Pathophysiology of extracellular haemoglobin: use of animal models to translate molecular mechanisms into clinical significance

The blood's major gas exchange is carried out by haemoglobin, a haeme protein that binds iron and oxygen and can have potentially dangerous side-effects due to redox reactions. Haemoglobin is a very abundant molecule with a concentration of 150 g/l in whole blood, resulting in almost one kg haemoglobin in an adult human body. Normal turnover of red blood cells results in significant haemoglobin release, and pathological conditions that involve haemolysis can lead to massive haemoglobin levels. To control for the potential threat of extracellular haemoglobin, several protective defence systems have evolved. Many pathological conditions, diseases as well as iatrogenic conditions, such as infusion of haemoglobin-based oxygen carriers, cerebral intraventricular haemorrhage, extracorporeal circulation and the pregnancy complication pre-eclampsia, involve abnormal levels of haemolysis and extracellular haemoglobin. Although quite different aetiology, the haemoglobin-induced damage often causes similar clinical sequelae and symptoms. Here, we will give an overview of the pathophysiological mechanisms of extracellular haemoglobin and its metabolites. Furthermore, we will highlight the use of animal models in advancing the understanding of these mechanisms and discuss how to utilize the knowledge in the development of new and better pharmaceutical therapies.