What are the highest and lowest possible values of pH, $paCO_2$, and $HCO_3$ in the (arterial or venous) blood of a living human being?

  • 1
    $\begingroup$ You should look for the standard reference ranges for arterial blood gases, which will give you your answer $\endgroup$
    – Rory M
    Commented Mar 2, 2014 at 21:07
  • $\begingroup$ These give the normal values in normal people, not the possible in any living human body. $\endgroup$
    – Orion
    Commented Mar 2, 2014 at 22:20
  • $\begingroup$ How would we know the numbers for any possible living body? The best you could rely on would be individual data points from someone whose blood happened to be checked for those quantities (i.e. highest and lowest recorded concentrations). Not a controlled experiment. And no way to tell if that's the highest possible. $\endgroup$
    – A. Kennard
    Commented Mar 3, 2014 at 9:43
  • $\begingroup$ Perhaps a more reasonable thing to find is the variance or standard deviation of the values of these quantities in normal patients. $\endgroup$
    – A. Kennard
    Commented Mar 3, 2014 at 9:54
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    $\begingroup$ Practically, in order for this to be answerable you'll need to comment on whether you consider a pulseless person during resuscitation efforts to be "alive." It's not uncommon to see a pH ~6.8 range drawn during CPR, but that's a different situation than normal "life." $\endgroup$
    – Susan
    Commented Sep 27, 2014 at 17:39

1 Answer 1


As discussed in the comments, there are theoretical and practical problems with arriving at such data. There have been controlled experiments using canines, but you have specifically stated your interest in humans. I offer a summary of a few observational studies that may be helpful.

In this study, 117 patients in intensive care units (who are likely to have the most abnormal blood gas parameters among the living) were monitored with continuous optode-based blood gas analysis system. Of the 1,341 measurements, the ranges were:1,2

  • pH: 7.14 to 7.64
  • PaCO2: 19 torr - 98 torr
  • PaO2: 38 torr – 413 torr3

Another study included a total of 487 samples were obtained in ICU patients with severe respiratory failure and found the following ranges:

  • pH 7.23 – 7.55
  • PaCO2: 19 torr – 83 torr
  • PaO2 30 torr – 522 torr3

These are reasonable ranges for pseudo-stable ICU patients. Contrast that with normal ranges (+/- 2 standard deviations):

  • pH: 7.35 – 7.45
  • PaCO2: 35 torr – 45 torr

On the other hand, you’re interested in the most extreme recorded values, which are likely going to occur during cardiac arrest. According to this case report and literature review, the lowest published arterial pH of an adult who survived cardiac arrest with neurologic recovery is 6.33. The caveat here is that this was a near-drowning victim, and the associated hypothermia provides neurologic protection. The authors go on to report several others including their own patient with pH in the range of 6.5 - 6.6 with neurologic recovery in the absence of hypothermia.


1. Bicarbonate measurements on whole blood are calculated rather than measured and so not included in such studies. The more accurate test for this measurement is serum chemistry. The bounds are usually defined by the assay. It’s possible for a living human to have HCO3 less than assay (5 mmol/L in most labs by serum chemistry) or greater than assay (~50 mmol/L by most labs).

2. All data provided are arterial. Venous blood gases are of little clinical utility except as a cop-out for obtaining an arterial blood gas.

3. These upper values are intubated patients breathing 100% FiO2. (The lower values are those about to be intubated breathing 100% FiO2....) The theoretical maximum PaO2 on room air is derived from the alveolar gas equation, ~100 torr.


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