Haemoglobin is the iron-containing protein attached to red blood cells that transports oxygen from the lungs to the rest of the body. Haemoglobin bonds with oxygen in the lungs, exchanges it for carbon dioxide at cellular level, and then transports the carbon dioxide back to the lungs to be exhaled.
Whether haemoglobin binds with oxygen or carbon dioxide depends on the relative concentration of each around the red blood cell. When it reaches the oxygen-rich lungs, it releases the less-abundant carbon dioxide to bind with oxygen; when it goes back out into the body where cells are producing carbon dioxide, it releases the oxygen and binds with carbon dioxide. This is called the Bohr effect.
When carbon monoxide is present, it competes with oxygen at the heme binding sites, and since haemoglobin is 200 times more likely to bond with carbon monoxide, forming a very bright red form of haemoglobin called carboxyhaemoglobin. A concentration of carbon monoxide as low as 0.02% in the air can cause nausea and headache; 0.1% causes unconsciousness and death. (compare that with the normal 20% oxygen saturation of the air!) Heavy smokers, who expose themselves regularly to carbon monoxide, may have as many as 20% of their haemoglobin's oxygen sites blocked by carbon monoxide.
Haemoglobin abnormalities result in very serious hereditary diseases, such as sickle-cell anaemia and thalassemia.
Haemoglobin is made up of four subunits, with a haem (iron-containing) group in each for oxygen binding. There are slightly different haemoglobins in adults when compared to children and foetuses.
High 2,3-diphosphoglycerate levels are found in people who live in high altitudes; this chemical allows a larger amount of oxygen to be delivered to the tissues, preventing altitude sickness.
Haemoglobin levels are common blood tests, and your haemoglobin level can tell a doctor a lot about your health in general.
Web Resources On Haemoglobin
An Overview of Hemoglobin
Hemoglobin and the Heme Group
Book Resources On Haemoglobin
Disorders of Hemoglobin: Genetics, Pathophysiology and Clinical Management by Martin H. Steinberg et al
Hemoglobin: Structure, function, evolution, and pathology by Richard Earl Dickerson