The kidneys depend upon a consistent and reliable blood flow to and through the organ. This flow goes by the name of the Glomerular Filtration Rate (GFR). When it is interrupted, problems arise:
1. Pre-Renal Failure: In this type there is a clinical disease before the kidneys. This often includes heart, liver or circulatory issues. Insufficient waste products are delivered to the kidney producing signs of kidney disease.
2. Renal Failure: This is when the anatomy and or physiology of the kidney is altered. This prevents the normal production of urine leading to signs of kidney failure.
3. Post-Renal Failure: This type of failure is always below the kidneys and is produced by some obstructive disease of the lower urinary tract; either in the ureters, bladder or urethra.
In acute renal failure, damage to the kidneys is often minimal and will improve rapidly with appropriate medical care. Dogs or cats with chronic renal failure are usually suffering from parenchymal damage to the kidney itself. It barely functions. Acute failure dogs will produce very little urine initially. This is a survival response. In chronic failure the animals drinks excessive amounts of water and since kidney function is basically non-existent, produces urine concentration just a step above straight water.