Hemoglobin (Hb) is an extremely important molecule in the body as it allows for the transport and delivery of oxygen. It consists of a protein, globin, as well as an iron-based molecule called heme, synthesized in the mitochondria of immature red blood cells. Oxygen dissolves very poorly in the blood with less than 1 ml O2/100 ml blood (or 1 vol %) at atmospheric pressures. However, it has a high affinity for hemoglobin, raising the O2 content of the blood to about 20 vol % when fully saturated. Oxygen content is defined as the amount of O2 in the blood at a certain partial pressure of oxygen (PO2) at a certain concentration of Hb (normally 15 g/100 ml blood = 15 gm %). PO2 represents the partial pressure of oxygen in various parts of the body. Normal alveolar PO2 = 150 mm Hg, arterial PO2 = 100 mm Hg, and venous PO2 = 40 mm Hg. Increasing the amount of oxygen in the air will act to increase alveolar PO2, which is directly related to air PO2.
P50 is an expression of affinity of Hb for O2 and represents the PO2 at which Hb is 50% saturated. Shifts in the Hb-O2 dissociation curve can alter this value.
Hb also acts as an important buffer for acids and CO2 through processes known as the Bohr and Haldane effects. These processes shift the Hb-O2 dissociation curve to the right and left, respectively.
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