Without gravity's load, the skeleton is underused and breaks down more bone than it builds. Astronauts lose on average 1 to 1.5% of the bone mass in their hips and spine for every month in orbit. Hence their daily hours of exercise on board. Bones need to bear weight to stay strong.
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.