efattah said:
Some comments:
- When I breathe into a plastic bag, my apnea reflex DOES kick in; everything happens as usual, including contractions
- Who says that contracting the spleen to 60-70% of its initial volume is fully contracted? Who says a trained diver can't contract it to 20% of its initial volume?
- Who says that the spleen only offers an 8% increase in RBC's? That depends on the size of your spleen. Some people with hypoxic disorders have HUGE spleens, several kilograms in size. Who says that an apnea diver cannot 'train' himself to develop such a huge spleen?
- Low oxygen DOES produce a breathing reflex, this is very easy to measure if you have a transcutaneous O2/CO2 monitor and try apneas with different amounts of hyperventilation. In fact, among my friends we routinely talk about how high CO2 contractions feel very different from low O2 contractions.
-Several studies claim, according to empirical evidence presented, that the actual act of holding your breath is needed for e.g. brachycardia (the diving response) to kick in. Quoting for example an article by Lindholm et al (1999) in Journal of Applied Physiology 87:2122-2127: "The marked differences between the cardiovascular responses to gradually increased hypoxia-hypercapnia with and without breathing movements support and extend the previously established notion that the respiratory arrest per se is required to elicit the cardiovascular responses to apnea in resting humans. Thus also during exercise, when additional inputs such as central command and proprioceptive afferents contribute to HR [heart rate] control, the presence or absence of regular respiratory movements appear to be critical for the maintenance of HR during a hypoxic period." Eric, if you're able to get your diving response to kick in even if you breath into a plastic bag, maybe you have developed, as a veteran freediver, a skill to activate the dive response upon willing for it. It would be interesting to see you present some empirical data on how e.g. your heart rate develops as you breathe into a bag and as you hold your breath. Then we could see whether the degree of brachycardia is the same in boh cases if you're the subject.
-The Croatian national apnea team, however good they are, don't seem to be able to contract their spleens more than 30-40%. This actually makes sense, since the primary function of the spleen is to act as a filter against pathogenic organisms in the bloodstream. The RBC storage function is only a secondary job. However, even if you managed to furher contract your spleen, let's say by up to 80%, this would only double the amount of RBC released from 8 to 16% of your total RBC count, which is still not very much, since you would get merely a 8% increase in your total oxygen storage capability.
-The spleen offers a 8% RBC increase in an average person, who does not, I am sure, rergularly and willingly hold his breath for long periods of time.
Thus the amount of RBC stored in the spleen may indeed increase with apnea training, let's hope so. One thing depressingly suggests that this will not happen, however. Aerobic athletes press the human endurance to the max and a huge RBC store is very advantageous in these sports also. Yet elite long distance runners for example do not have expanded spleens. Horses increase their aerobic performance by spleen contraction, since they have big spleens with lots of stored RBC, see how analogous this is to the dive response?
-Very low SaO2 does indeed fire the breathing reflex, since sensory organs in the walls of major veins (e.g. aortic bodies) react to very low SaO2. These sensors cannot however be the activators of the dive response or the spleen contraction, since both event happen long before very low SaO2 is reached.
ADR,
You will not see the spleen contraction as a spike in the SaO2 graph, since the spleen contracts in the beginning of apnea already, when the SaO2 is still in the normal level.
DeepThought,
In the article by Bakovic et al they conclude: "We have shown that, in simulated apnea diving, the reduction of the spleen volume is fast with unchanged flow in the splenic artery. This rules out the possibility of passive collapse and shows that in apnea diving the spleen is not the part of the periphery with reduced blood flow secondary to elevated sympathetic tone. The spleen contracts immediately on the onset of apnea, in parallel with simultaneous increase in the heart rate, when arterial blood gases are yet unaffected. This rapidity of the splenic response to apnea diving argues
against peripheral triggers and favors the existence of a centrally mediated feed-forward mechanism. The splenic contraction was only moderately greater
in trained than in untrained persons, which suggests that apnea training does not much influence the ability of spleen to participate in the diving response."
And we do not KNOW whether the human splenic RBCs are 100% oxygenated. We just assume so, until proven otherwise.
