Involved in oxygen transport from the lung to the various peripheral tissues. Hemopressin acts as an antagonist peptide of the cannabinoid receptor CNR1. Hemopressin-binding efficiently blocks cannabinoid receptor CNR1 and subsequent signaling. Heterotetramer of two alpha chains and two beta chains. Two external cysteine residues at beta-16 and beta-52 cause reversible polymerization to octamers and most likely irreversible formation of higher polymers. Red blood cells. Belongs to the globin family.