Any physicists out there?

Here's something thats bothering me related to work. Sad, eh? You would have thought I could find someone who could answer this without any doubt, wouldnt you

With my METI Human Patient Simulator I can show students how increasing deadspace affects respiration (amongst other things). This is most easily thought of as increasing the volume by using a stupidly long snorkel tube.

Now the classic thinking on snorkel tubes is that they can only be so long (about 40cm) as:
1. They increase deadspace
2. Too deep and you cant inhale anyhow due to pressure on chest cavity
3. To wide and you cant easily clear them.

But focussing on the deadspace issue, we normally say that its the increase in deadspace volume that causes problems breathing through tube. Dont try it but believe me it will. But although you cant breathe properly through a 3m tube that is say 1cm wide as deadspace is then an extra 300ml (your own is about 200ml), obviously you could have a tube 30cm long and 10cm wide, which would be fine but the volume is the same.
So it must surely be more complex than just volume, with length being the critical factor below a certain tube diameter. And a diameter gets very small I would imagine so would length. In part this must be due to friction on surface versus flow but that must only be part of it.

So basically what is it? Length or volume? What's the relationship?

Gawd....

Having said that apparently some say the trachea of the giraffe is very narrow, hence reducing deadspace volume. On the other hand it has other adaptations in its ventilatory system to deal with increase deadspace, so it breathes harder anyhow.

Sorry you`ve lost me... I know this this is a serious issue for you but i`m more used to answering the length v. girth question, not the length v. volume

Originally Posted by Speedy Claire

Sorry you`ve lost me... I know this this is a serious issue for you but i`m more used to answering the length v. girth question, not the length v. volume

Well that does come into it

lol well i could quite happily enter into a debate on the length v. girth but feel that won`t help you much.

However, not sure if it would help but one of my friends works in a sleep apnoea and respiratory disorder unit in a research capacity and she works alongside some professor or other. I could always ask her to have a word with him

I think you'll find with a wide short tube turbulence at the large surface will lead to sufficient mixing of the "dead" air with the fresh air. Therefore the actual "dead" volume is in fact very small. With a long thin tube this becomes much less of an effect. I'm not sure at what L/D ratio, or which other dimensionless group actually determines the exact cut off point, but there is bound to be one.

Any help for you?

Edit: I'm a chemical engineer and have a little knowledge of fluid dynamics but am no expert.

Originally Posted by Tim in Belgium

II'm not sure at what L/D ratio, or which other dimensionless group actually determines the exact cut off point, but there is bound to be one.

Yes the mixing and the point that there will be a cut off somehow determined makes sense. But volume per se is definitely misleading, so I am happy with that.
Its going to be a function of surface for exchange with length so L and D as you say.

And Claire, I guess surface area is v important

This is interesting and perhaps part of the answer is in the question? But what exactly do you mean by deadspace? It could be a description of the fluid dynamics or a literal term about that part which is alien to the body. If your looking for the optimum ratio between L and D in the fluid dynamic sense then surely then answer is in the ratio achieved through evolution and already seen within the human trachea? That's what evolution does, finds the best solution. If your looking to work around that because you have to, i.e emergency then clearly you have to be inventive. So, what adaptations does the giraffe have for example that could be exploited to help people in need?

Originally Posted by Dr Rich

Here's someth...

*Sorry, it's the student response.*

[yorkie_chris]

Originally Posted by Dr Rich

But although you cant breathe properly through a 3m tube that is say 1cm wide as deadspace is then an extra 300ml (your own is about 200ml), obviously you could have a tube 30cm long and 10cm wide, which would be fine but the volume is the same

IMO gas mixing is a factor in very wide tubes, meaning a 10cm wide tube wouldn't actually act as deadspace.
Also what are you meaning 3cm wide etc, surely you mean cross section (cm^2 in that case)

I can't think of how you'd calculate gas mixing as a factor, it's bound to be very dependant on the "nozzle" giving some degree of swirl to the gas into the tube, as well as velocity, humidity etc.

IMO you need to consider, what is the capacity of human lungs? What %age reduction in "swept volume" (to butcher an engine term..) are you actually causing? If for example you are getting 300cc less gas into a 3000cc lung, then that 10% is probably not a factor at rest, but would be noticeable at higher workload.

The long thin tube is the real bad one, the pressure to overcome viscosity is quite significant, though the total volume will not change, work will have to be done to inhale and exhale.
Velocity is the key term here as it's a factor in the reynolds number of the flow, which is a way to tell whether the flow will be laminar or turbulent, obviously a turbulent boundary layer condition through the whole pipe is going to have much more friction, ReN is also dependant on diameter of the pipe.

If you wanted to experiment with pressure loss then you could easily make a snorkel with negligble, simply 2 tubes, with a non return valve on each, like a rebreather loop, without the rebreather