Final answer:
When afferent arteriole blood pressure increases, the GFR remains stable due to the contraction of smooth muscle cells in the arteriole wall. Conversely, if blood pressure decreases, the smooth muscle cells relax, maintaining a constant GFR.
Step-by-step explanation:
When the afferent arteriole blood pressure increases, the smooth muscle cells in the wall of the arteriole respond by contracting to resist the pressure, resulting in little change in glomerular filtration rate (GFR). This is due to the myogenic mechanism, where stretching of smooth muscle cells causes contraction. However, if blood pressure decreases, the same smooth muscle cells relax to lower resistance, allowing a continued even flow of blood and maintaining a stable GFR.