Out of curiosity what is the tipping point where partial pressure (moles) of O2 is greater than the metabolic capacity?
Hello,
Lots of good comments in this thread, but this question comes closest to pointing at the complete answer to the OP's question. It is actually quite a tricky one to get one's head around, but if you're interested, here it is.
To understand it, you need to understand some fundamental concepts of oxygen transport.
Oxygen diffuses from alveolar gas (in the lungs) into blood, and initially (like any other gas) is dissolved in the plasma. From there it would be carried off around the body. The problem with 'transport as a dissolved gas' is that the solubility of oxygen in the plasma is too low for enough oxygen to be transported in dissolved form. To put some perspective on this, every time a liter of blood passes through tissues from the arteries to the veins, about 50 ml (cc) of oxygen is extracted into the tissues. Breathing air at surface pressure there is only about 3 ml per liter oxygen dissolved in plasma. This obviously represents a massive shortfall between what can be dissolved in plasma breathing air (3 ml per liter) and what the tissues need (50 ml per liter).
This shortfall is overcome by having hemoglobin (Hb) in the blood (in the red cells to be accurate). As has been pointed out, Hb is a complex molecule that has binding sites for oxygen and can carry a lot of it. In contrast to the above numbers for dissolved oxygen, when breathing air at surface pressure, the Hb in the blood (assuming normal levels of Hb) will absorb about 200 ml of oxygen per litre of blood as it passes through the lungs. It is important to appreciate (and it should not be surprising) that we have evolved so that Hb absorbs as much oxygen as it can carry when we are breathing air at surface pressure. Breathing more oxygen does not improve oxygen carriage by Hb.
Then, as the arterial blood passes through a tissue where oxygen is being consumed and the oxygen partial pressure is low, the following things happen. First, the oxygen dissolved in plasma will naturally start to diffuse from blood into the tissue. As mentioned before, this is only a small amount, but this outward diffusion lowers the partial pressure of oxygen in the plasma. Second, (and this is important) that fall in plasma oxygen partial pressure is a 'signal' to Hb to start unloading oxygen. As mentioned above, it unloads about 50 ml oxygen per liter of blood as it passes through the tissue.
So, how does this change when you breathe oxygen at higher pressures, as in diving.
As above, Hb absorbs as much oxygen as possible even when breathing air at surface pressure. So, breathing a higher pressure of oxygen during a dive does not increase oxygen carriage on Hb (its still around 200 ml per liter. However, the higher pressures of oxygen we breathe in diving will increase the amount of dissolved oxygen in plasma. It will still not be enough to supply the 50 ml per liter requirement of tissue, but it will come closer. For example, breathing oxygen at 1.3 ATA you dissolve about 25 ml of oxygen in each liter of plasma as it leaves the lungs in the arterial blood. This means that the same process as described above will occur when blood enters the tissue - dissolved oxygen will diffuse into tissue, the partial pressure of oxygen in the plasma will fall, and this will signal Hb to unload some oxygen to make up the shortfall in delivery (remember you need to deliver about 50 ml per liter of blood passing through tissues). In this setting (with 25 ml per liter dissolved oxygen) the Hb won't have to unload quite as much oxygen as normal to meet the tissue requirements.
It is crucial to understand that at this point, even breathing 1.3 ATA of oxygen, we are still supplying just enough oxygen to meet the metabolic requirements of the tissues. The pressure of oxygen in the tissue will change very little (because there is still only just enough delivery to meet metabolic requirements) and there is no opportunity for oxygen to accumulate in tissues in the way a non-metabolised gas like nitrogen does.
However, now consider what would happen if we breathed oxygen at a high enough pressure to supply
more than the tissue metabolic requirement (50 ml per liter) from dissolved oxygen alone. For example, if you breath 3 ATA oxygen there will be about 67 ml per liter of dissolved oxygen in plasma. As the blood passes through the tissue this will more than meet the tissue's 50 ml per liter oxygen requirements from dissolved oxygen alone, the Hb won't unload at all, and crucially, it is now possible for oxygen to start accumulating in the tissue as dissolved gas like the inert gases (nitrogen and helium) do. Under these circumstances it is possible that a decompression could result in bubbles forming from accumulated dissolved oxygen as can happen with nitrogen.
However, to DDM's point, this is only possible at inspired pressures of oxygen that we would never use underwater because of the risks of cerebral oxygen toxicity. To answer the question at the start of this post, the 'tipping point' is the pressure of inspired oxygen that more than meets tissue oxygen requirements from dissolved oxygen alone, which is somewhere around 2.5 ATA of 100% oxygen. Not something we would ever do.
I hope that puts it in a digestible form!
This answer is a slightly simplified version of real physiology, but it is fundamentally accurate. In case you are wondering, the 50 ml per liter oxygen extraction I quote is different in some organs, and changes a bit during exercise - though the greater delivery of oxygen required is largely met by increasing the blood flow through tissues.
Simon M
