Alveolar respiration efficiency

[svetliy]

I was recently reading through the "Manual" and 2 numbers caught my attention 21% and 16%, the amount of O2 in the normal air and exhaled air. I was wondering if there is any method to reduce these 16% - to make your alveoli extract the air more efficiently?and if there is a reason for their inefficient work?Does it have something to do with the fact that not all of inhaled air is used for respiration? I have read that alveolar ventilation depends on tidal volume and the rate of breathing/but what about actual respiration?

[trux]

This is quite well explained here: Gas exchange - Wikipedia, the free encyclopedia

Basically, the oxygen in lungs diffuses through the alveolar wall into the alveolar arteries. To enable the diffusion, the PAO2 (partial alveolar pressure of oxygen) must be higher than PvO2 (partial venous pressure of oxygen). And since the PvO2 cannot drop too much without you blacking out, in the moment the level of oxygen in your lungs drops close to the minimal PvO2, the diffusion is inefficient and the blood gets little new oxygen. Furthermore, the inefficiency is amplified by increasing CO2 which reduces the affinity of oxygen to hemoglobin.

In other words, you simply cannot spend all oxygen in your lungs.

[svetliy]

Thanks, trux! I got the point. Can you please also tell me if packing considerably increases the air pressure inside the lungs (i believe there must be a limit to rib cage expansion and max lung volume)? Or lungs collapse before any considerate pressure change happens?Or it is physically impossible?Thank you in advance.

[trux]

I recommend having a look at this document: it gives you quit good details:
Transpulmonary pressures and lung mechanics with glossopharyngeal insufflation and exsufflation beyond normal lung volumes in competitive breath-hold divers -- Loring et al. 102 (3): 841 -- Journal of Applied Physiology

A previous similar study by G. Simpson, J. Ferns, and Stephan Murat concluded that packing paradoxically does not increase the transpulmonary pressure, but the above mentioned study demonstrates the opposite, and claims the older one is inconsistent with pulmonary mechanics.

The measured values are quite individual, but one measured subject was able of packing more than 4 liters above his TLC (to 181% of his predicted TLC). The measured water vapor pressure raised from atmospheric 64 cmH2O to 80 cmH2O - that's increase of some 30%. At this subject though, pneumomediastinum (lung damage) was diagnosed. Usually, when packing, the percentual increase of TLC, and hence of the pressure is lower than that.

Roughly half of the packed air expands the lungs beyond their TLC volume, and the rest increases the pressure. What is more important though from the practical point of view for the theoretically achievable breath-hold time, is not really the pressure inccrease, but rather the increase of oxygen stores - at the measured subjects (all competitive freedivers) the increase was between 32% and 79%, which is rather huge, but on the other hand you have to keep on mind that packing has also negative effects - besides the risk of lung squeeze at depth disciplines, it may suppress or delay the diving response, increases the heart rate, adds to discomfort and muscle tension, etc. So although the oxygen stores are considerably higher, its consumption may rise considerably too, hence the outcome is usually lower than the theoretic numbers indicate.

PS: more interesting documents on packing can be found here: packing @ APNEA.cz

[svetliy]

Thank you for the informative replies and for the links,trux! I will have a careful read through them. I believe i will have much more questions along the way-human body and its mechanics is just one huge fascination machine!

[chrismar]

A study done on me and some fellow club members measured an increase in pulmonary pressure. I posted the full spirometry results elsewhere on the site. I believe the increase in pressure was not enought to account for the extra volume, which suggests that both lung expansion and pressurisation is occurring.