red blood cells

Meaning: Red blood cells (erythrocytes) carry O₂ bound to hemoglobin, a protein with four subunits, each with a heme group holding an iron ion. Mature mammalian RBCs lack a nucleus and mitochondria, so they make ATP by glycolysis and do not consume the O₂ they carry. The biconcave disc gives a high surface-area-to-volume ratio for gas exchange and flexibility in narrow capillaries. Carbonic anhydrase in them helps carry much of the CO₂ as bicarbonate. They form in red bone marrow and are removed in the spleen and liver after about 120 days.

On the MCAT: Low blood O₂, as at high altitude, raises erythropoietin (from the kidney), increasing RBC production and hematocrit. Other items use anemia, sickle cell disease (a hemoglobin mutation), or the O₂-hemoglobin curve, which shifts right with higher CO₂, lower pH or higher temperature, so hemoglobin releases more O₂ to active tissue. No nucleus predicts no new protein synthesis; no mitochondria predicts lactate production.

Common confusion: Students mix RBCs up with white blood cells (immune cells, nucleated) and platelets (cell fragments for clotting). Another slip is giving RBCs aerobic respiration; without mitochondria they rely on glycolysis. This holds for mature mammalian RBCs; those of birds and fish keep a nucleus.