What is the primary function of red blood cells?

The primary function of red blood cells, also known as erythrocytes, is to transport oxygen from the lungs to the tissues throughout the body. This process is facilitated by hemoglobin, a protein within red blood cells that binds to oxygen molecules. When oxygen is inhaled into the lungs, it diffuses into the bloodstream and attaches to hemoglobin in red blood cells. Once these oxygen-rich red blood cells circulate to the body's tissues, they release the oxygen to be used for cellular metabolism.

In addition to their role in oxygen transport, red blood cells also play a role in transporting a smaller amount of carbon dioxide from the body's tissues back to the lungs. However, their primary and most crucial function remains to carry oxygen, making this choice the most accurate in the context of the question.