Final answer:
Mature red blood cells are filled with hemoglobin molecules, which allow for the transport of oxygen and carbon dioxide throughout the body.
Step-by-step explanation:
Mature red blood cells in the circulating blood are filled with hemoglobin molecules. This iron-containing protein is crucial for the transport of oxygen and carbon dioxide. As erythrocytes (another name for red blood cells) mature, they extrude their nucleus and most organelles such as mitochondria and smooth endoplasmic reticulum, which maximizes the space available for hemoglobin. Lacking a nucleus and other organelles, they are adapted for their primary function of oxygen transport, and they rely on anaerobic respiration since they lack mitochondria. This adaptation ensures that none of the oxygen they carry is used for metabolic processes within the red blood cells themselves.