If you could look inside yourself with magical X-ray vision, you'd see some blood and a lot of squishy, shiny organs. If you have situs inversus, or reversal of organs, you'd see the same thing -- only your organs might be in the exact opposite place they're supposed to be.
So what happens when your insides are all switched around? Sometimes, nothing -- you might not even know about it until you get examined for something completely unrelated. But other times, the condition can lead to serious side effects. When the heart is on the right side of the body instead of the left, the great arteries can also be transposed. This means that the strongest part of the heart is pumping to the lungs instead of the body, leaving the weaker ventricle to take care of the body's needs.
People who have all of their organs flipped can suffer from congenital heart disease, male sterility and lung abnormalities. Another kind of organ reversal results in either no spleen or a spleen on both sides of the body -- neither of which is a good thing.