Final answer:
Vasoconstriction of afferent arterioles caused by the sympathetic nervous system or hormones like angiotensin II decreases glomerular filtration rate by reducing blood flow into the kidneys. Autoregulatory mechanisms, however, partially counteract these effects to maintain a relatively constant GFR.
Step-by-step explanation:
Vasoconstriction of afferent arterioles impacts the glomerular filtration rate (GFR) by reducing the blood flow into the glomeruli of the kidneys. The constriction of these vessels is often triggered by signals from the sympathetic nervous system or the release of hormones like norepinephrine, epinephrine, and angiotensin II during stress or changes in blood pressure. Such a constriction leads to a decrease in glomerular hydrostatic pressure and subsequently a reduced GFR. Conversely, factors that induce vasodilation of the afferent arterioles, such as the actions of atrial natriuretic peptide (ANP) or a decrease in sympathetic stimulation, can increase blood flow and thus raise GFR.
The kidneys' ability to maintain a consistent GFR despite fluctuating systemic blood pressures is due to intrinsic autoregulatory mechanisms like the myogenic response and tubuloglomerular feedback. These mechanisms ensure that changes in systemic blood pressure or blood volume do not excessively affect the GFR and consequently the filtration of plasma and solutes into the urine.