Dense (aged) circulating red cells contain normal concentrations of adenosine triphosphate (ATP).

The densest 0.1%--1% of circulating red cells were separated from fresh blood, and the ATP content of a representative sample of such cells was determined. The dense ("old") cells had decreased amounts of ATP relative to unfractionated cells. However, the dense cells were also smaller, and the concentration of ATP in these cells was the same as in controls. Therefore, it seems unlikely that loss of cellular ATP is a causative factor in removal of senescent red cells from the circulation.