We propose to determine the mechanisms that cause vasoconstriction when cell free hemoglobins are introduced into the circulation as the O2 carrier of an O2-carrying plasma expander (OCPE) by studying the vascular effects produced by polyethylene glycol (PEG) surface decorated hemoglobins (HbS) developed in this program. We will study the presser response to top loads of various formulations and quantify the reactions of the microcirculation in the hamster skinfold model, which can be studied without anesthesia, in the awake condition for periods of up to 2 weeks. The principal microvascular parameters to be evaluated in vivo are functional capillary density and intravascular oxygen tension distribution. Tests will analyze different hypothesis on the genesis of vasoactivity due to molecular hemoglobin in the circulation, namely: 1) NO scavenging by hemoglobin; 2) lowered viscosity in hemodilution, leading to lowered shear stress and production of endothelial relaxing factors; 3) Increased facilitated diffusion and O2 autoregulation by oxyhemoglobin; and, 4) Extent of hemoglobin surface shielding by PEG molecules. Efficacy of the OCPEs will be determined in conditions of isovolemic hemodilution and hemorrhagic shock. An effective OCPE must also insure sufficiently elevated blood viscosity, which is necessary for the maintenance of adequate microvascular function, and condition that can be obtained with PEG-Hbs. In hemodilution these molecules increase plasma viscosity, causing redistribution of hydraulic pressure in the circulation, decreasing systemic viscosity dependent pressure losses and increasing peripheral resistance. Additionally the O2 dissociation curve for these modified hemoglobins should be left shifted, so O2 release occurs only in anoxic regions and not from arterioles and where tissue oxygenation is adequate. Our goal is to obtain an understanding of vasoactivity in support of OCPE development that prioritizes maintenance of microvascular function in terms of capillary perfusion, which is as critical for tissue survival as adequate oxygenation, by using methods for the analysis at the cellular microscopic level, where blood performs its vital functions.