What is the basic function of red blood cells in animals?

The primary function of red blood cells (RBCs), also known as erythrocytes, is to transport oxygen from the lungs to the tissues of the body and to carry carbon dioxide, a waste product of metabolism, from the tissues back to the lungs for exhalation. This critical role is facilitated by hemoglobin, a protein within the red blood cells that binds to oxygen and carbon dioxide, enabling efficient gas exchange.

Transporting nutrients is typically the role of other components in the blood, such as plasma, which carries various nutrients from the digestive system to cells throughout the body. Body temperature regulation primarily involves mechanisms related to blood flow and the functioning of the skin rather than red blood cells themselves. Lastly, while the immune response and fighting infections are vital functions in the animal body, this task is carried out by white blood cells, not red blood cells. Thus, the correct response highlights the essential role of red blood cells in respiratory gas transport.