Kidney failure has many possible causes. Some lead to a rapid decline in kidney function (acute kidney injury, also called acute renal failure). Others lead to a gradual decline in kidney function (chronic kidney disease, also called chronic renal failure). In addition to the kidneys being unable to filter metabolic waste products (such as creatinine and urea nitrogen) from the blood, the kidneys are less able to control the amount and distribution of water in the body (fluid balance) and the levels of electrolytes (sodium, potassium, calcium, phosphate) and acid in the blood.
When kidney failure has lasted for some time, blood pressure often rises. The kidneys lose their ability to produce sufficient amounts of a hormone ( erythropoietin) that stimulates the formation of new red blood cells, resulting in a low red blood cell count (anemia). The kidneys also lose their ability to produce sufficient calcitriol (the active form of vitamin D), which is vital to bone health. In children, kidney failure affects the growth of bones. In both children and adults, kidney failure can lead to weaker, abnormal bones.
Although kidney function can decline in people of all ages, both acute kidney injury and chronic kidney disease are more common in older than in younger people. Many disorders that cause a decline in kidney function can be treated, and kidney function may recover. The availability of dialysis and kidney transplantation has transformed kidney failure from a fatal disease to one that is manageable.