Oxygen Dissolving at Low Temperatures

[RH8448]

A Khan Academy question investigates what would decrease the rate of oxygen dissolving in the lungs. The question asks what would decrease the rate of oxygen diffusion into the blood stream. The multiple choice answers are: turbulent flow, gravitational effects, additional capillary surface area, and "low temperatures near the cells"

Turbulent flow is the answer. They say it decreases the diffusion rate of oxygen.

This question could seem trivial, however, I would think that lower temperature would lower the rate of diffusion. They argue in the hints that, "A thermal gradient would push molecules towards lower temperature areas, increasing the opportunities for oxygen molecules to be taken up into blood."

I just don't know what concept or theory to tie this concept with. Any insight? Thank you. Also, this question can be found in context in the Physical Sciences Khan Academy Questions labeled "Pressure Regulation and Fluid Dynamics of the Respiratory System"

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[ElectricNoogie]

The tendency of higher temperature leading to greater solubility is true when speaking of dissolving a solid into a liquid. However, when dissolving gases in liquids, it is a different story. Think about leaving a bottle of soda out in the sun to get warm. As it gets warmed, the CO2 solubility decreases, and the soda goes flat as the CO2 converts to gas and escapes. Keep the soda cold, and it stays nice and bubbly longer because the gas stays dissolved in the liquid longer.

Though it's a bit outside the scope of the MCAT, when a gas is added to water, a cavity develops around the gas molecules. The size and shape of this cavity is dependent on the size and conformation of the gas solute but it will lead to a stimulation of attractive forces between the gas and water.

With pressure, it goes the other way. As ambient P increases, gas solubility will increase. This phenomena helps explains Barometric trauma (aka the bends) in divers who come up too quickly. Elevated pressure underwater increases the solubility of the oxygen breathed from a diver's tank. If the diver rises too quickly, where the pressure of the atmosphere is less than the pressure underwater, the solubility of the gases decreases and the gas bubbles out of the bloodstream.

It's rare the MCAT tests turbulent (non-ideal) fluid flow, but if you think of turbulence as chaos in the fluid, then more turbulent flow may not allow for the same level of attractive forces between the liquid and the gas, which leads to decreased solubility, though I am not 100% on this.

long story short: Solid in liquid solubility is the opposite of gas in liquid solubility.

An increase in T --> decrease in gas solubility in water, while a decrease in T --> increase of gas solubility in water.