The respiratory pigment haemoglobin present in human red blood cells contains the metal

Haemoglobin is a protein with an iron-containing haem group at each of its four subunits, and iron binds oxygen reversibly, so oxygen is carried from the lungs to the tissues. Magnesium is found in chlorophyll, and copper in the blood pigment haemocyanin of some molluscs. Iron deficiency therefore leads to anaemia.