Abstract: The present invention provides superoxygenating compositions that comprise hemoglobin and guanosine 3′:5′-cyclic monophosphate (cyclic GMP) generating compounds, and methods for treatment of diseases or medical conditions which utilize the superoxygenating compositions as biocolloids, i.e. hemodiluents, blood substitutes, plasma expanders, or resuscitative fluids. The cardiac output-increasing compositions comprise a simple mixture of hemoglobin and the cyclic GMP generating compounds or hemoglobin physically or chemically coupled to the cyclic GMP generating compounds. The invention also provides time-controlled superoxygenating compositions of hemoglobin and cyclic GMP generating compounds that would be used for treatment of specific diseases or medical conditions requiring time-dependent use of biocolloids, i.e. hemodiluents, blood substitutes, plasma expanders, or resuscitative fluids.