The Iceni
Medical Moderator
Hi Jorgen,
I am sure Dr Deco, many other more knowlegible contributers to this forum and indeed any physiology text book can answer your questions far better than me (a mere interloper) but this is an explanation in what I hope are layman's terms.
As you say, normal respiration is stimulated, not by low Oxygen levels but by excess Carbon Dioxide above a trigger level. This is detected by chemoreceptors, which are thought to measure the local acidity produced by free Carbon Dioxide molecules which react with water to produce Carbonic Acid. These receptors are found in the respiratory centre of the mid brain.
Even at rest the body continuously uses Oxygen and produces Carbon Dioxide so this makes considerable sense as Oxygen itself is effectively undetectable as it does not cause a change to the acidity of the blood, while for each molecule of oxygen used a molecule of potentially acidic Carbon Dioxide is produced. If there appears to be "excess" Carbon Dioxide (or any acidity) in the blood the receptors interprete this as having been produced by the consumption of Oxygen, molecule for molecule, so more Oxygen must be needed and the only way to get it is to take deep breath! (In fact, low Oxygen levels can stimulate respiration but only in certain circumstances)
Hyperventilation is very much a relative term. What I meant was that compared with the amount of gas moved by the lungs at any given level of activity on the surface, when submerged, humans move more gas.
If you hold your breath on the surface there comes a point when the Carbon Dioxide level reaches the point at which the reflex stimulus to take a breath is overwhelming and overrides your conscious attempts to hold your breath any longer so you have no choice but to take a breath.
When you hyperventilate you remove more Carbon Dioxide from the body than is produced but do not take on more Oxygen (as the haemoglobin is always almost fully saturated as it leaves the lungs in any case). If you were, consciously, to hold your breath after a period of such excesssive hyperventilation, as before Oxygen continues to be consumed so after only a short while it's level falls below that needed for normal brain function and this arrives BEFORE the level of Carbon Dioxide rises sufficently to stimulate reflex respiration and overide concious breathholding.
Unconciousness results. Active breathholding ceases; Normal breathing resumes; The Oxygen supply is restored and conciousness returns ( at least on land!)
Scuba diving is undertaken in an alien environment. At the most basic level the equiment produces "dead space" which causes some retention of Carbon Dioxide and so stimulates a relative hyperventilation but there are other causes, such as the reaction to cold and to immersion itself. Because od this divers are routinely exposed to higher levels of Carbon Dioxide so the brain's trigger level is gradually raised as the respiratory center "learns" the higher average blood acidity and sets new trigger level so experienced divers, as a group, can hold their breath for longer than non-divers.
Free divers hyperventilate before they dive so they can breath-hold for longer and so dive deeper. This breathholding causes CO2 to build up, of course, they therefore routinely expose their bodies to very high levels of Cardon Dioxide and reset the trigger even higher. As you can imagine, they run the considerable risks of hypoxia as they return to the surface from each dive and many suffer
"shallow water blackout"
which is extremely dangerous. :drown:
At a very technical level I discussed some of the further effects the Carbon Dioxide molecule, with respect to Oxygen toxicity on this forum at
http://www.scubaboard.com/t4425/s.html
Headache is one feature of fatigue but I suspoect in your case that is not the main cause.
I hope this answers you questions satisfactorily.
I am sure Dr Deco, many other more knowlegible contributers to this forum and indeed any physiology text book can answer your questions far better than me (a mere interloper) but this is an explanation in what I hope are layman's terms.
As you say, normal respiration is stimulated, not by low Oxygen levels but by excess Carbon Dioxide above a trigger level. This is detected by chemoreceptors, which are thought to measure the local acidity produced by free Carbon Dioxide molecules which react with water to produce Carbonic Acid. These receptors are found in the respiratory centre of the mid brain.
Even at rest the body continuously uses Oxygen and produces Carbon Dioxide so this makes considerable sense as Oxygen itself is effectively undetectable as it does not cause a change to the acidity of the blood, while for each molecule of oxygen used a molecule of potentially acidic Carbon Dioxide is produced. If there appears to be "excess" Carbon Dioxide (or any acidity) in the blood the receptors interprete this as having been produced by the consumption of Oxygen, molecule for molecule, so more Oxygen must be needed and the only way to get it is to take deep breath! (In fact, low Oxygen levels can stimulate respiration but only in certain circumstances)
Hyperventilation is very much a relative term. What I meant was that compared with the amount of gas moved by the lungs at any given level of activity on the surface, when submerged, humans move more gas.
If you hold your breath on the surface there comes a point when the Carbon Dioxide level reaches the point at which the reflex stimulus to take a breath is overwhelming and overrides your conscious attempts to hold your breath any longer so you have no choice but to take a breath.
When you hyperventilate you remove more Carbon Dioxide from the body than is produced but do not take on more Oxygen (as the haemoglobin is always almost fully saturated as it leaves the lungs in any case). If you were, consciously, to hold your breath after a period of such excesssive hyperventilation, as before Oxygen continues to be consumed so after only a short while it's level falls below that needed for normal brain function and this arrives BEFORE the level of Carbon Dioxide rises sufficently to stimulate reflex respiration and overide concious breathholding.
Unconciousness results. Active breathholding ceases; Normal breathing resumes; The Oxygen supply is restored and conciousness returns ( at least on land!)
Scuba diving is undertaken in an alien environment. At the most basic level the equiment produces "dead space" which causes some retention of Carbon Dioxide and so stimulates a relative hyperventilation but there are other causes, such as the reaction to cold and to immersion itself. Because od this divers are routinely exposed to higher levels of Carbon Dioxide so the brain's trigger level is gradually raised as the respiratory center "learns" the higher average blood acidity and sets new trigger level so experienced divers, as a group, can hold their breath for longer than non-divers.
Free divers hyperventilate before they dive so they can breath-hold for longer and so dive deeper. This breathholding causes CO2 to build up, of course, they therefore routinely expose their bodies to very high levels of Cardon Dioxide and reset the trigger even higher. As you can imagine, they run the considerable risks of hypoxia as they return to the surface from each dive and many suffer
"shallow water blackout"
which is extremely dangerous. :drown:
At a very technical level I discussed some of the further effects the Carbon Dioxide molecule, with respect to Oxygen toxicity on this forum at
http://www.scubaboard.com/t4425/s.html
Headache is one feature of fatigue but I suspoect in your case that is not the main cause.
I hope this answers you questions satisfactorily.
