What is the primary function of red blood cells?

The primary function of red blood cells is to transport oxygen from the lungs to various tissues and organs throughout the body. This is accomplished through the presence of hemoglobin, a protein that binds to oxygen molecules. As red blood cells travel through the bloodstream, they pick up oxygen in the lungs, where oxygen concentration is high, and release it in tissues where the concentration is lower, thereby facilitating cellular respiration and energy production.

The other functions listed, such as fighting infections or transporting nutrients, are typically associated with different types of cells or components in the blood. For example, white blood cells are primarily responsible for fighting infections, while plasma proteins and other components are involved in nutrient transport. Additionally, regulating body temperature is generally a function of the body's overall physiology, involving factors like blood flow and metabolism, rather than a specific role of red blood cells themselves.