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Can Henry's law be applied to find $\ce{[O2]}$ in alveoli blood?
This kind of question appeared in an another forum and I've been struggling to find a definitive answer. My initial thought was 'no', because Zumdahl's Chemical Principles book says that

Henry's law is obeyed most accurately for dilute solutions of gases that do not dissociate or react with the solvent. $\ce{O2}$ will bind to the haemoglobin in the blood. Given this reaction in the solvent, $\ce{O2_{(g)}}$ in blood does not follow Henry's law.

However, a quick google search turned up multiple sources that use Henry's law directly to find the dissolved oxygen concentration (which is about $2\%$ of total oxygen):

http://www.umich.edu/~projbnb/cvr/O2transport.pdf https://www.ncbi.nlm.nih.gov/books/NBK54103/

What I failed to understand is why the dissolved oxygen is linearly proportional to the partial pressure while the oxygen dissociation curve is not. Is there something I'm missing?

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Henry's law states that the amount of a gas that dissolves in a liquid is directly proportional to the partial pressure of that gas.

Oxygen has a larger partial pressure gradient to diffuse into the bloodstream, so it's lower solubility in blood doesn't hinder it during gas exchange. Therefore, based on the properties of Henry's law, both the partial pressure and solubility of the oxygen and carbon dioxide determine how they will behave during gas exchange.

I encourage you to read the article below.

Source: Boundless. “Henry's Law.” Boundless Anatomy and Physiology. Boundless, 21 Sep. 2016. Retrieved 16 Oct. 2016

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  • $\begingroup$ Thanks, the article was very insightful but did not properly address my question. But it got me thinking that air pressure in the lungs might stay fairly constant and thus the oxygen dissociating into hemoglobin will not affect the dissolved oxygen predicted by Henry's law. Might this be a correct line of thought? $\endgroup$
    – A. La
    Oct 16, 2016 at 20:55
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Old, but I'll take a stab at explaining my take on your question about linear proportionality for any future readers:

Henry's law gives us the direct proportionality for dissolved oxygen in the plasma. The key here is understanding that we are talking about dissolved oxygen in the plasma. This is distinct from oxygen bound to hemoglobin. As oxygen partial pressure increases, more of it is "pushed" into the plasma. If you just had plasma with no RBCs, that would be it. There would be a specific coefficient of Henry's law for the plasma, but it would otherwise be no different than dissolved oxygen in water.

The nonlinear part comes in with the Hemoglobin. Hemoglobin is not directly effected by the partial pressure of oxygen, but by the amount of oxygen available to it in the plasma (which we know is driven by partial pressure of oxygen). Hemoglobin essentially removes takes some of the dissolved oxygen from the plasma. At low pressures, there isn't much oxygen around, so hemoglobin can only slowly (and nearly linearly) increase its uptake. The more available to it, the more it will take (to a point). s partial pressure of oxygen increases, more oxygen dissolves in the plasma, so hemoglobin has more to take, and it does The cool thing about hemoglobin is the "positive feedback" that occurs. Once one molecule of oxygen is bound the next is more likely to bind. This gives the curve its exponential-like region. But remember, it will only continue to bind to a point. It maxes out at 4 molecules oxygen per molecule of hemoglobin. This is why the sigmoid curve has an upper asymptote. As more and more molecules of hemoglobin "max out", the curve flattens out.

Figure 1 in this article is a nice graph of the two distinct mechanisms of oxygen carrying by the blood.

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