25B13: Exam Report

  1. What is lymph and what is its normal volume? (10% of marks).
  2. Outline the following with respect to lymph:
    1. composition (20% of marks)
    2. circulation (45% of marks)
    3. functions (25% of marks)

69% of candidates passed this question.

This topic is less commonly examined; however, it is well covered in the recommended physiology texts and was generally well done.

The question breakdown provided a clear structure to approach this question and was mostly well utilised by candidates.

Details that were commonly omitted included:

  • the contribution of starling forces to the production of lymph
  • differences between lymph and plasma
  • the difference in protein and fat content depending on the areas drained

I1iv / 25B13: 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
  • Water (primary component)
  • Proteins (small amounts; more in thoracic duct lymph) – higher in the cisterna chyli and thoracic duct 
  • Lymph coming from liver will have higher [protein] because hepatic sinusoids have low reflection coefficient
  • Electrolytes (Na⁺, K⁺, Cl⁻, etc.)
  • Lymphocytes (mainly T and B cells)
  • Macrophages and Antigen-presenting cells
  • 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

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
    • Flow: lymphatic capillaries -> plexuses -> afferent lymphatic vessel -> lymph node -> efferent lymphatic vessel -> lymphatic trunks -> lymphatic ducts 
      • Four pairs of lymphatic trunks (lumbar, bronchomediastinal, subclavian and jugular)
      • One unpaired intestinal lymphatic trunk -> drains into cisterna chylii -> thoracic duct
    • Right Lymphatic Duct: Drains the right upper limb, right thorax, and right side of the head and neck → empties into the right subclavian vein.
    • Thoracic Duct: Drains the rest of the body → empties into the left subclavian vein.
    • 80% of lymph enters via the thoracic duct, 20% enters within the right lymphatic duct

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