During pregnancy, cells from the fetus cross the placenta and settle in the mother's body. Some survive for a remarkably long time: they've been found 27 years after a birth, and even in the brains of elderly women. This “microchimerism” makes every mother a genetic blend, quite literally.
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.