What is the primary function of red blood cells?

The primary function of red blood cells, or erythrocytes, is to transport oxygen from the lungs to the tissues and organs throughout the body, and to carry carbon dioxide back from the tissues to the lungs for exhalation. Hemoglobin, the protein within red blood cells, binds to oxygen molecules and facilitates this critical exchange. This role is essential for cellular respiration, which provides the energy needed for cells to function effectively.

While other options like transporting nutrients and hormones are important functions of other components in the blood—such as plasma and certain specialized cells—these are not the primary roles of red blood cells. Additionally, regulating blood pressure is mainly controlled by the circulatory system and specific hormones, but is not a direct function of red blood cells themselves. Thus, focusing on the unique and vital role of red blood cells in gas exchange clarifies why transporting oxygen and carbon dioxide is their primary function.