Iron deficiency, or anemia, is one of the most common nutritional deficiencies in the world. You have about 2 grams of iron in your body normally if you're a women, and up to 6 grams if you're a man. This iron can be divided into a functional group & a storage group. The iron in the functional group is what you use daily - 80% of this being in the hemoglobin of your red blood cells. This is what binds to oxygen when the blood passes through your lungs, circulating this necessary nutrient throughout the body. You can see a picture of hemoglobin on the right, with the 4 protein subunits in red and blue. The small green units are heme groups, which contain an iron atom at their center (in orange).
You lose about 1 - 2 milligrams of iron each day, mainly through shedding of dead epithelial cells (both your skin & the lining of your intestines). This means you have to absorb 1 - 2 mg of iron each day to remain balanced. You have to eat about 10 - 20 mg of iron each day though since you only absorb about 20% of what you eat (this decreases to 1 - 2% of the iron found in vegetables though, so it's best to take a multivitamin & mineral each day if you don't eat animal products).