# Oedema Mechanisms > Oedema mechanisms in MBBS Physiology — raised capillary pressure, low plasma albumin, permeability change and lymphatic failure with Guyton safety factors. - Canonical URL: https://prepelephant.com/topics/mbbs/physiology/oedema-mechanisms - Exam / course: MBBS · Subject: Physiology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Oedema Mechanisms", PrepElephant, https://prepelephant.com/topics/mbbs/physiology/oedema-mechanisms ## Direct answer Oedema is excess interstitial fluid, and every cause ultimately works through one of four mechanisms: raised capillary hydrostatic pressure, lowered plasma colloid osmotic pressure, increased capillary permeability, or lymphatic obstruction. Fluid does not appear the moment Starling forces tilt, because the interstitium has a combined safety factor of about 17 mmHg — low interstitial compliance, a lymph flow that can rise ten-fold, and washdown of interstitial protein — so oedema declares itself only once this buffer is exhausted. ## What you must remember - Raised Pc: congestive heart failure, venous thrombosis, venous obstruction, fluid overload — dependent, pitting oedema. - Lowered πp: serum albumin below about 2.5 g/dL in nephrotic syndrome, cirrhosis, protein-losing enteropathy and kwashiorkor — generalised, periorbital, soft. - Raised permeability: burns, sepsis, histamine and bradykinin release, angioedema — local, may be non-pitting; Kf rises and σ falls. - Lymphatic obstruction: filariasis, malignancy, post-axillary-dissection arm — non-pitting, protein-rich, brawny. - Guyton's three safety factors total roughly 17 mmHg: high interstitial compliance resistance (fluid pressure rises steeply), increased lymph flow (up to 10–50 times), and protein washdown lowering πi. - Pulmonary oedema appears when pulmonary capillary pressure exceeds about 25 mmHg acutely (mitral stenosis patients can adapt up to nearly 40 mmHg as safety factors enlarge). - Non-pitting swellings to quote in viva: lymphoedema, myxoedema (mucopolysaccharide binding), elephantiasis, angioedema. - Cerebral oedema has its own taxonomy: vasogenic (permeability), cytotoxic (cell swelling in ischaemia), interstitial (hydrocephalic). ## A morning face that pinpoints the kidney A 24-year-old wakes with periorbital puffiness; urine shows 4+ protein and serum albumin is 1.8 g/dL. Reason the sequence rather than memorise it. Glomerular albumin leakage lowers πp below the point where even venular capillaries can absorb, so filtration continues around the clock at every capillary. Why the face and not the ankles? Because the loose periorbital interstitium offers the least resistance, and overnight recumbency removes the gravity gradient that would otherwise pool fluid in the legs. Contrast the 60-year-old with right heart failure: there the ankles swell by evening because raised venous pressure acts only on dependent capillaries, and the sacrum takes over when he lies flat overnight — the distribution itself is a diagnostic clue. Add the second-hit concept: the nephrotic kidney also retains sodium avidly (a primary renal, "overfill", component in addition to the underfill response to hypoproteinaemia), so both the Starling side and the volume side of the balance are disturbed. Treatment logic follows the mechanism — sodium restriction and diuretics reduce ECF volume regardless of cause, but albumin infusion helps only the low-πp states, and compression stockings help only the gravity-dependent ones. ## One-mark traps in oedema questions "Which oedema is non-pitting?" is answered fastest by mechanism, not memory: anything where the fluid is protein-rich or bound in matrix — lymphoedema, myxoedema, elephantiasis, angioedema — whereas transudative Starling oedema pits. Beware the pulmonary numbers: systemic oedema thresholds do not apply to the lung, whose capillary pressure is normally 7 mmHg and which floods near 25 mmHg acutely. Finally, do not attribute all swelling to Starling forces in a post-mastectomy arm or a filarial limb — the mechanism there is mechanical obstruction, and no amount of salt restriction will drain protein-laden lymph. ## Frequently asked questions ### What is Guyton's safety factor against oedema? About 17 mmHg, contributed by low interstitial compliance, increased lymph flow and washdown of interstitial protein, so capillary pressure must rise substantially before visible oedema appears. ### At what serum albumin level does generalised oedema typically appear? Below about 2.5 g/dL, when plasma colloid osmotic pressure falls enough to abolish net absorption at the venular end of capillaries. ### Why is cardiac oedema dependent while renal oedema is periorbital? Raised venous pressure acts on dependent capillaries, whereas hypoalbuminaemia filters at all capillaries and the loose periorbital tissue shows it first, especially after recumbency. ### Why does pulmonary oedema develop at a lower pressure threshold? Pulmonary capillaries normally run at only about 7 mmHg and the interstitial safety margin is smaller, so left atrial pressures above roughly 25 mmHg acutely flood the lungs. ### Which type of cerebral oedema follows head injury, and which follows ischaemia? Vasogenic oedema from capillary permeability in trauma and tumours; cytotoxic oedema from failure of cellular ion pumps in ischaemia.