ABSTRACT
PHYSIOLOGY OF RENAL FUNCTION The kidney regulates fluid and electrolytes by filtration, secretion and re-absorption. Renal blood flow is around 20% of cardiac output (1000 ml/min in an adult) and renal plasma flow (RPF) is approximately 600 ml/min. The glomerulus filters 125 ml/min of renal plasma. This glomerular filtration rate (GFR) is a much more reliable marker of renal function than plasma creatinine. Most of this fluid is re-absorbed, with only 1% passed as urine (0.5-1 ml/kg/h). The kidney autoregulates its plasma flow over a wide range of mean arterial pressure (MAP). However at low MAP, RPF and GFR become supply dependent and urine output decreases – this is the kidney protecting itself from further reduction in perfusion pressure (Fig. 9.1). This is reversible in the short term but the kidney is much more vulnerable to other insults, particularly the tubular cells deep in the medulla, which is more poorly perfused than the cortex.
