Erythrocytes are primarily responsible for oxygen transport. They contain hemoglobin, the iron-containing protein that binds oxygen in the lungs and releases it to tissues. Platelets are involved in hemostasis and clot formation, not oxygen transport. Leukocytes are white blood cells that support immune defense. Plasma proteins perform functions such as maintaining oncotic pressure, transporting substances, and supporting clotting through proteins such as fibrinogen, but they are not the primary oxygen carriers. This foundational knowledge matters because phlebotomy technicians work with specimens used to evaluate red blood cell count, hemoglobin, hematocrit, indices, anemia, blood loss, and hemolysis. If a specimen is hemolyzed, red blood cells rupture and release intracellular contents, which can alter laboratory values. Understanding blood components helps the technician connect collection technique to specimen integrity. For example, rough handling, excessive suction, improper needle size, or vigorous shaking can damage erythrocytes and compromise results. Reference topics: Phlebotomy Fundamentals; blood components; erythrocytes; hemoglobin; CBC testing.