Glomerular Filtration Barrier
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Direct answer
Three layers stand between blood and Bowman's space: fenestrated glomerular endothelium (70-100 nm pores carrying a negatively charged glycocalyx), the glomerular basement membrane of type IV collagen, laminin and heparan sulphate proteoglycans, and podocyte foot processes joined by the slit diaphragm built of nephrin and podocin. Filtration is size-selective (molecules under about 7000 Da or 1.8 nm effective radius pass freely; above roughly 70,000 Da or 3.6 nm almost nothing does) and charge-selective (anionic albumin, 69 kDa, is largely repelled by the negative charges). The result: 180 litres of protein-free ultrafiltrate daily, with normal albumin loss under 30 mg — achieved across a net filtration pressure of only 10 mmHg (60 − 18 − 32) through a filtration coefficient of about 12.5 mL/min/mmHg, yielding the 125 mL/min GFR.
What you must remember
- Layer one — endothelium: 70-100 nm fenestrae block cells but not proteins; the glycocalyx adds the first negative charge.
- Layer two — GBM: type IV collagen network (with laminin, nidogen and the heparan sulphate proteoglycans agrin and perlecan) — the mechanical scaffold and a major charge filter.
- Layer three — podocytes: foot processes with slit diaphragms of nephrin (the NPHS1 protein), podocin (NPHS2) and CD2AP — effacement of foot processes is the lesion of minimal change disease; the filtration slit is about 25-40 nm wide.
- Size cut-offs: free filtration under 7000 Da, complete exclusion above 70,000 Da effective molecular radius beyond about 3.6 nm; albumin at 69 kDa sits at the edge and is excluded chiefly by charge.
- Charge selectivity: anionic molecules filter least, neutral intermediately, cationic most — loss of the negative charge alone (early diabetic nephropathy) spills albumin before any structural breach.
- Starling arithmetic: glomerular capillary hydrostatic 60, Bowman's hydrostatic 18, plasma oncotic 32 — net 10 mmHg, times Kf 12.5 mL/min/mmHg equals 125 mL/min; the glomerular capillary is some 400 times more water-permeable than a systemic capillary.
- Mesangial cells: contractile (angiotensin II squeezes them, lowering Kf), phagocytic, and the seat of IgA deposition.
- Normal protein limits: total protein under 150 mg/day, albumin under 30 mg; microalbuminuria (30-300 mg/day) is the earliest diabetic nephropathy marker, screened annually.
- Disease map to layers: minimal change — podocyte effacement with selective albuminuria, steroid-responsive; membranous — anti-PLA2R subepithelial deposits; Alport — type IV collagen alpha-3/4/5 mutation (X-linked in 85 per cent) with deafness and lenticonus; thin basement membrane disease — benign familial haematuria.
A typical case: periorbital oedema in a child
A four-year-old develops periorbital puffiness over a week; urine shows 4+ proteinuria in the nephrotic range, but blood pressure, complement and creatinine are normal without haematuria. Light microscopy of the kidney is normal; electron microscopy shows podocyte foot process effacement. Read it through the barrier: the filter's structure is intact, but the podocyte has lost its slit-diaphragm architecture — believed to be caused by a circulating permeability factor (possibly related to anti-nephrin antibody activity) — and, critically, the leak is selective: albumin escapes while larger proteins like IgG do not, proving the size barrier's coarse architecture still stands with the fine charge-and-slit layer gone. Steroids close the leak in over 90 per cent of children. Contrast the diabetic adult whose microalbuminuria precedes any fall in GFR: there the negative charge is stripped first, which is why annual albumin screening catches nephropathy before creatinine moves.
How the exam frames it
Two conceptual questions carry the marks. Why does normal urine contain almost no albumin — because at 69 kDa, 3.6 nm and strongly anionic, albumin fails all three layers at once: too large for the slit, sieved by the GBM, repelled by charge; when charge alone is lost, albuminuria appears with normal architecture on light microscopy. And what angiotensin II does to Kf — mesangial contraction reduces the filtering surface, one reason ACE inhibitors preserve GFR in the long term by relaxing it.
Frequently asked questions
Name the three layers of the glomerular filtration barrier.
Fenestrated glomerular endothelium with its glycocalyx, the glomerular basement membrane (type IV collagen, laminin, heparan sulphate proteoglycans), and podocyte foot processes with the nephrin-podocin slit diaphragm.
Why is albumin normally excluded from the filtrate?
At 69 kDa and 3.6 nm effective radius it is near the size limit and strongly anionic, so GBM negative charges and the slit diaphragm exclude it together — charge loss alone allows albuminuria.
Which proteins form the slit diaphragm, and what disease follows their loss?
Nephrin (NPHS1) and podocin (NPHS2) with CD2AP; nephrin deficiency causes congenital nephrotic syndrome of the Finnish type, and foot-process effacement underlies minimal change disease.
Calculate net filtration pressure and GFR from the standard Starling forces.
Net pressure = 60 (capillary hydrostatic) − 18 (Bowman's hydrostatic) − 32 (plasma oncotic) = 10 mmHg; GFR = net pressure × Kf = 10 × 12.5 = 125 mL/min.
What is microalbuminuria and why is it screened in diabetics?
Urinary albumin of 30-300 mg/day — the earliest sign of charge-selectivity loss in diabetic nephropathy, appearing years before GFR falls when ACE-inhibitor or ARB therapy is most protective.