Extracellular Hemoglobin (Extracellular Hb)

Oxygen transport via extracellular hemoglobin refers to the process by which hemoglobin molecules, located outside of red blood cells (RBCs), bind and deliver oxygen to tissues. While in most vertebrates, hemoglobin is contained within RBCs, some organisms (such as certain annelids like lugworms) possess large, multi-subunit extracellular hemoglobins that circulate freely in the plasma. Additionally, under pathological or experimental conditions in humans and other mammals, free (extracellular) hemoglobin can be present due to RBC lysis or as engineered oxygen carriers. When free/extracellular Hb appears in human plasma due to cell lysis or transfusion products, it can scavenge nitric oxide (NO), disrupting vascular tone regulation and leading to vasoconstriction or hypertension. It undergoes autoxidation more readily than intracellular Hb, generating reactive oxygen species (ROS) such as superoxide and hydrogen peroxide, increasing oxidative stress if not neutralized by antioxidants or scavenger proteins like haptoglobin.

Other names

Free HemoglobinPlasma Hemoglobin