Question 4

Describe how interstitial fluid recirculates to the vascular system. 

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College Answer

Candidates had a limited understanding of this area of the syllabus. It was expected that answers would describe important concepts including the anatomy of venous structures, valves and lymphatics, permeability and factors which influence permeability. A description of hydrostatic forces, other pressures involved, and the role of osmotic and electric forces were required.  


Take it from somebody who knows the syllabus and routinely wracks his brain trying to somehow fit these questions into it;  there is no area of syllabus for this question. Presumably, Section I1(i) of the 2017 CICM Primary Syllabus would fit, as it expects the candidates to "explain the distribution and movement of body fluids". So, if "movement of body fluids" is what we are categorising this as, then it ends up in the same bucket as circulation of CSF and micturition. In short, taxonomy is hell; no wonder of 10% of you passed. So, instead, "Outline the composition and functions of lymph" is how we are going to play this.

Formation of interstitial fluid is through the balance of  Starling forces in the capillaries, where plasma fluid is ultrafiltered into the interstitial space

Capillary Starling equation Jv = Lp S [ (Pc - Pi) - σ(Πesl - Πb) ]  describes a situation where the net balance of forces (capillary hydrostatic pressure and low interstitial oncotic pressure) favours the movement of fluid into the interstitial space, as an ultrafiltrate.

Composition of interstitial fluid is regionally variable, and generally poor in protein (oncotic pressure ~ 5 mmHg)

Electrolytes of the interstitial fluid are different from plasma due to the Gibbs Donnan effect (for example, interstitial sodium = 0.95 ×plasma sodium) 

Lymphatic vasculature transports interstitial fluid back into the circulation

  • About 3 litres of lymph are produced every day
  • That makes about 120ml per hour (100mls of which returns via the thoracic duct)

Flow of lymph is via progressively larger valve lymphatic vessels:

  • Lymphatic capillaries (blind open-ended vessels)
  • Pre-collecting lymphatics (primary valves)
  • Collecting lymphatics (secondary bicuspid valves)
  • Lymphangions is the term given to length of lymphatic vessels between two valves
  • Lymph nodes
  • Thoracic duct empties into the central venous circulation
    • Left thoracic duct: 83% of total flow, empties into the junction of the left IJ and subclavian veins,
    • Right side of the head, right chest and right arm all empty into the right subclavian vein. 

Flow of lymph is propelled by:

  • Contraction of contractile smooth muscle of the lymphatic vessels, which is peristalsis-like
  • Contraction of skeletal muscle surrounding the lymphatics (thus, increases with exercise)
  • Transmitted pulsation of neighbouring arterial structures
  • Presence of one-way valves
  • Decreased intrathoracic pressure associated with normal breathing
  • Postural changes (i.e. with sleep)


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