Your head makes up about 10% of your body's surface area and loses roughly as much heat as any other uncovered patch of the same size. The “80%” idea seems to stem from an old military experiment in which volunteers wore insulated suits… but no hats. Naturally, that's where the heat escaped.
Every day, your kidneys filter around 180 liters of blood plasma, about a bathtub's worth. Yet you only have about 3 liters of plasma, so it passes through your kidneys some sixty times a day. Over 99% of the filtered water is reabsorbed, leaving just one to two liters of urine.
A 70 kg adult is made of about 7 × 10²⁷ atoms, or seven billion billion billion. The observable universe is thought to hold on the order of 10²² to 10²⁴ stars. Even with the high estimate, you contain thousands of times more atoms than there are stars in the entire visible cosmos.