For example: if 2 molecules of O2 are bound to 1 hemoglobin, it is said to be 50% saturated because it can bind a max of 4 O2s. The most important factor that determines how much O2 binds to hemoglobin is its partial pressure (PO2). The higher the PO2, the more O2 combines with and is tightly held to hemoglobin. Said another way, Hb has a high affinity for O2. The lower the PO2, the less O2 is held and as a result is dropped off. For example: in pulmonary capillaries (lungs) where PO2 is high, a lot of O2 binds to hemoglobin. In tissue capillaries where PO2 is low, hemoglobin does not hold as much O2, and the O2 is unloaded via diffusion into tissue cells. Hemoglobin is only 75% saturated with O2 at a PO2 of 40 mmHg which is the average for tissue cells in a horse at rest. Therefore, only 25% of the O2 is unloaded and is used by tissue cells in a horse at rest. If PO2 is between 60 and 100 mmHg, Hemoglobin picks up more O2 (90% or more saturated). Thus, blood picks up nearly a full load of O2 in the lungs even at PO2 of 60 mmHg . This explains why some horses can still perform well at high altitudes even when pressures drop to 60 mmHg. In active tissues such as contracting skeletal muscles, PO2 will drop well below 40 mmHg. This causes a large percentage of O2 to be dropped off at those tissues providing more O2 to metabolically active tissues.
Other factors that affect the Affinity of Hemoglobin for Oxygen
First rule to remember : metabolically active tissue needs O2, produces acids, CO2, and heat as waste products. 1) Acidity (pH): as acidity increases (pH decreases), the affinity hemoglobin has for O2 decreases and O2 dissociates more readily into tissues. 2) Partial Pressure of Carbon Dioxide: CO2 can also bind to hemoglobin. When PCO2 rises, hemoglobin releases O2 more readily
3) Temperature : as temperature increases, so does the amount of O2 released from hemoglobin.
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