Artificial blood for clinical use is not yet available therefore, we previously developed artificial oxygen carriers (capsules) and showed their functionality in vitro and biocompatibility in vivo. Herein, we assessed the functionality of the capsules in vivo in a normovolemic hemodilution rat-model. We stepwise exchanged the blood of male Wistar-rats with medium either in the presence of capsules (treatment) or in their absence (control). We investigated tissue hypoxia thoroughly through online biomonitoring, determination of enzyme activity and pancreatic hormones in plasma, histochemical and immunohistochemical staining of small intestine, heart, liver and spleen as well as in situ hybridization of kidneys. After hemodilution, treated animals show higher arterial blood pressure and have a stable body temperature. Additionally, they show a more stable pH, a higher oxygen partial pressure (pO 2), and a lower carbon dioxide partial pressure (pCO 2). Interestingly, blood-glucose-levels drop severely in treated animals, presumably due to glucose consumption. Creatine kinase values in these animals are increased and isoenzyme analysis indicates the spleen as origin. Moreover, the small intestine of treated animals show reduced hypoxic injury compared to controls and the kidneys have reduced expression of the hypoxia-inducible erythropoietin mRNA. In conclusion, our capsules can prevent hypoxic tissue damage. The results provide a proof of concept for capsules as adequate erythrocyte substitute. The need to develop synthetic products for blood replacement has become more and more important during the last decades. On the one hand, the demographic change leads to increasing requirements for blood, and on the other hand, simultaneously the willingness for blood donation declines 1. Blood with its various characteristics fulfils many functions, recent developments of synthetic blood substitutes focus on its most important function: the transport of physiological gases, in particular of oxygen. Additional key requirements for artificial blood substitutes are optimal size, sterility, biocompatibility, and the ability to be stored without loss of functionality. Moreover, artificial oxygen carriers (AOCs) should have a long intravascular circulation time and universal usage independent of the blood group characteristics 2,3. There are two main approaches to realize the transport of essential gases. The first is the use of the physiological oxygen carrier hemoglobin (hemoglobin-based artificial oxygen carriers, HBOCs); a concept realized e.g. in the prominent product hemopure, that gained pharmaceutical approval in South Africa and Mexico. Further HBOCs (e.g. sanguinate) are currently tested in clinical trials 4. So far, most of the products showed severe side effects so that the manufacturing had to be stopped 5. The second is the work with synthetic materials like perfluorocarbons [perfluorocarbon-based artificial oxygen carriers (PFOCs)]. Depending on the actual partial pressure, PFOCs are able to solve gases physically 6...