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 (P_O2) at a certain concentration of Hb (normally 15 g/100 ml blood = 15 gm %). P_O2 represents the partial pressure of oxygen in various parts of the body. Normal alveolar P_O2 = 150 mm Hg, arterial P_O2 = 100 mm Hg, and venous P_O2 = 40 mm Hg. Increasing the amount of oxygen in the air will act to increase alveolar P_O2, which is directly related to air P_O2.

P₅₀ is an expression of affinity of Hb for O₂ and represents the P_O2 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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