The term 'Iron Carrier in Blood' refers to Heme, a four letter word. Heme is an iron-containing compound found in hemoglobin, the protein responsible for transporting oxygen in red blood cells. Heme binds to oxygen molecules and carries them throughout the body, allowing cells to use oxygen for energy production. Heme also plays a role in the regulation of iron levels in the body, as it helps to store and transport iron.