In human bodies, coppers is an important constituent of many essential enzymes, including ceruloplasmin (which carries Cu and oxidizes Fe in human plasma), cytochrome c oxidase, metallothionein and superoxide dismutase 1. Serum in human blood is typically Cu-depleted by ~0.8‰ relative to erythrocytes (i.e., red blood cells). In a study of 49 male and female blood donors, the average δCu value of the donors' blood serum was -0.26 ± 0.40‰, while that of their erythrocytes was +0.56 ± 0.50‰. In a separate study, δCu values of serum in 20 healthy patients ranged from -0.39 to +0.38‰, while the δCu values of their erythrocytes ranged from +0.57 to +1.24‰. To balance Cu loss due to menstruation, a large portion of Cu in the blood of menstruating women comes from their liver. Due to fractionation associated with Cu transport from the liver to the blood, the total blood of pre-menopausal women is generally Cu-depleted relative to that of males and non-menstruating women. The δCu values of healthy human liver tissue in 7 patients ranged from -0.45 to -0.11‰.