This chapter discusses applied renal physiology, where normal renal function is so important for homeostasis that situations in which renal function is impaired can be life-threatening. Rapid deterioration of renal function may be due to disorders within or outside the kidneys. The causes of acute renal failure are sometimes divided into pre-renal, renal and post-renal. Important renal causes of acute renal failure are nephrotoxins such as carbon tetrachloride and heavy metals such as mercury. Occasionally renal failure may be precipitated by postrenal causes such as prostatic hypertrophy, urinary calculi, or traumatic rupture of the urinary bladder. The basic defect in acute renal failure is a severe reduction in glomerular filtration rate. Chronic renal failure results from the irreversible loss of a large number of nephrons. A patient with acute renal failure usually develops sodium and water retention, metabolic acidosis and hyperkalaemia.