What is the primary function of red blood cells?

Red blood cells (RBCs), or erythrocytes, serve the primary function of transporting oxygen from the lungs to the tissues and organs throughout the body. They contain hemoglobin, a specialized protein that binds to oxygen molecules, allowing RBCs to carry oxygen efficiently. In addition to oxygen transport, they also collect carbon dioxide—a waste product of metabolism—and transport it back to the lungs for exhalation.

This dual role of facilitating the exchange of gases is crucial for maintaining cellular respiration and overall metabolic function. Without adequate red blood cells and their ability to transport these gases, the body's tissues would lack the necessary oxygen and build up carbon dioxide, leading to serious physiological imbalances.

The other options provide roles associated with different components of the blood or other cells. Nutrient transport is primarily the function of plasma and various types of blood cells, not solely RBCs. The immune response is mainly facilitated by white blood cells, which fight infections and manage inflammatory responses. Blood clotting, or hemostasis, involves platelets and clotting factors, not red blood cells specifically. Therefore, the primary function of red blood cells distinctly centers around the transport of oxygen and carbon dioxide.