Hemoglobin is composed of four iron heme subunits that can each individually bind oxygen. These four hemes participate in cooperative binding, which means that when one heme subunit binds oxygen, the others are more likely to bind.
Lots of hemoglobin is needed in red blood cells because of the high oxygen demands of the body.
High O2 levels increases hemoglobin's binding affinity for oxygen, allowing it to pick up oxygen in areas like the alveolus.
High temperature also reduces oxygen-binding affinity in hemoglobin as part of the physiological response to hyperthermia.
High carbon dioxide levels reduce hemoglobin's binding affinity for oxygen, allowing it to drop oxygen off at tissues. Because tissues are undergoing cellular metabolism, they produces CO2 as a byproduct and use O2 as an electron acceptor.
Low pH reduces O2 binding in a condition called acidosis. If respiration levels are low and CO2 is not being exhaled fast enough, blood becomes acidic and O2 binding in hemoglobin is reduced.
Myoglobin is a hemoglobin derivative found only in muscle cells. It has one subunit instead of four.
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