But urea excretion presents a challenge for the kidney by virtue of the osmotic forces that the compound generates. The basic structural and functional unit of the kidney is the nephron.

Urea is the main waste product removed in the urine, as it is not reabsorbed in the kidney. Each kidney contains over one million microscopic filtering units called nephrons. Each nephron is made ...

Each nephron is composed of a highly coiled tubule ... The fluid entering the descending limb contains sodium chloride and other salts, urea and other chemicals that have been filtered out ...

The molecules which are not selectively reabsorbed (the urea, excess water and ions) continue along the nephron tubule to the collecting ducts as urine. This eventually passes down to the bladder.