From what I know, there doesn't seem to be a huge difference in the rate of oxygen decrease in the blood between (typical, healthy) men and women.
Here's an interesting reference I came across:
Energy expenditure (EE) during treadmill walking under normal conditions (normobaric normoxia, 21% O[2] ) and moderate hypoxia (13% O[2] ) was measured. Ten healthy young men and ten healthy young women walked on a level (0°) gradient a range ...
www.ncbi.nlm.nih.gov
Also, this is worth reading:
Women may experience greater pulmonary gas exchange impairment during exercise than men. To test this we used the multiple inert gas elimination technique to study eight women and seven men matched for age, height and V˙[O[2] max] (∼48 ...
www.ncbi.nlm.nih.gov
However, that does
not mean there are no gender differences (the above articles make that clear, as do others below) - it just seems that, even though there are differences, the way it all works out means the oxygen use tends to pretty much balance out overall.
Here's some more research that demonstrates both of these points:
ched men. Additionally, ovarian hormones, namely progesterone and estrogen, are known to modify and influence the pulmonary system. These differences may have an effect on airway responsiveness, ventilation, respiratory muscle work, and pulmonary gas exchange during exercise. Recent evidence...
journals.lww.com
. Women use a greater fraction of their ventilatory reserve and have a higher metabolic cost of breathing....
journals.lww.com
The purpose of this study was to determine whether there are sex differences in the cardiorespiratory and sympathetic neurocirculatory responses to central, peripheral, and combined central and peripheral chemoreflex activation. Ten women (29 ± 6 ...
www.ncbi.nlm.nih.gov
Abstract. CO2 single breaths have been performed in 7 men and 7 women in conditions of normoxia (FICO2≈0.13; FIO2≈0.21; FIN2 6≈ 0.66) and of hyperoxia (FICO2≈0.13; FIO2≈0.87). Ventilatory responses of the subjects and modifications of breathing pattern in the course of the CO2 tests were also...
karger.com
This study used a modified CO(2) rebreathing procedure to examine the effect of gender on the chemoreflex control of breathing during wakefulness in healthy men (n = 14) and women (n = 14). Women were tested in the follicular phase of the menstrual cycle. During rebreathing trials, subjects...
pubmed.ncbi.nlm.nih.gov
Anyway, I hope that's some interesting bedtime reading...
One further thing...
The
other recent thread here on iron deficiency did make me realise that could be another link with fainting - so some kind of anaemic condition would likely also having some effect on susceptibility to blackout (since it's clearly linked to oxygen transport & delivery in the blood).
So I guess if you are anaemic, which could have fainting as a potential symptom, then I think it would be wise to be somewhat more careful when freediving.
I guess all of this means if you do find you are prone to fainting, you should see if there could be some kind of reason for it (e.g. cardiac-related, or anaemia, or something else I've not considered offhand).
But, to repeat what I said above, unless you're an experienced breathholder, and if you are in decent health (i.e. not suffering from a condition like above) it's unlikely you will hold your breath long enough to get so near the point of blackout. (Again, you can do some tests on that for yourself via some dry static breathholds with a pulse oximeter.)
And I'll also say again, for the benefit of anyone else who may happen to read this:
If you are diving with a well-trained buddy then a blackout isn't a particularly big issue. (Assuming it's not happening regularly!)
Conversely, though, if you don't then it is...