<P> Hemoglobin exists in two forms, a taut (tense) form (T) and a relaxed form (R). Various factors such as low pH, high CO and high 2, 3 BPG at the level of the tissues favor the taut form, which has low oxygen affinity and releases oxygen in the tissues . Conversely, a high pH, low CO, or low 2, 3 BPG favors the relaxed form, which can better bind oxygen . The partial pressure of the system also affects O affinity where, at high partial pressures of oxygen (such as those present in the alveoli), the relaxed (high affinity, R) state is favoured . Inversely, at low partial pressures (such as those present in respiring tissues), the (low affinity, T) tense state is favoured . Additionally, the binding of oxygen to the iron (II) heme pulls the iron into the plane of the porphyrin ring, causing a slight conformational shift . The shift encourages oxygen to bind to the three remaining heme units within hemoglobin (thus, oxygen binding is cooperative). </P> <P> Deoxygenated hemoglobin is the form of hemoglobin without the bound oxygen . The absorption spectra of oxyhemoglobin and deoxyhemoglobin differ . The oxyhemoglobin has significantly lower absorption of the 660 nm wavelength than deoxyhemoglobin, while at 940 nm its absorption is slightly higher . This difference is used for the measurement of the amount of oxygen in a patient's blood by an instrument called a pulse oximeter . This difference also accounts for the presentation of cyanosis, the blue to purplish color that tissues develop during hypoxia . </P> <P> Deoxygenated hemoglobin is paramagnetic; it is weakly attracted to magnetic fields . In contrast, oxygenated hemoglobin exhibits diamagnetism, a weak repulsion from a magnetic field . </P> <P> Scientists agree that the event that separated myoglobin from hemoglobin occurred after lampreys diverged from jawed vertebrates . This separation of myoglobin and hemoglobin allowed for the different functions of the two molecules to arise and develop: myoglobin has more to do with oxygen storage while hemoglobin is tasked with oxygen transport . The α - and β - like globin genes encode the individual subunits of the protein . The predecessors of these genes arose through another duplication event also after the gnathosome common ancestor derived from jawless fish, approximately 450--500 million years ago . The development of α and β genes created the potential for hemoglobin to be composed of multiple subunits, a physical composition central to hemoglobin's ability to transport oxygen . Having multiple subunits contributes to hemoglobin's ability to bind oxygen cooperatively as well as be regulated allosterically . Subsequently, the α gene also underwent a duplication event to form the HBA1 and HBA2 genes . These further duplications and divergences have created a diverse range of α - and β - like globin genes that are regulated so that certain forms occur at different stages of development . </P>

Oxygen and carbon dioxide bind to different parts of hemoglobin