What is the primary function of red blood cells?

The primary function of red blood cells is the transportation of gases, specifically oxygen and carbon dioxide. Red blood cells contain hemoglobin, a protein that binds to oxygen in the lungs and carries it to the tissues throughout the body. When they pass through the tissues, hemoglobin releases the oxygen, which is then utilized by cells for metabolic processes. After delivering oxygen, red blood cells pick up carbon dioxide, a waste product of metabolism, from the tissues and transport it back to the lungs for exhalation. This gas exchange is vital for maintaining the body's homeostasis and ensuring that cells receive the oxygen they need to function efficiently.

While other options mention important functions like immune defense, nutrient transport, and temperature regulation, these functions are carried out by other components of blood, such as white blood cells for defense, plasma for nutrients, and various mechanisms for temperature regulation. In contrast, the unique role of red blood cells is specifically focused on gas transport, making this the correct answer.