Normal adult kidney size is 9-12 cm in length.

Normal parenchymal thickness is 2-2.5 cm.

Enlarged kidneys with thick parenchyma are usually indicative of an acute disorder, whereas small kidneys with thin parenchyma are often present in chronic renal disease. Causes of ARF are classically placed in one of three categories

Prerenal etiologies account for approximately 70% of all ARF cases and should be diagnosed by nonradiologic methods (↑ blood urea nitrogen [BUN] and creatinine with inherently normal renal function). Hypoperfusion secondary to volume depletion, sepsis, cardiac failure, liver failure, burns, bilateral renal artery stenosis, or pharmacologic agents (such as cyclooxygenase [COX] inhibitors or angiotensin-converting enzyme [ACE] inhibitors) are the most common causes of prerenal failure.

Renal etiologies result from damage to the kidney parenchyma itself. Acute tubular necrosis is the most common type of intrinsic renal disease and is found in cases of prolonged hypotension, secondary to gram-negative sepsis, trauma, hemorrhage, or direct toxins (e.g., mercuric chloride). Other causes include large vessel disease (thrombosis, emboli, dissection), small vessel disease (vasculitis, thrombotic thrombocytopenic purpura [TTP], and disseminated intravascular coagulation [DIC]), interstitial nephritis (urate, myeloma, drugs), and cortical necrosis. Glomerulonephritis is a relatively uncommon intrinsic renal disease, which may occasionally cause ARF but more commonly presents in the context of chronic renal failure (CRF).

Postrenal failure is secondary to obstruction of urine outflow from the kidneys. To cause renal failure, obstruction must be bilateral unless the contralateral kidney is absent or diseased. The most common sources of obstruction include stricture, prostatic hyperplasia, and bladder neck obstruction. Postrenal failure is a relatively uncommon cause of ARF, accounting for less than 5% of cases.