blood cells

Meaning: Blood cells arise from hematopoietic stem cells in red bone marrow. Red blood cells (erythrocytes) are biconcave discs filled with hemoglobin, a shape that gives high surface area for gas exchange. Mature mammalian RBCs have no nucleus or mitochondria and survive about 120 days before the spleen and liver remove them. White blood cells (leukocytes) include neutrophils, eosinophils, basophils, monocytes (which become macrophages) and lymphocytes (B and T cells). Platelets are fragments of megakaryocytes and take part in clotting.

On the MCAT: Passages ask which cell type fits a function or finding. Low blood O₂, as at high altitude, raises erythropoietin release from the kidney, which increases RBC production. Infection typically raises the white cell count. Anemia means reduced oxygen-carrying capacity. Hematocrit is the fraction of blood volume made up of red cells. B cells mature in bone marrow and T cells in the thymus.

Common confusion: Because RBCs lack mitochondria, they make ATP by glycolysis and lactate fermentation, so they do not consume the O₂ they carry. Platelets are not whole cells, since they are anucleate fragments. Plasma is the fluid portion of blood, not a cell type.