Your urge to breathe is triggered mainly by rising carbon dioxide in the blood, detected by sensors in the brainstem, not by falling oxygen. The burning need to breathe while holding your breath comes from built-up CO₂. It's also why hyperventilating before a dive is dangerous: it lowers CO₂, so the alarm sounds too late.
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.