9.4 Blood

The four components of blood — red blood cells (haemoglobin transports O₂), white blood cells (defence: phagocytosis + antibody production), platelets (clotting), plasma (transport of cells, ions, glucose, urea, hormones, CO₂). Two roles of clotting: prevent blood loss + prevent entry of pathogens. HL extends to distinguishing phagocytes (multi-lobed nucleus, engulf) from lymphocytes (single round nucleus, antibodies), and the clotting reaction: soluble fibrinogen → insoluble fibrin mesh.