It was long claimed that the gut lining, unfolded, would cover 200 to 300 square meters. In 2014, researchers measured it more rigorously: about 32 square meters, nearly 30 of them in the small intestine. That's half a badminton court. Less spectacular, but still sixteen times the area of your skin.
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.