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    One question that I had and cannot find an answer for is this.

    Why does the percentage saturation of haemoglobin with oxygen never reach 100%?

    This question I have searched multiple times but Yahoo answers mentions foetal haemoglobin for no apparent reason.

    Is there a reason that fits an AS level explaination?

    Thanks.

    Also,
    This question.

    Tissue fluid accumulates in the tissues of people who do not eat enough protein. Explainwhy.

    I said that there are fewer plasma proteins in the blood so that the water potential at the arteriole end is higher than in the tissue fluid, so water does not leave from the tissues, but I'm not sure if that is right.
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    I'm not sure if this is going to be helpful but here's how I understand it.
    The ability of haemoglobin to bind with oxygen is a function of the partial pressure of oxygen.So if it's a 100% saturated,haemoglobin carries 4 units of oxygen but this can theoretically only happen in the lungs when the partial pressure is a 100mmHg.The thing is,partial pressure in the normal (active) tissues is around 40mmHg which means that haemoglobin gives up around 1/4th of its oxygen due to the lower partial pressure and so at any point of time,haemoglobin can never actually be 100% percent saturated in the active tissues.So this mechanism is basically designed to allow haemoglobin to deliver the bound oxygen molecules to the tissues.
    Try looking up the oxygen dissociation curve!Could be helpful.


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    Your answer to the tissue fluid question is almost right. But you should mention the water potential gradient being less steep so equilibrium reaches quicker and thus less is reabsorbed and the fact that proteins lower the WP in the capillary.
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    It gets increasingly difficult for oxygen to bind to the Hb as it gets more full, it's because once, say when 3/4 of the haem groups already have oxygen in them, there is a reduced probability for anymore oxygen to bind to it as only 1/4 of the sites are available for oxygen. Also what Anj said was right, the pO2 will never be truly 100%.

    Also with the tissue fluid question, you are right but as Rkm said, you need to state key terms such as water potential gradient and equilibrium as generally those marks are underlined and you must state them terms to get all the marks
 
 
 
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