Quiz - 26 - Diving Knowledge Workbook - Diving Physiology

[spoolin01]

 

[Seaweed Doc]

Interesting discussion about differential vs. difference. I teach plant physiology, and the concept of differential was clear to me. E.g., when modelling diffusion we often talk about "concentration gradients" and "charge gradients."

Actually the term "gradient" here is used slightly improperly, as the pressure gradient is the ratio between the pressure difference and the distance "d" of the two points where you did measure the pressure:
grad(p) = (p1-p2)/d
Well, for being pedantic, the gradient is obtained as the limit of the above ratio when d is reduced towards zero...
So the correct wording should have been "pressure difference", not "pressure gradient".
Perhaps this confused some people, also because the term "gradient" is not really of common use outside a physics classroom. "Difference" is much simpler, and maintains the correct meaning...

Interesting discussion about differential vs. difference. I teach plant physiology, and the concept of gradients was clear to me. E.g., when modelling diffusion we often talk about "concentration gradients" and "charge gradients" across membranes.

Using your equation for gradient above, if d and p1 are held constant, but p2 is decreased, can't we say the gradient increased? (p1 = partial pressure of N in tissue, p2 = partial pressure of N in alveoli, d is whatever distance estimate you're using)

 

[Angelo Farina]

Not quite. The Bühlmann (and VPM) model doesn't determine a "safe rate of offgassing" but a safe pressure difference between the pressure of nitrogen (and helium) in the tissue and the ambient pressure at the diver's depth.
Yes, increasing the pressure difference (by ascending to a shallower depth) will increase that rate and exceed the tolerable pressure difference limit, but the rate of offgassing can also be increased safely without ascending by breathing a gas with less nitrogen and helium (and more oxygen). The distinction is subtle but is incredibly important when it comes to choosing deco gasses.

The limit for avoiding DCS is set on the "pressure ratio", not a "pressure difference" between the pressure of gas inside the tissue and the ambient pressure...
That is, p(tissue)/p(ambient), not p(tissue)-p(ambient)...

 

[Angelo Farina]

View attachment 593799

That is in an homogenous medium. But inside the alveoli, we have a liquid in contact with a gas. The Nitrogen leaves the liquid and goes into the gas driven by the pressure difference, NOT by the gradient...
Henry's law, not Fick's law...

 

[spoolin01]

That is in an homogenous medium. But inside the alveoli, we have a liquid in contact with a gas. The Nitrogen leaves the liquid and goes into the gas driven by the pressure difference, NOT by the gradient...
Henry's law, not Fick's law...

It was a technical example of the colloquial generality that except where more narrowly defined by some particular discipline, gradient and difference can be fairly interchangeable. Beyond that though, the idea that either the tissue compartment model for gas perfusion and DCS risk, or however else you might imagine it works physiologically, does not square with even a more restrictive definition of gradient seems hard to defend. You have physical separation at varying degrees, and perfusion driven in part by concentration differences. That sounds like a gradient (not that you can't also find plenty of examples describing gradient across a membrane). Nothing in the question confined the applicability of the gradient concept entirely to the alveolar interface, even if you think that Henry's law for some reason precludes applicability of the term.

 

[Angelo Farina]

It was a technical example of the colloquial generality that except where more narrowly defined by some particular discipline, gradient and difference can be fairly interchangeable. Beyond that though, the idea that either the tissue compartment model for gas perfusion and DCS risk, or however else you might imagine it works physiologically, does not square with even a more restrictive definition of gradient seems hard to defend. You have physical separation at varying degrees, and perfusion driven in part by concentration differences. That sounds like a gradient (not that you can't also find plenty of examples describing gradient across a membrane). Nothing in the question confined the applicability of the gradient concept entirely to the alveolar interface, even if you think that Henry's law for some reason precludes applicability of the term.

I understand this entirely, but my point is another: a gradient is a vectorial differential operator, and understanding what a "gradient" is, requires to have studied some advanced physics and maths. Most people do not have this in their background.
Of course I have nothing against educating people, and explaining them the Henry and Fick laws, and making them understanding what a gradient is. But is this really necessary for explaining gas exchanges in our lungs?
In my opinion, no. The concept of DIFFERENCE is known to everyone, is very simple and effective. So why people insist talking about gradient, when the simpler concept of "difference" is better understood, and explains perfectly what's happening?
Why making the things more complex than necessary?
Please, re-read the PADI official answer in the Spoiler:
"the difference in the pressure within a liquid (gas tension) and the gas in contact with it is referred to as the pressure gradient. When this condition occurs, the gas within the liquid and that in contact with the liquid will attempt to equalize. A large pressure gradient is said to create a high driving force, meaning the gas exchange will take place rapidly"
So the DIFFERENCE is the driving force, not the gradient...
The term "gradient" is introduced as equivalent to difference (which of course is wrong, the units are different: Pa for the difference, Pa/m for the gradient). Wasn't simpler just saying that the difference between the two pressures is the driving force?

 

[spoolin01]

Gradient is a perfectly commonplace word, and while commonplace words can also become more narrowly defined for precision in esoteric contexts, that doesn't make other uses wrong. If it's understood, it's by definition a correct usage, and time only cements that status. Perhaps someone with knowledge of the etymology of the word could shed light on this divergence you are refereeing! What usage has this word seen back through history. Who gets to claim ownership. Some technical terms become misused as they become more colloquially common (parameter ~= variable?). I've even just read that soon we may have to accept that "literally" has come to mean "figuratively". Thank you, American educational system. But more often I think it goes the other way as disciplines develop. Maybe the Germans have a better way of handling this. How is it done in Italian?

I still am not sure what your quarrel with this usage is. What aspect of the system or model here does not meet your definition? Isn't the presence of nitrogen gradients throughout the tissues the basis of on- and off-gassing models? Is it the jump from tissue to gas (is that even a good way to describe what happens at the molecular level)? The PADI question doesn't inherently narrow the consideration to that aspect I don't think. What is it about either Henry or Fick that precludes applicability of the term gradient to real systems?

 

[Angelo Farina]

Sorry, I suppose this is a language offset. I am Italian...
In Italy absolutely no one ever uses the term "gradiente" in normal language. It is a scientific word, you can only hear this term used in an advanced physics classroom.
And for most people, years after high school or university, if you ask what is a "gradiente" the answer you get is usually that it is a vector.
It is never associated with the concept of difference.
I studied in UK, and also there I do not remember of hearing it used outside the university.
Perhaps in US it is different, and people use commonly "gradient" as a substitute for "difference".
It is always very useful for me to learn these subtle linguistic differences.
And I want yo thank again @Pedro Burrito for these quizzes, they are really stimulating!

 

[spoolin01]

I was schooled with a physical science orientation, and continued that through university and career. It is possible the word seems more common to me than it really is in English, but that is not my expectation.

 

[-Sven-]

I‘m from Germany and here no one outside of university uses the term ‚gradient‘ and in university only science people use it, not in the arts departments.