The primary function of red blood cells is to:

Red blood cells (RBCs), also known as erythrocytes, are specialized cells in the blood responsible for carrying oxygen from the lungs to the body's tissues and returning carbon dioxide from the tissues to the lungs. RBCs contain hemoglobin, a protein that binds to oxygen molecules. Hemoglobin is composed of four subunits, each containing an iron ion that can bind one molecule of oxygen. This binding and release of oxygen are critical for cellular respiration, which provides energy for the body's metabolic processes. The unique biconcave shape of RBCs increases their surface area for efficient gas exchange and allows them to navigate through the narrow capillaries.