That is:

Urinary excretion rate = Filtration rate – Reabsorption rate + Secretion rateAlthough the strictest sense of the word excretion with respect to the urinary system is urination itself, renal clearance is also conventionally called excretion (for example, in the set term fractional excretion of sodium). Filtration
The blood is filtered by nephrons, the functional units of the kidney. Each nephron begins in a renal corpuscle, which is composed of a glomerulus enclosed in a Bowmans capsule. Cells, proteins, and other large molecules are filtered out of the glomerulus by a process of ultrafiltration, leaving an ultrafiltrate that resembles plasma (except that the ultrafiltrate has negligible plasma proteins) to enter Bowmans space. Filtration is driven by Starling forces. The ultrafiltrate is passed through, in turn, the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and a series of collecting ducts to form urine. Reabsorption
Tubular reabsorption is the process by which solutes and water are removed from the tubular fluid and transported into the blood. It is called reabsorption (and not absorption) both because these substances have already been absorbed once (particularly in the intestines) and because the body is reclaiming them from a postglomerular fluid stream that is well on its way to becoming urine (that is, they will soon be lost to the urine unless they are reclaimed).
Urinary bladder disease includes urinary bladder inflammation such as cystitis, bladder rupture and bladder obstruction (tamponade). Cystitis is common, sometimes referred to as urinary tract infection (UTI) caused by bacteria, bladder rupture occurs when the bladder is overfilled and not emptied while bladder tamponade is a result of blood clot formation near the bladder outlet. Cystitis
Cystitis is a urinary bladder inflammation that results from any one of a number of distinct syndromes. It is most commonly caused by a bacterial infection in which case it is referred to as a urinary tract infection. Bladder trauma
Bladder rupture (rupture of bladder, N32.4) may occur if the bladder is overfilled and not emptied. This can occur in the case of binge drinkers who have consumed large quantities of fluids, but are not conscious of the need to urinate due to stupor. This condition is very rare in women, but does occur. Signs and symptoms include localized pain and uraemia (poisoning due to reabsorbed waste). Intra- and extraperitoneal rupture
Bladder rupture is divided into intraperitoneal and extraperitoneal rupture, with the latter encompassing 85% of post-traumatic bladder rupture. Intraperitoneal rupture is both more rare and associated with greater morbidity, requiring surgical repair due to the risk of non-healing and gram negative sepsis. Bladder tamponade
Bladder tamponade is obstruction of the bladder outlet due to heavy blood clot formation within it. It generally requires surgery. Such heavy bleeding is usually due to bladder cancer. See also
Underactive bladder
Overactive bladder

References
External links

http://kidney.niddk.nih.gov/kudiseases/topics/bladder.asp