So someone at school questioned why, if our cells are dividing constantly, we dont split in half/double up etc. And the reason is that cells die. In the average human, 50-70 billion cells die each day. In Apoptosis, one form of programmed cell death, biochemical events can morph cells and cause their death. If the process (apoptosis) is insufficient, it that which causes cancer. Necrosis is the premature death of cells brought about by external factors such as infection. This is not a beneficial process and necrotic tissue often needs to be surgically removed. The third process, autophagy, entails degrading cell components. Anyway, the basic gist of it is, that cells are dying, whilst others are dividing. And these dead cells and cell components become part of the fluid surrounding the living cells. The lymphatic system (running parallel to the circulatory system, carrying a fluid called lymph) collects dead cells (along with urea etc) and removes them as waste. Externally of course, we have dead skin cells which flake off. Same deal.