25,000,000 of your cells died while you read this sentence. It's okay though, your body made more than 300 billion new ones today. The cells that make up your body are dying and being replaced all the time. This happens so often that about every 7 years you have a completely new body of cells. While this is true for most cells, such as the cells lining the stomach, some of the most important cells in your body (those of your bones, your brain, and your heart) don't regenerate much at all, so treat them with care.