What is the primary function of red blood cells?

The primary function of red blood cells is to transport oxygen from the lungs to the tissues throughout the body. This process occurs due to the presence of hemoglobin, a protein within red blood cells that binds to oxygen molecules. When blood passes through the lungs, oxygen molecules diffuse across the alveolar membrane into the red blood cells, where they are then bound by hemoglobin.

Once the red blood cells circulate through the body, they release this oxygen to the tissues, which is essential for cellular metabolism and energy production. The efficient delivery of oxygen is crucial for maintaining overall metabolic activity in the body's organs and muscles during various physical activities.

While red blood cells do play a role in transporting some carbon dioxide back to the lungs for exhalation, their primary role is unequivocally the transport of oxygen. Other options such as protecting against infection and regulating body temperature are functions associated more with other components of the blood, such as white blood cells and plasma, respectively. These functions highlight the specialized roles within the circulatory system, defining the unique importance of red blood cells in oxygen transport.