Post-renal obstruction affects kidney function by:

• A. Obstruction raises intratubular pressure and lowers GFR
• B. Obstruction lowers intratubular pressure and raises GFR
• C. Obstruction has no effect on GFR
• D. Obstruction improves hydronephrosis

Answer: (A)

When urine flow is blocked after the kidney, backpressure builds up inside the nephron. This raises the pressure in the tubular system and especially in Bowman's space. Since glomerular filtration depends on the pressure difference across the glomerular filter, this increased intratubular (Bowman's space) pressure reduces the net filtration pressure, so the glomerular filtration rate falls.

In other words, obstruction creates backpressure that makes it harder for the kidney to form filtrate, lowering GFR. If the obstruction is relieved early, function can recover; if it’s prolonged, the backpressure can cause damage to tubules and sustained loss of function. Hydronephrosis reflects dilation from this backpressure, not an improvement from obstruction.