As part of the maturation process, human red blood cells destroy their cell nuclei. They do this in order to carry as much oxygen as possible and still stay small enough to fit through narrow blood capillaries, thereby maximizing the oxygen delivery. In fact, humans have some of the smallest red blood cells of all vertebrates, thanks in part to the destruction of the nucleus. Most mammals have red blood cells without nuclei, while all other types of vertebrates do have nuclei in their red blood cells. However, all red blood cells, including human, must start with DNA, as DNA contains the code that tells each cell how to construct itself in the first place. Human red blood cells simply destroy their nucleus once it is no longer needed as part of the maturation process. A ring of actin within a maturing red blood cell pinches and splits the cell into two parts: one part with the DNA and one part without. Red blood cell enucleation is therefore a special type of cell division. Macrophages then come along and gobble up the parts containing DNA, leaving only the red blood cell parts that don't have DNA. Note that there is much more in blood than red blood cells. As a result, a blood sample does contain DNA due to the presence of other kinds of cells.