Considering the importance of GFR in urinary output and the renal excretory function, it is no surprise that there are physiological controls that can alter the GFR depending on needs. This control is achieved by changing the diameters of the afferent and/or the efferent arterioles. By decreasing the diameter of the afferent arteriole, the hydrostatic pressure is decreased and thus the GFR is decreased. On the other hand, decreasing the diameter of the efferent arteriole, results in an increase of capillary hydrostatic pressure resulting in an increase of the GFR.

Sympathetic stimulation

Increased sympathetic stimulation results in vasoconstriction of the afferent arteriole. Sympathetic stimulation occurs during the fight or fight mechanism and/or in cases of shock (severe hemorrhage or cardiac shock). In all these cases the increased need to shunt blood towards the central portion of the body (heart, to keep the B.P. and brain, to remain conscious) is assisted by the decrease in GFR and the resultant decrease in urine formation.

Renal Autoregulation

In an isolated kidney (a kidney without its nerve supply), the pressure within the glomerular capillaries is kept constant despite large changes in the cardiac blood pressure (within the limits 90mmHg to 180mmHg systolic pressure). Thus the pressure within the capillaries is kept at around 70-80mmHg. This occurs through autoregulatory mechanism.

These autoregulatory mechanisms involve the dilation of the afferent arteriole by the action of released local hormones when the central blood pressure falls.

Autoregulation is also achieved by a direct negative feedback relationship between the filtrate volume and afferent arteriole. An increased flow of filtrate is sensed by a group of special cells called the macula densa in the region of the distal tubule that is nearest to glomerulus. The macula densa is part of the juxtaglomerular apparatus. When the macula densa senses an increase in filtrate flow, it sends signals to the afferent arteriole to constrict and thus decrease the GFR.