Mechanism
Hemoglobin (Hb) molecule is the combination of protein globin and nonprotein portion called haem and responsible for essentially transporting of the oxygen in the blood. Hemoglobin is composed of four subunits: two alpha subunits and two beta subunits, each of which contains a heme group and globin chain. The heme group contains an iron atom as ferrous form(Fe 2+) at its core that binds one oxygen molecule, allowing one hemoglobin tetramer to bind four oxygen molecules.[2] Hemoglobin exists in two states: the T (deoxygenated-tense) state and the R (oxygenated-relaxed) state. The two states differ in their affinity to bind oxygen. In an unbound state, hemoglobin exists in the T state, and binding of oxygen occurs with low affinity. The T-state hemoglobin thus requires a higher partial pressure of oxygen (pO2) to facilitate the binding of an oxygen molecule. The binding of single oxygen induces a conformational change that destabilizes the T state and facilitates the transition of the other subunits to the high-affinity R state. The binding of the first oxygen allows the second, third, and fourth oxygen molecules to subsequently bind with increasing ease. This relationship is an example of positive cooperativity.[3]
In a set of normal lungs, the partial pressure of oxygen is naturally high at the alveolar-capillary junction. Therefore, exposing the T state hemoglobin to vast amounts of oxygen and facilitating oxygen loading. This process occurs very quickly and allows for hemoglobin to saturate to 100% before the end of the capillary bed.[1] Upon exiting the pulmonic system, the hemoglobin is now in the oxygenated R state. However, the partial pressure of oxygen is naturally lower in peripheral tissue, which aids in the release of oxygen. In the peripheral tissue, the T state of hemoglobin is preferred as there are a lower pO2 and less oxygen bound, which results in a quick release of the other three molecules of oxygen. Throughout the bloodstream, there are different pO2 levels that participate in a continuous equilibrating transition between T and R states.
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The relationship between pO2 and SaO2 can be represented by the oxygen dissociation curve, which represents oxygen saturation (y-axis) as a function of the partial pressure of oxygen (x-axis). The sigmoid or S-shape of the curve is due to the positive cooperativity of hemoglobin.[4] In the pulmonary capillaries, the partial pressure of oxygen is high allowing more molecules of oxygen to bind hemoglobin until reaching the maximum concentration. At this point, little additional binding occurs and the curve flattens out representing hemoglobin saturation. At the systemic capillaries, pO2 is lower and can result in large amounts of oxygen released by hemoglobin for metabolically active cells, which is represented by a steeper slope of the dissociation curve.
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