Flexi Says: Cells are the smallest units of tissues like blood. Humans have an average total of 25 trillion red blood cells (about 1/3 of all the cells in the body). Cells are made up of complex molecules, which are in turn made up of atoms. Scientists estimate that an average human cell has 100 trillion atoms. Using multiplication, we can estimate that an average human has around 2.5 x 1027 atoms in their red blood cells.