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Hemoglobin is very crucial to our bloods capacity for storing and transporting oxygen As oxygen is relatively insoluble in blood plasma 3 milliliters of oxygen per 100 milliliters of plasma respiratory pigments such as hemoglobin and hemocyanin arthropods and mollusks may raise the oxygen transporting capacity of the blood as much as seventy fold In most of the invertebrates respiratory pigments are merely dissolved in the blood plasma which in contrast to vertebrates and echinoderms use highly developed red blood cells or erithrocites to carry their respiratory pigments Hemoglobins ability to carry oxygen lies in its structure constructed of four subunits each subunit contains a heme unit and a polypeptide chain The heme unit comprises of a porphyrin ring with one atom of iron in the center which can then be combined with a molecule of O2 making for a total of four molecules per hemoglobin Combination of the first subunit Hb with oxygen increases the affinity of the next for oxygen which when oxidized affects the affinity for the next This is caused by the two beta polypeptide chains of hemoglobin moving closer together which as a result changes affinity of the hemoglobin for oxygen Because of this the curve the curve relating the uptake of oxygen to the partial pressure of O2 is not directly proportional but in a sigmoid shape When fully saturated hemoglobin allows our bloodstream to carry about 65 times as much oxygen as an equal amount of plasma alone This saturation of hemoglobin is dependent on the partial pressure of oxygen Po2 in the surrounding blood plasma A good example of this would be how oxygen gets into the blood stream in the first place Oxygen diffuses from the air into the alveolar capillaries Here the higher Po2 causes most of the hemoglobin to be combined with oxygen However in the tissues where oxygen is used in cellular respiration the P02 is lower and oxygen is released from
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