In humans and other mammals red blood cells lack a nucleus and most of the cell organelles, this provides maximum space to carry hemoglobin and RBC thus can transport oxygen gas with full capacity.
∙ The lack of a nucleus also gives the RBCs their characteristic biconcave shape which increases the cell’s surface area there by allowing more hemoglobin to be stored in the cell.
∙ The larger surface area of RBCs helps in better exchange of oxygen and carbon dioxide.