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  1. #1
    sam16231 is offline Newbie 510 points
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    Partial pressure of gases in various parts of the respiratory system

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    Hello everyone,

    Ok to understand the respiratory system, proper understanding of this diagram is essential. Something I don't have, so if anyone can help me with these questions I would be very greatful. Thanks

    [email protected]@us/img515/8923/[email protected]

    (replace @ with . )

    The colour of blood for arteries and veins is mixed up in the diagram. So excuse that.

    1. Anatomic dead space and residual volume?
    I know what anatomic dead space is but residual volume is confusing me. Is it in the conducting zone as well and non technically isn't it like some kind of anatomic dead space.

    2. Why does the alveoli have less partial pressure of oxygen than inspired air? I can understand how addition of water vapour can reduce it but residual volume just confused me. Does oxygen from residual volume get mixed with this as well increasing P02.

    3. Why does the expired air have higher P02 of oxygen than alveoli?
    I can understand how mixing with dead space would archieve it. Does the residual volume affect it as well. Residual volume is confusing me.

    4. What creates the P02 in arterial blood and why it is 95?
    So it is basically oxygen dissolved in blood right that causes the partial pressure of oxygen.We consider the haemoglobin bind oxygen as part of dissolved blood right? 95 is due to shunting right?

    5. Why is PC02 in alveoli 40 and what creates PC02 in arterial blood?
    Is it 40 in alveoli because CO2 diffuses into it from the capillaries. So C02 from tissues goes to capillarie-->veins--->capillaries---->alveoli so from where does arterial PC02 come from.

    Your help would be greatly appreciated. Thanks
    Last edited by sam16231; 11-04-2009 at 07:24 PM.

  2. #2
    atropine is offline Member 512 points
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    1. Anatomic dead space and residual volume?
    I know what anatomic dead space is but residual volume is confusing me. Is it in the conducting zone as well and non technically isn't it like some kind of anatomic dead space.
    Residual volume is the amount that you can't exhale, and it's not really deadspace because it exchanges gas with blood.

    2. Why does the alveoli have less partial pressure of oxygen than inspired air? I can understand how addition of water vapour can reduce it but residual volume just confused me. Does oxygen from residual volume get mixed with this as well increasing P02.
    It has less due to mixing with CO2 in the alveoli. You should know the alveolar gas equation for sure, which includes this. It's inspired air - PaCO2/RQ (RQ=respiratory quotient).

    3. Why does the expired air have higher P02 of oxygen than alveoli?
    I can understand how mixing with dead space would archieve it. Does the residual volume affect it as well. Residual volume is confusing me.
    Not sure about this one, because I always thought expired PO2 was much lower than what you've got in that diagram.

    4. What creates the P02 in arterial blood and why it is 95?
    So it is basically oxygen dissolved in blood right that causes the partial pressure of oxygen.We consider the haemoglobin bind oxygen as part of dissolved blood right? 95 is due to shunting right?
    This is a common area of confusion. Your PO2 only refers to the amount dissolved directly in the plasma, so not including what is attached to hemoglobin. The amount dissolved (usually expressed in mls o2 / 100 mL plasma) is directly proportional to the PO2, which is why the two terms are interchangeable. You are correct about shunting being a cause for the slightly lower PO2, namely from the bronchial circulation.

    It's important to remember though, that your PO2 is indirectly associated with your hemoglobin's oxygen saturation (remember that oxyhemoglobin dissociation curve). The numbers are pretty consistent except under conditions of acidosis, hypercapnia, 2,3-BPG, and the other things which shift the curve left or right.

    5. Why is PC02 in alveoli 40 and what creates PC02 in arterial blood?
    Is it 40 in alveoli because CO2 diffuses into it from the capillaries. So C02 from tissues goes to capillarie-->veins--->capillaries---->alveoli so from where does arterial PC02 come from.
    Venous CO2 is about 45-46ish, and arterial is 40 after it exchanges with the alveoli.
    Last edited by atropine; 11-10-2009 at 03:07 PM.

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