Body & HealthFact
Your kidneys filter 180 litres a day but keep almost all of it.
About 20 percent of your cardiac output flows through the kidneys. Their roughly 2 million nephrons press out around 180 litres of filtrate a day, nearly sixty times your blood plasma. The tubules then reclaim over 99 percent of it, including water, salts and sugar. What remains is about 1 to 2 litres of urine.
Useful: So urine isn't what's left over, it's what the body has deliberately chosen to discard.
Source: en.wikipedia.org