What are the components of the glomerular filtration barrier?

• A. Fenestrated endothelium, glomerular basement membrane, and podocytes with slit diaphragms.
• B. Endothelial cells only
• C. Glomerular basement membrane only
• D. Podocytes only

Answer: (A)

The glomerular filtration barrier is built from three layers that together determine what is filtered from blood into the urine: the fenestrated endothelium of the glomerular capillaries, the glomerular basement membrane, and the podocytes with their slit diaphragms. The endothelial layer serves as the first hurdle, allowing plasma to pass while preventing blood cells from crossing, with its fenestrations and a surface glycocalyx that contribute to selectivity. The basement membrane provides size and charge selectivity, acting as a dense barrier enriched with negatively charged components like heparan sulfate to restrict passage of large and negatively charged molecules. The final layer consists of podocytes, whose interdigitating foot processes create narrow filtration slits lined by slit diaphragms; proteins such as nephrin and related proteins form this diaphragm to prevent protein leakage while still letting small solutes through. Damaging any part of this tri-layer disrupts filtration, which can lead to proteinuria or impaired renal filtration.