wrote:

Hey dave,
Ever swam under water?
Ever hypervent your self before taking that last breath so you can stay
down longer?
What would you call that?


You are eliminating the Carbon Dioxide in your body to VERY low levels,
which helps forestall the URGE to breath until they build back up to
higher levels.

Hyperventilation does not increase oxygen capacity over and above what
normal breathing does. It simply clears out waste products. All of this
is again a function of the partial pressures and pressure gradients that
were discussed elsewhere in this thread.

On room air, your pO2 in arterial blood is in the area of 80-100 mmhg.
Your pC02 is in the neighborhood of 40 mmhg. Oxygen Saturation is in the
90-100% range.

Mixed venous blood gases (which are sampled from the pulmonary artery,
in patients with the proper monitoring equipment) on room air tends to
be in the 40-50 mmHg range. This corresponds with about ONE of the FOUR
oxygen molecules on each hemoglobin molecule being extracted, and a
saturation in the 60-75% range. Without a fresh supply of oxygen in the
lungs to extract from, this "desaturated" blood makes a second trip
around, and because of the laws of nature, it is even more difficult for
the remaining oxygen to be extracted. For practical purposes, its
essentially NOT going to release more than two of the four
hemoglobin-bound oxygen molecules. Your arterial oxygen saturation gets
too low and you will end up getting confused, euphoric, or just unconscious.

Hyperventilation in an otherwise healthy individual will not increase
the p02, but can/will drive the pCO2 in arterial blood down to the 20's.
Prolonged hyperventilation will cause vasoconstriction in the brain
(as well as everywhere else), resulting in lightheadedness, dizziness,
cramping in the fingers/toes, and chest pain. I deal with persons on
breathing machines EVERY DAY for a living. If their oxygen level is low,
we dont turn up the rate or volume, we turn up the oxygen concentration.
If their CO2 level is high, THEN we turn up rate/volume to increase
the amount of VENTILATION. Ventilation is the removal of waste products.
Oxygenation is the delivery of oxygen. They are TWO completely different
functions, and not very related to each other, despite the fact they are
happening in the same place at the same time.

It is still possible to pass out from Hypoxia without building up CO2
levels back to the amount needed to trigger the chemoreceptors to tell
your brain "I GOTTA BREATHE". It has been the cause of more than one
drowning/diving accident. If you want further reference on this, a
knowledgeable professional diver, an aerospace medicine MD or a
hyperbaric technician should be able to validate this information.

Dave