Lymph: Composition, Circulation & Function

Definition and normal volume

• Lymph is interstitial fluid after it enters the lymphatic vessels.
• Normal lymph flow is approximately 2–3 L/day (about 2 mL/min at rest), and increases substantially with exercise and increased interstitial filtration.

Composition

Lymph is derived from interstitial fluid and therefore resembles interstitial fluid more closely than plasma.

Regional variation

• Hepatic lymph: relatively protein-rich because hepatic sinusoids are highly permeable to protein.
• Intestinal lymph: fat-rich after meals because lacteals absorb chylomicrons.
• Peripheral tissue lymph is generally lower in protein.

Circulation

Formation

Fluid moves from capillary plasma into the interstitium according to Starling forces. Net capillary filtration produces interstitial fluid; excess fluid and escaped protein are returned through the lymphatic system.

J_v = fluid flux; κ = filtration coefficient; P = hydrostatic pressure; π = oncotic pressure; σ = protein reflection coefficient. Open in Formula Library

Entry into lymphatics

• Initial lymphatic capillaries are thin-walled, blind-ending vessels.
• Overlapping endothelial cells act as one-way flap valves.
• Rising interstitial pressure and anchoring filaments open the flaps.
• Fluid, protein and larger particles can therefore enter lymphatic capillaries.

Pathway

Interstitial fluid → lymphatic capillaries → collecting lymphatics → lymph nodes → lymphatic trunks → lymphatic ducts → systemic veins.

• Thoracic duct: drains both lower limbs, abdomen, left thorax, left upper limb and left head/neck; enters the venous circulation near the junction of the left internal jugular and subclavian veins.
• Right lymphatic duct: drains the right head/neck, right upper limb and right thorax; enters at the right venous angle.

Factors promoting lymph flow

• Intrinsic contraction of lymphatic smooth muscle.
• Skeletal muscle contraction.
• Respiratory pressure changes.
• Arterial pulsation and external tissue compression.
• One-way valves prevent backflow.

Functions

1. Return of excess interstitial fluid

• Returns net-filtered fluid to the vascular circulation.
• Failure of lymph drainage causes interstitial fluid accumulation and oedema.

2. Return of interstitial protein

• Collects protein that has escaped from plasma and returns it to the circulation.
• This limits accumulation of interstitial protein and helps maintain tissue fluid balance.

3. Absorption and transport of fat

• Intestinal lacteals absorb dietary long-chain lipid as chylomicrons.
• Chyle enters the thoracic duct and then the systemic circulation, initially bypassing the portal circulation.

4. Immune function

• Transports lymphocytes, antigen-presenting cells and antigens.
• Lymph nodes filter lymph and facilitate macrophage activity and adaptive immune responses.