Describe how interstitial fluid recirculates to the vascular system.
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
Flow of lymph is via progressively larger valve lymphatic vessels:
Flow of lymph is propelled by:
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