Regulation of Renal Hemodynamics

B eginning the renal physiology section with a detailed consideration of renal microvascular dynamics allows a smooth transition from cardiovascular-related topics to kidney-related topics. In addition, reductions in renal blood flow (RBF) and glomerular filtration rate (GFR) are often associated with many pathophysiological conditions including hypertension, renal failure, heart failure, and diabetes, and it is essential for the student to obtain a solid foundation regarding the principal mechanisms regulating renal hemodynamics. The following learning objectives provide a clear guide for the student. 1) Be able to define and calculate the renal fraction of the cardiac output and the factors that influence it. 2) Know the average values for RBF and GFR in adult humans. 3) Be able to define and calculate the filtration fraction. 4) Know the major sites of renal vascular resistance and describe the hydrostatic pressure profile along the renal vasculature. 5) Describe the roles of hydrostatic and colloid osmotic pressures in regulating glomerular and peritubular capillary dynamics. 6) Explain in a quantitative manner the determinants of GFR and how these are regulated. 7) Identify the extrinsic, neural, and hormonal systems that regulate renal hemodynamics. 8) Describe the roles of the major paracrine systems that participate in the intrinsic control of renal vascular resistance. 9) Define the phenomenon of renal autoregulation and describe the myogenic mechanism and the tubuloglomerular feedback mechanism in mediating autoregulatory behavior. It is not possible to discuss all these learning objectives in the present article. Consequently, attention has been focused on those aspects that are conceptually more difficult or that cause confusion among the students. In particular, the recent recognition of multiple humoral and paracrine factors regulating renal microvascular dynamics requires a very discriminating selection of specific topics.