aids in lubrication
To reduce friction between the bones. It also lubricates the joints and allows smooth movements.
it depends on which body fluid you are refering to can your rephrase your question to be more specific?
The kidney is the organ that actually sends extra fluids and toxins to the bladder, however there are many other organs that serve in the process such as the liver, the pancreas, and the spleen.
Mainly water.