If the diameter of the afferent arterioles leading to the glomerulus decreases (vasoconstriction) which of the following is not likely to occur?

A Net filtration pressure will decrease.

B Urine output will decrease.

C Systemic blood pressure will decrease.

Answer: C. Systemic blood pressure will decrease.

Explanation:

Arteriole Myogenic Mechanism

The myogenic mechanism (initiatiated by the muscle cell instead of an external stimulus) is one of two internal auto-regulatory mechanisms that operate in regulating blood flow within the kidney. It works based on a characteristic shared by most smooth muscle cells of the body. When stretched, a smooth muscle cell contracts; when the tension is removed, it relaxes, restoring its resting length.

This mechanism works in the afferent arteriole that supplies the glomerulus and can regulate the blood flow into the glomerulus. When systemic blood pressure increases, smooth muscle cells in the wall of the arteriole are stretched and respond by constricting (become smaller in diameter) to resist the pressure, resulting in little change in flow. The vasoconstriction of the afferent arteriole acts to reduce excess filtrate formation, maintaining normal Net Filtration Pressure (NFP) and Glomerular Filtration Rate (GFR).

Therefore, vasoconstriction of afferent arterioles are as a result of an increase and not a decrease in systemic blood pressure.