Why doesn't dissolved gas in blood come out of solution?

Question

From "Ganongs Review of Medical Physiology" (26th Edition)

The partial pressure of a gas in a liquid is the pressure that, in the gaseous phase in equilibrium with the liquid, would produce the concentration of gas molecules found in the liquid.

My understanding from this is that partial pressure of a gas in solution is defined as the partial pressure of the gas over the liquid in a confined space, which is proportional to the concentration of the gas that is dissolved in that liquid in accordance with Henry's law (at least at dilute concentrations).

i.e. If, say, the partial pressure of CO2 in a water solution is 40mmHg, then if I took that water, put it in a box and measured the partial pressure of CO2 over that water, it would be 40mmHg.

Please correct me if that understanding of partial pressure of a dissolved gas is wrong. From my own research it seems to be correct and was also answered here: What does "partial pressure" mean in the setting of arterial CO₂? and What does pO2 of blood mean and why do we use it?

Assuming it is correct (or correct for the most part) and ignoring hemoglobin, why doesn't gas that is dissolved in blood come out of solution, and create gas bubbles in our vasculature. Doesn't that gas have to exist in some equilibrium, in accordance to Henry's law, with the gas that is out of solution over the liquid? There obviously is no gas over the blood in our circulatory system.

I think I have an answer, but please confirm or deny it. The partial pressures of the gases in our blood are all lower than the partial pressures we find in the atmosphere. Our circulatory system is actually open to the atmosphere and, therefore, is in fact in some kind of equilibrium with the dissolved gas in our blood. The gas in the atmosphere is the "gas out of solution over the liquid", and that liquid is our blood.

Thanks in advance!!!

Answer 1

I think you're kind of on track, but I'd explain it a different way. Similar to this Q&A: Why doesn't water enter the venous system if injury happens far underwater? I think the main principle you're missing is the importance of absolute and relative pressures.

You might be used to seeing blood pressures in the range of 0-120 mmHg; near zero in the veins, near 120 mmHg for a healthy/normal systolic arterial pressure. But these are relative pressures, specifically, relative to the outside atmosphere. Even at "0 mmHg" of relative pressure, your blood is always under atmospheric pressure (about 760 mmHg at sea level). Your body isn't that rigid. The atmosphere is pushing on all of the exposed surfaces on your body which in turn push on each other, putting everything under pressure. Skin and tissue isn't impermeable to gases, it's not just your blood that has dissolved gas, all tissue has dissolved gas.

That lack of rigidity and permeability makes the whole system "open" to the atmosphere, like you wrote, but not just the circulatory system and not just because of mechanisms to vastly increase the surface area of the opening (that is, the lungs).

So, in the circulatory system, the blood is always under approximately atmospheric pressure from the outside, so unless you dissolve higher pressure gas than atmospheric pressure (for example, by breathing pressurized gas during a dive), there is no favorable drive to create gas bubbles.

Answer 2

Your understanding is correct. The partial pressure of a gas in solution is indeed the pressure it would exert in the gas phase in equilibrium with the liquid.

Regarding why gas doesn’t form bubbles in the blood: our circulatory system is in a dynamic equilibrium with the atmospheric gases we breathe. The partial pressures in the blood are balanced with the gases in the air we inhale, preventing bubble formation. Gas embolism typically occurs only when external pressures change rapidly, such as in diving accidents.