By measuring the carbon-14 that nuclear tests of the 1950s and 1960s left in DNA, researchers dated heart muscle cells. The verdict: about 1% are replaced each year at age 25, under 0.5% by 75. Over a lifetime, fewer than half are ever swapped out, so most of your heart is the original.
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.