A 30-year-old man presents to his physician for a follow-up appointment because of a blood pressure of 140/90 mm Hg during his last visit. He was advised to record his blood pressure at home with an automated device twice every day. He recorded a wide range of blood pressure values in the past week, ranging from 110/70 mm Hg to 135/84 mm Hg. His medical history is unremarkable and he takes no medications. He occasionally drinks alcohol after work but denies smoking and illicit drug use. What factors are responsible for maintaining a near-normal renal blood flow over a wide range of systemic blood pressures?