Haemoglobin is the red substance inside red blood cells. It contains iron, and that iron is what grabs oxygen in the lungs and carries it around the body. It is also why blood looks red. A blood test often measures haemoglobin (you may see Hb on a report). If it is too low, a person may feel tired and weak; that is one kind of anaemia. The word is haemo- (blood) + globin (a protein that can carry something). Though actually, people sometimes talk as if iron just floats in the blood. Most of that iron is locked inside haemoglobin. The British spelling keeps ae; Americans write hemoglobin. Same substance, two spellings.