Final answer:
The sympathetic nervous system causes vasoconstriction of the afferent arterioles, leading to reduced glomerular blood flow and GFR, especially under stress, to redirect blood to vital organs and to maintain blood pressure and volume.
Step-by-step explanation:
The sympathetic nervous system causes vasoconstriction of the afferent arterioles, reducing glomerular blood flow and the GFR (glomerular filtration rate). During times of stress or when there is a need to redirect blood to more vital organs, the sympathetic nervous system increases its activity. This leads to the release of norepinephrine, which directly causes the afferent arterioles to constrict, thereby reducing the blood flow through the kidneys and lowering the GFR. Additionally, the adrenal medulla may release epinephrine, contributing to the vasoconstriction and further reducing kidney perfusion. These responses help in regulating blood pressure and redirecting blood to areas in immediate need during stress.
A fall in blood pressure activates sympathetic nerves to release renin, which in turn increases angiotensin II, a potent vasoconstrictor that helps to preserve blood volume by reducing GFR and renal blood flow, when necessary. The autoregulatory mechanisms of arteriole myogenic response and tubuloglomerular feedback also contribute to the fine-tuning of GFR and are closely linked with the autonomic nervous response.