A 60-year old patient underwent renal aery doppler which shows narrowing and turbulence in right renal aery. If radius of the aery is reduced by 1/3rd, resistance to blood flow in right kidney would have increased by:: (A) 3 times, (B) 9 times, (C) 16 times, (D) 81 times
Answer is D. "Fouh power law" of radius ~ Blood flow varies directly and resistance inversely with the fouh power of the radius (r 4). If the radius is changed by a factor of 3 (reduced by 1/3rd), resistance would change by (34).