I1iv: Outline the composition and functions of lymph

Defintions

Lymph = fluid which has entered lymphatic vessels from interstitial spaces

Lymph Fluid Content

  • Derived from ISF → very similar composition
  • Contains all clotting factors (so can clot)
  • No platelets
  • Low fibrinogen (because larger MW 340,000)
  • After high fat meal, lymph draining bowel will have ‘milky’ appearance due to presence of chylomicrons
  • Lymph coming from liver will have higher [protein] because hepatic sinusoids have low reflection coefficient

Anatomy

  • Lymphatic capillaries = everywhere EXCEPT CNS, bone & cartilage
  • Lymph vessels are blind-ending
  • Made of endothelial cells with a highly permeable BM
  • Possess flap valves between endothelial cells → permit one way entry of ISF
  • Lymph caps drain into larger lymph vessels
  • All lymph passes through LNs
  • Lymph returns to circulation via:
    • THORACIC DUCT → junction of L) subclavian & LIJ
    • R) LYMPHATIC DUCT → junction of R) subclavian & RIJ

Function

1) Return Fluid

  • Starling’s Forces result in:
    • Net Filtration ~20mL/min at arterial end
    • Net reabsorption ~18mL/min
    • ∴2mL/min escapes into interstitium
  • This fluid is returned to circulation by lmph at a rate of 2mL/min (at rest)

2) Return Protein

    • Protein of lymph is low & mimics ISF [protein] EXCEPT liver, which has low ‘s’
    • Protein is returned to circulation by lymph vessels → NB: this is the only way that any protein which has escaped vascular compartment can be returned because it cannot re-enter vessels

3) Immunological Role

  • LNs contain macrophages
  • Filter & phagocytose bacteria & debris
  • LCs also present → proliferate in response to Ag’s

4) Fat Transport

    • Fat transported as chylomicrons reach circulation by thoracic duct (bypassing portal circulation)

Lymph Flow

  • Lymph flow 2mL/min (to return fluid from ISF)
  • EXTRINSIC & INTRINSIC factors promote this
    • Extrinsic: tissue pressure, skeletal m. activity, arterial pulsations → all compress lymph vessels
    • Intrinsic: smooth m. wall contraction in large lymph vessels & presence of 1-way-valves → ensure unidirectional flow
  • ↑flow with exercise
  • If ISF volume > lymph drainage → OEDEMA