The air around you is about 21% oxygen. The air you exhale still contains roughly 16%: your lungs only take a small share with each breath. That's exactly why mouth-to-mouth resuscitation works, since a rescuer's exhaled breath carries enough oxygen to supply another person's blood.
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.