Why is hemoglobin said to combine reversibly with oxygen?

Answer 1

As I understand it, and very simplistically, I think it means that the bonding is easily reversible. In other words, the O2 is readily bound to haemoglobin but is also readily released depending on the partial saturation levels of Oxygen (PO2) that are encountered by the RBC. * In the pulmonary capillaries the PO2 is high and therefore the amount of O2 bound to Hb is also high (fully saturated). In the systemic capillaries the PO2 is low and hence the O2 dissociates with the Hb and diffuses into the tissue cells. (The Hb is then said to be partially saturated). Although PO2 is the major factor in the binding of O2 to Hb, other factors also effect this process. * Acidity: Lactic acid and carbonic acid generated during exercise lowers the pH of the blood which promotes the release of O2 from Hb. Hence the exercising tissues which require extra O2 help create a situation where more O2 is available. * PCO2 effects the transport and delivery of O2. Increased metabolic activity of the tissue cells produces more CO2 as waste, which promotes release of O2 from Hb through an associated decrease in pH (see above point) and also as part of an O2/CO2 cycle. It goes something like this:

In the pulmonary capillaries, O2 diffuses from the alveolus into the RBC. It binds to Hb to create oxyhaemoglobin and H+ ions. Bicarbonate ions combine with the H+ ions to create carbonic acid which, under the influence of the enzyme carbonic anhydrase, dissociates into CO2 and water. The CO2 diffuses into the alveoli and is expelled by the lungs.

The RBC then travels in the blood to the systemic capillaries and CO2 diffuses from the tissues (as a waste product of tissue cell metabolism) into the RBC. A small percentage of the CO2 binds to the globin in Hb, forming carbaminohaemoglobin, this causes O2 to dissociate from the haem part of the Hb. The O2 then diffuses from the RBC into the tissue cells.

Most of the CO2, under the influence of the enzyme carbonic anhydrase, combines with H2O to become Carbonic acid which then dissociates into Bicarbonate ions (HCO3-) and H+ ions. The H+ ions promote the dissociation of O2 from the haemoglobin by taking the place of the O2 in the Hb molecule. * Increased temperature also has a minor effect of promoting the release of O2 from the Hb. Anatomy and physiology books such as "Anatomy and Physiology - From Science to Life" by Jenkins Kemnitz and Tortora is a fabulous resource for understanding the intricacies of this process.