Renal physiology: blood flow, glomerular filtration, and plasma clearance

The homeostatic and excretory functions of the kidney are dependent on its perfusion, totalling 20–25% of cardiac output, and the process of glomerular ultrafiltration. Renal blood flow (RBF) is directly proportional to the trans-renal gradient which is autoregulated across a mean arterial pressure of 50–150 mmHg in a normotensive person. Selective molecular filtration in the glomerulus is achieved by the glomerular filtration barrier and is related to the size, shape, and electrical charge of molecules. The process of ultrafiltration is determined by the balance between hydrostatic and colloid osmotic pressures in the glomerular capillary and Bowman's space, and is affected by renal plasma flow, altered surface area and changes in afferent and efferent renal arteriole vascular resistance. The phenomenon of renal plasma flow autoregulation minimizes changes in the volume of ultrafiltration through myogenic and tubuloglomerular feedback mechanisms. Glomerular filtration rate can be measured using exogenous inulin, or estimated (eGFR) from creatinine clearance, several equations can be used to calculate eGFR but their limitations in estimating the true excretory function of the kidney need to be taken into consideration.