Which protein primarily delivers iron throughout the bloodstream?

Transferrin is the primary protein responsible for delivering iron throughout the bloodstream. It binds to iron ions and transports them to various tissues, including the bone marrow, liver, and muscles, where iron is utilized for various physiological processes, such as hemoglobin synthesis for red blood cells. Transferrin plays a crucial role in iron homeostasis, ensuring that iron is delivered in a safe and controlled manner, thereby preventing free iron, which can catalyze harmful reactions through the Fenton reaction.

Hemosiderin is an intracellular storage form of iron, primarily found in macrophages and hepatocytes, and is not involved in iron transport in the bloodstream. Ferritin is another protein that stores iron but does not transport it through the bloodstream; instead, it sequesters iron, making it available for use while preventing excess free iron accumulation. Ferrous iron refers to the reduced form of iron (Fe²⁺) rather than a transport protein, and it does not participate directly in the transport process in the bloodstream.

Hence, the accurate selection is transferrin due to its specific role in circulating and delivering iron effectively throughout the body.