Renal blood flow, glomerular filtration and plasma clearance

Homeostatic and excretory functions of the kidney depend on blood flow (∼25% cardiac output) and glomerular ultrafiltration (∼20% renal plasma flow). Blood flow distribution is not uniform, with only 10% reaching the medulla. Selectivity of ultrafiltration is related to molecular size, shape and electrostatic charge of molecules, and structure of the glomerular capillary barrier with its negatively charged glycoproteins. Ultrafiltration, determined by the balance between hydrostatic and colloid osmotic pressures (Starling forces) in the glomerular capillary and Bowman’s space, occurs along the length of the capillary: hydrostatic pressure is relatively unchanged and always exceeds plasma colloid osmotic pressure plus pressure in Bowman’s space. Ultrafiltration is influenced by changes in renal plasma flow, altered surface area (mesangial cell activity) and changes in vascular resistance afforded by afferent and efferent arterioles (mediated by sympathetic nerve activity, vasoconstrictors and vasodilators). Autoregulation of renal plasma flow minimises changes in ultrafiltration (hence, filtered load). Myogenic and tubuloglomerular feedback mechanisms are responsible for autoregulation but their relative contribution is yet to be resolved. Clearance measurements are used to assess renal plasma flow (PAH), glomerular filtration rate (creatinine, inulin), filtration fraction (GFR/RPF), net reabsorption or secretion, and proximal and distal nephron function (lithium, free-water) but all have their limitations.