Arteriole leading into the glomerular capillary. It is the location of the juxtaglomerular cells as well as intrarenal baroreceptors. At first glance, it is merely a blood vessel, but with the help of the aforementioned JG cells and the macula densa, as well as the sympathetic nervous system, it regulates the pressure and therefore the flow of blood entering the glomerular capillary. This regulation is done mostly via the renin-angiotensin system, where vasoconstriction is caused by angiotensin II.

Through these mechanisms, the glomerular filtration rate is regulated in such a way that it does not change substantially unless the systemic arterial pressure is less than 80 mmHg or greater than 180 mmHg.

These are my interpretation of my lecture notes, but I may have used some references from Hole's Anatomy and Physiology (Shier, Butler, Lewis) and Human Physiology (Vander, Sherman, Luciano)