Urine is the byproduct or fluid secreted by the kidneys, transported by
the ureters to the urinary bladder where it is stored until it is
voided through the urethra. It is a transparent solution that is clear
to amber in colour, and usually is light yellow. Urine is made up of a
watery solution of metabolic wastes (such as urea), dissolved salts and
organic materials. Fluid and materials being filtered by the kidneys,
destined to become urine, comes from the blood or interstitial fluid.
The composition of urine is adjusted in the process of reabsorption
when essential molecules needed by the body, such as glucose, are
reabsorbed back into the blood stream via carrier molecules. The
remaining fluid contains high concentrations of urea and other excess
or potentially toxic substances that will be released from the body via
urination. Urine flows through these structures: the kidney, ureter,
bladder, and finally the urethra. Urine is produced by a process of
filtration, reabsorption, and tubular secretion.

Urine contains large amounts of urea, an excellent source of nitrogen
for plants. As such it is a useful accelerator for compost. Urea is
10,000 times less toxic than ammonia and is a byproduct of deamination
(2 NH3 molecules) and cellular respiration's (1 CO2 molecule) products
combining together. Other components include various inorganic salts
such as sodium chloride (the discharge of sodium through urine is known
as "natriuresis".)