Quiz - 13 - Diving Knowledge Workbook - Diving Physiology

[Pedro Burrito]

From the Diving Physiology Section of the PADI Diving Knowledge Workbook Version 2.02 © PADI 2009:

Objective 2.4 - Explain the physiological mechanism that causes a "shallow-water blackout" and why this condition usually occurs during ascent rather than descent.

Question 3

Blackout normally occurs on ascent because:

a. the diver's energy expenditure increases.

b. the partial pressure of the alveolar oxygen rapidly decreases.

c. the partial pressure of the alveolar carbon dioxide rapidly decreases.

d. the decreasing pressure causes a carotid sinus reflex. ​

I will post a daily question from my exams to help newer divers and to encourage more experienced divers to interact gracefully and helpfully with the newer divers.

Reminder - this is a post in the Basic Forum and it is a green zone. Please be nice and on topic.

Thank you for your patience while we try to give people something to discuss other than Covid-19 and/or Politics. I will post the answer covered by the spoiler tag later today.

 

[TMHeimer]

Think I finally missed one.

 

[Storker]

Good thing I didn't follow up the previous quiz question like I almost did. That would've spoiled this one.

 

[Angelo Farina]

We are again dealing with free diving, I see...

 

[Storker]

We are again dealing with free diving, I see...

And?

Free diving, scuba diving. It's all diving. And the same physics/physiology.

 

[Angelo Farina]

And?

Free diving, scuba diving. It's all diving. And the same physics/physiology.

Well, at least here in Italy freediving is more than "just diving". It is a philosophy, it is a religion, it is a dream. Our generation was grown under the myths of Raimondo Bucher, Enzo Majorca, Jacques Majol, Andrea Pellizzari.
Their vision of the man and of the world had a great influence. Scuba diving is just employing some mechanical device for breathing underwater, freediving was the way of discovering the marine mammal which is hidden in our DNA.
Reducing freediving to just physics/physiology is truly reductive, in my opinion.
I was a freediver, before giving up, many years ago. Scuba diving, for me, is something intrinsically inferior. Purely mechanics, not a mystic experience.
But this vision is impossible to understand for people who are "just" scuba divers...

 

[Storker]

at least here in Italy freediving is more than "just diving". It is a philosophy, it is a religion, it is a dream.

Well, yeah. A lot of the freedivers I've met have a more... alternative... outlook on the world than I have. They still venture underwater, though. So they're divers. While I might disagree rather fundamentally on their metaphysical outlook on the world, we're still limited by the same physics and physiology.

 

[Dizzi Lizzi]

Whoops, I think in rereading this I got it wrong...

 

[Pedro Burrito]

Spoiler: Answer

b. the partial pressure of the alveolar oxygen rapidly decreases.

At depth, the elevated partial pressure of alveolar oxygen can be sufficient to meet the tissues' demand. However, when the diver ascends, the oxygen partial pressure quickly drops (for example, an ascent from 10 meters/33 feet will result in a reduction of the partial pressure by 50%). Now, suppose the diver barely had enough oxygen at depth to remain conscious and functional. A severe drop would be enough to decrease the level to a point the body could not tolerate. So, although the diver could continue to function at depth, he/she will black out as he/she ascends to the surface due to abrupt decrease in the oxygen partial pressure.

 

[Angelo Farina]

While I might disagree rather fundamentally on their metaphysical outlook on the world, we're still limited by the same physics and physiology.

This is what doctors and engineers were telling to Majorca and Majol, predicting physiologic limits to freediving at 40m, then at 50, and so on...
And these recordmen were systematically exceeding these limits, down to 100m, where physics was predicting immediate death due to the pressure crunching the chest...
During those years, Jacques Majol developed his theory about the "homo delphinus".
Watch the film Le Grand Bleu by Luc Besson...
A freediver practicing deep diving with this approach is not just diving, is touching the depth of his soul in an experience which is comparable only to some mystic experiences provided by zen meditation. The technicalities of diving are almost irrelevant, the focus is about interaction between your brain (or perhaps your soul) and your body.