Kidney failure means loss of some (but not all) of the filtration capacity of the kidneys, which can be caused by a fall in blood pressure, a blockage of the blood circulation to the kidneys, blockage of urine outflow, or by disease of the kidneys themselves. Many different kinds of kidney disease are recognized, all of which cause loss of filtration capacity, but some of which are rapidly reversible. These reversible types of kidney failure are known as acute kidney failure.
Acute kidney failure can be caused by drugs toxic to the kidneys, by a severe reduction in kidney blood flow (for example, during surgery), and by many other causes. Urine output usually falls drastically, and waste products accumulate in the blood. But amazingly, complete recovery can occur within a few weeks. Patients often need dialysis temporarily.
Chronic kidney failure is generally not reversible, but often (though not always) gets progressively worse. When about two-thirds of filtration capacity is lost, symptoms of kidney failure begin to appear (see Chapter 3). When seven-eighths or so is lost, survival depends on either starting dialysis or transplanting a new kidney. This is called end-stage renal disease (ESRD).
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