Thrombopoietin physiology

The mechanism by which TPO regulates platelet production from megakaryocytes. TPO (size of large arrows indicates relative concentration) is produced constitutively by the liver and enters the circulation. Left side: When the platelet count is normal, high affinity TPO receptors on the platelets clear most of the TPO and produce a normal plasma TPO concentration, thereby providing basal stimulation of bone marrow megakaryocytes and a normal rate of platelet production. Right side: When platelet production and the platelet count are low, the overall clearance of TPO is reduced, subsequently increasing the plasma TPO concentration and megakaryocyte and platelet production.