow.

Clinical Relevance

• Lymphedema = failure of transport capacity > filtration load.
• Compression therapy augments extrinsic pump.
• Manual lymph drainage stimulates intrinsic pump.