Lung or diaphram muscle memory

[Jake]

I'm curious about something:

If I inhale only half-way while sitting here at my desk near sea level, my body gets the impression that I need to breathe deeper. If I don't, I feel uncomfortable until I take a full breath.

Let's assume I do the same thing at a depth of 66 feet while breathing air. Even though I am only taking half of a breath, I am most likely fulfilling my body's oxygen requirements because of the higher partial pressures involved. However, my brain still sends sort of a discomfort signal indicating that I haven't taken a good enough breath.

Is this the result of simple muscle memory of the lungs and diaphram (that is, lungs expanded to capacity equals good to my brain), or is there a more complex respiratory/circulatory reason for this?

Anyone care to hazard a guess? My guess is that it's the former; a full breath is better for oxygen and carbon dioxide exchange and reduces the risk of hypoxia. Therefore, it's a survival instinct unrelated to a direct chemical trigger (like CO2 buildup triggering breathing response).

This seems supported by rebreather accidents in which damaged equipment has resulted in black outs as a result of CO2 buildup in the loop. The diver felt that they were breathing normally as a result of full breaths, and it was only until hypoxia occurred that the system became overtly aware of the gas problem. I've never tried it, but I would imagine my body would still signal a problem with a half breath even if I were breathing 100% O2....

Just curious. Thanks!

 

[TSandM]

The body tightly regulates the blood level of carbon dioxide, in large part because it has significant effects on the pH of the blood. By far and away the major determinant of blood CO2 is the effective volume of air exchanged per minute in the lungs. By effective volume, I mean the part that actually gets to the gas exchange portion of the lung structure -- Gas in the trachea (windpipe) and large bronchi doesn't exchange, and is called "dead space".

If you only inhale a third of the way, you have sucked the air in your dead space down into your lungs, but not really brought in much new, low CO2/highO2 gas. Therefore, you aren't clearing your CO2 well, and you will quickly feel a need to breathe more. (The body has very poor ability to sense low O2, which can become a problem with rebreathers.)

There are stretch receptors in the lung lining, but if I remember correctly, they primarily function to limit lung expansion, not to mandate it.

 

[Sideband]

TSandM hit it on the head.

Joe

 

[archman]

I thought this was covered in PADI Basic OW?

maybe it's just my own classes...

 

[shakeybrainsurgeon]

I agree that carbon dioxide and not oxygen is the primary stimulus for respiratory rate, but not the only determinant of respiratory volume. There is a thing called the sigh reflex that forces us to fully expand our lungs every so many breaths. This prevents atelectasis, the collapse of airspaces. If we suppress that reflex, by using breath volume to control buoyancy, we get uncomfortable. The sigh reflex seems to be a mechanical phenomenon, not related to gas exchange.

 

[spectrum]

I agree that carbon dioxide and not oxygen is the primary stimulus for respiratory rate, but not the only determinant of respiratory volume. There is a thing called the sigh reflex that forces us to fully expand our lungs every so many breaths. This prevents atelectasis, the collapse of airspaces. If we suppress that reflex, by using breath volume to control buoyancy, we get uncomfortable. The sigh reflex seems to be a mechanical phenomenon, not related to gas exchange.

That's what I was going to describe but I didn't know it was called the "sigh reflex". I notice this If I pay attention to my breathing while falling asleep.

So slow deep breaths are the way to go. Time them right and the buoyancy changes should not make you porpoise up and down too much. Modified lung volume is fine for temporaty excursions from your naturally buoyant depth but for prolonged excursions adjust your BC/Wing contents and breath deeply.

Pete

 

[Jake]

I thought this was covered in PADI Basic OW?

maybe it's just my own classes...

You covered partial gas pressures and autonomic gas exchanges in your open water class? Either you had a physician for an instructor, or I asked a badly worded question. Probably the latter.

 

[archman]

You covered partial gas pressures and autonomic gas exchanges in your open water class? Either you had a physician for an instructor, or I asked a badly worded question. Probably the latter.

1. Partial gas pressures. Slightly.
2. "Autonomic gas exchange". We explain the CO2 accumulation pitfalls derived from shallow breathing. Like TS&M said, minus the details about dead spaces.

 

[anakin]

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