The round figure of 100 billion circulated for decades without a real count behind it. In 2009, researchers turned brains into a uniform “soup” and counted the cell nuclei: about 86 billion neurons. The missing 14 billion? That's more than twice the neurons in an entire rhesus monkey brain.
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.