A nearly continuous cycle whereby iron from organic and inorganic sources is converted to form iron-porphyrin compounds, which can be utilized by the body. One such compound, termed a heme, is hemoglobin; more than 60% of the iron in the body is used in hemoglobin metabolism. Iron is also essential for other heme compounds, such as myoglobin and cytochromes, and for a wide variety of nonheme enzymes, including many in the citric acid cycle.