Your heart sits almost in the middle of your chest, behind the breastbone, between your lungs. It's simply tilted, with its tip pointing left, which puts about two-thirds of its mass left of the body's midline. That tip is what you feel beating, hence the impression that it's on the left.
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.