Body Fluids — Interstitial Oedema

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2011A Q18

Exam question

Explain the physiological processes involved in the development of tissue interstitial oedema.

CICMWrecks answer

Master answer

Interstitial oedema occurs due to increased permeation of fluid from intravascular to interstitial spaces, and inability of the lymphatics to reabsorb this additional fluid

Starling Forces

The NET flux across the membrane is the balance of hydrostatic pressure and oncotic pressure, as defined by the Classic Starling Equation:

J_v={\kappa \; ([P_{capil} – P_{interstit}] – \sigma \; [\pi_{plasma} – \pi_{interstit}])}

where
J_v is the trans endothelial solvent filtration volume per second
( [ P_c – P_i ] – σ [ π_p – π_i ] ) is the net driving force
P = hydrostatic pressure
π = oncotic pressure
σ = Staverman’s reflection coefficient ie. Permeability of membrane to protein
κ = filtration constant = L_pS = Hydraulic conductivity x Surface Area

P_{cap} \; \propto \; {{Post-capil\;resist} \over {Pre-capil \; resist}}

Typically quoted values for the variables in the classic Starling equation:

Hydrostatic pressure	Oncotic pressure
Pressure moving fluid	pressure exerted by proteins which draw water into and keep it within a compartment
P_c ~35 → 15mmHg (Arterial → venous) Capillary hydrostatic pressure Pressure moving fluid out of capillary	π_p ~ 20mmHg Plasma oncotic pressure Pressure keeping fluid within capillary
P_if = 5mmHg Interstitial hydrostatic pressure Pressure moving fluid into capillary	π_if ~ 0mmHg Interstitial fluid oncotic pressure Pressure keeping fluid out of capillary

In general,
- at the arterial end of capillary NFP is positive (filtration) +10mmHg
- At the venous end NFP is negative (absorption) -10mmHg
Approx. 24L fluid filtered / day
- 85% reabsorbed into capillaries
- Rest reabsorbed via lymphatics (~3.5L/day) = Net fluid loss from filtration

Tissue Interstitial Oedema

Increased κ promotes oedema	– Inflammation
Increased P_c promotes oedema	Increased resistance to venous return – Diastolic HF – Increased R atrial pressures – Venous obstruction (DVT, mass) – Loss of one way valves – Loss of veno-muscular pump (in bed-bound patients)
	Increased intravascular volume – Activation of RAAS in CHF, cirrhosis, nephrotic syndrome – Crystalloid administration in ICU
	Gravity and posture – Lower limbs if standing – Sacrum if supine
Decreased π_c promotes oedema	– Hepatic failure and decreased plasma proteins – Nephrotic syndrome
Endothelial glycocalyx – damage promotes oedema	– Glycoprotein layer on inner surface of capillaries which sequester plasma proteins
Decreased lymphatic flow promotes oedema	Obstruction – Compression – Tumour – Infection – filariasis
Increased P_i inhibits oedema	– Compression stockings – Deep sea diving, increased atmospheric pressure

Sakurai / JC 2020

Reusable content

Formulae used in this answer

Capillary pressure — resistance relationship

Capillary pressure vs resistance ratio

CWF-0126

P_{cap} \; \propto \; {{Post-capil\;resist} \over {Pre-capil \; resist}}

Starling fluid flux

CWF-0125

J_v={\kappa \; ([P_{capil} - P_{interstit}] - \sigma \; [\pi_{plasma} - \pi_{interstit}])}

Answer by JC, Sakurai

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Exam appearances

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Exam	Exact exam wording	Candidate success
2011A Q18	Explain the physiological processes involved in the development of tissue interstitial oedema.	17%