abitpissy
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What is Partial pressure of oxygen and what does it have to do with the saturation of haemoglobin?
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anosmianAcrimony
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The partial pressure of oxygen is a measure of the concentration of oxygen in a solution; each of the gases dissolved in a solution exert a partial pressure. The higher the partial pressure of oxygen, the more saturated haemoglobin will be with it, but the relationship exhibits a very interesting sigmoidal relationship that allows Hb to change its affinity for oxygen quite drastically over a small range of values. This means that Hb is very efficient at taking up oxygen in the lungs (where the partial pressure of oxygen is high) and also very efficient at releasing it in respiring tissues around the body (where the partial pressure of oxygen is low). Hb's affinity for oxygen is also influenced by various allosteric effectors, but if I start talking about those, I won't stop.
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abitpissy
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In my textbook it talks about how the lugworm's haemoglobin is fully loaded with oxygen even with little available.

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However, with the lugworm respiring surely the CO2 level is high and the O2 is limitled. With the CO2 being acidic as a solution, hence causing the tertiary shape to change in the haemoglobin. This causes a low affinity for oxygen. Am I correct in thinking so? Then why would the haemoglobin of the lugworm be packed full with oxygen?
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anosmianAcrimony
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The lugworm has specifically evolved to produce haemoglobin with a massive affinity for oxygen, as a result of living in an environment with very little oxygen available. In other words, it would take a much lower concentration of O2 to cause it to dissociate from lugworm haemoglobin. Their haemoglobin probably works differently from ours, with a much smaller response to H+ concentration.
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