What is the primary role of erythrocytes in the blood?

Erythrocytes, commonly known as red blood cells, have a primary function of transporting oxygen from the lungs to various tissues throughout the body. This oxygen transport is facilitated by hemoglobin, a protein within the erythrocytes that binds to oxygen molecules. When blood is oxygen-rich, it takes on a bright red color, while deoxygenated blood appears darker.

This essential role in oxygen delivery is crucial for cellular respiration, where cells use oxygen to produce energy in the form of ATP. The efficient functioning of all bodily systems depends on the availability of oxygen, making the role of erythrocytes vital for sustaining life.

The other functions listed in the options pertain to different types of blood cells or components. For instance, white blood cells are primarily responsible for defending against infections, while platelets (thrombocytes) play a key role in blood clotting. Meanwhile, signaling molecules are typically transmitted by other cells or hormones rather than by erythrocytes. Therefore, option B accurately captures the fundamental purpose of erythrocytes in the bloodstream.