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Let's say you accidentally walk into a room pressurized with pure nitrogen (or you're jettisoned into space). Within a couple of seconds, the partial pressure of oxygen within your lungs drops to 0. But there's still a (small?) reserve within your bloodstream.

Now, will the oxygenated blood travel to your lungs? If so, will the usual gas exchange reverse, with oxygen going from your blood to the oxygen-poor environment?

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Yes. Blood will continue to circulate (as long as you're alive and your heart is beating), so will eventually come in contact with your lungs. Once there, the gasses (O2, CO2) will cross out into the lung and be exhaled. All diffusion is driven down a concentration gradient.

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Okay, two questions I think need to be explicitly asked: (1) In my basic research I read here that something goes on with 2,3-BPG near the heart which allows deoxygenated RBCs to be separated from the oxygenated ones – does that play a role here? (2) Can the concentration gradient just flat-out strip the oxygen from hemoglobin? –  rdhs Dec 29 '11 at 14:26
    
There are many factors - BPG concentration, hydronium concentration (i.e., pH), cooperativity - that modulate the actual Kd for oxygen of hemoglobin (described here employees.csbsju.edu/hjakubowski/classes/ch331/bind/… as well as other places), but the physics/chemistry is unavoidable: if one moves to an atmosphere devoid of oxygen, oxygen will leech out of the blood. olemiss.edu/depts/chemistry/courses/chem471_99/L12_6.pdfhas some graphs that show binding as a function of pO2. –  KAM Dec 29 '11 at 18:00
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