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 :lol:

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....

Re: Any physicists out there?
 

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.

:confused:

Re: Any physicists out there?
 

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 :confused:

Re: Any physicists out there?
 

Well that does come into it :lol:

Re: Any physicists out there?
 

lol :D 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

Re: Any physicists out there?
 

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.

Re: Any physicists out there?
 

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 :lol:

Re: Any physicists out there?
 

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?

Re: Any physicists out there?
 

:smt015

*Sorry, it's the student response.*:p

Re: Any physicists out there?
 

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

Re: Any physicists out there?
 

Forgot to mention ... you silly medic, physicists are no good in the real world, you want an engineer :-D

Re: Any physicists out there?
 

the volume of those two tubes is not the same though, note that the volume of a cylinder (or your circular tube) is proportional to it's length, but proportional to the square of it's diameter

volume of your tube equals length multiplied by pi multiplied by the radius squared

therefore tube one is 3m long and has an internal diameter (for argument's sake) of 0.005m (half of 1cm, remember radius not diameter) - the volume is 3.142 * 3 * (0.005)^2 = 0.000236cu.m

tube two has length 0.3m and internal diameter 0.1m, therefore the volume is 0.00236cu.m

so tube two actually has ten times the volume of tube one...

i'm not understanding what you mean with dead space however, if you want to explain it to a physicist with no medical knowlegde i'll have a stab though:D

Re: Any physicists out there?
 

Surely the ultimate question is whether you could still make that annoying slurpy-suckey noise whilst attempting to drain the last dregs of cola from the bottom of a glass by using a 3m long straw?

Re: Any physicists out there?
 

Yes but it would depend on the surface tension of cola, and the distance between the edge of the tube and the bottom of the glass. I also have a suspicion that the shape of the tube walls would play a part as well as the usual wall conditions determining pressure loss, and hence possible flow...

Oh wait, you were taking the pi$$....

Re: Any physicists out there?
 

the relevant contact angle wouldn't just depend on the surface tension of the cola, you also need to consider the relative hydrophobicity/hydrophilicity of the material used to construct the container or straw and their microscopic structure...:thumright:

Re: Any physicists out there?
 

Ah yes, the volume calcs are wrong, doh. However I think the problem remains the same as if volume were the determinant as you commonly read then tube 2 would be much much worse in this scenario, which it isnt for the reasons well put.

As someone said above not all the fresh air we breath in gets to the area where gas is exchanged or is used in exchange. Deadspace is the volume of the respiratory system that is not involved in gas exchange. Anatomical deadspace is the trachea and bronchi, where you will not absorb O2 and shed CO2. That deaspace contains a stale air mix which gets taken back into the lungs in the next breath, also. Alveolar deadspace is unperfused or unventilated alveoli (where gas is exchanged), and can be considered negligible for the purposes of this in a normal individual. Deadspace is about 150 to 200ml. So of a typical 500ml breath in we only use about 350ml or so in gas exchange. So the tube adds deadspace to that value, in the case of a tube maybe another 350ml, the total deadspace is 500ml. As a typical inspiration is 500ml (this varies of course) then in this case the total inspired air is deadspace air, and there will only be minimal mixing of fresh and stale air, not sufficient to properly oxygenate the alveoli and remove CO2.

[my respiratory physiology isnt that strong, btw, I'm best once we get stuff inside the body, so to speak, and my physics is **** poor]

Re: Any physicists out there?
 

Go away and caluculate the deadspace of the horse, sorry 'pony'.

*Standard lecturer response faced with veteriniary students.

Re: Any physicists out there?
 

Oh, and the reason I am worrying about this is people mostly say volume of tube and some say length and I am trying to work out the determinant.

Re: Any physicists out there?
 

Dont they all just fiddle with computers these days





and mend TVs and washing machines

Re: Any physicists out there?
 

Just like bad porn movies... bowchickawowwow...
wait this porn films not ready yet...
Why is that man fixing that washing machine...?

Re: Any physicists out there?
 

:lol: :lol: :lol:

Re: Any physicists out there?
 

No really, this is serious.

See, very important question - as is how long a tube it is possible to blow bubbles into a half empty glass of semi-flat cola in a vain attempt to make it fizzy again? ;)


OK, not that bit I understood - kinda.

Did yu know that the posterior half of a snake's right lung is totally dead space. Competely avascular.
S'cos they don't have a diaphragm, so internal pressure regulation and breathing is done by this section combined with the body muscles. It also acts as a reserve tank when they are eating as it can take minutes for food to clear past the lung.

The left lung is almost completely vestigial by the way. Makes room for the gut.


Not that you really needed to know that. But ner. :smt006

Well, make your mind up, which one, horse or pony - one's bigger than the other sir. :silent::rolleyes:

Re: Any physicists out there?
 

That's one good thing about teaching vets, comparative anatomy and physiology is interesting. Mind you whatever you do know some bright spark will come up with something odd....like the liver of the ocelot or something.

When you've done horse, concentrate on giraffe while I read the newspaper and check Facebook. Oh sorry, its saturday and I'm not at work :lol:

Re: Any physicists out there?
 

Not really a practical limit on that one, you will be able to exert some static pressure on any length of tube, the important thing is the depth of cola, much like the snorkelling example you have to be able to equal the pressure at depth before you can form a bubble.

Re: Any physicists out there?
 

ok, i'm still not sure exactly what you're asking, i had a late night and i think you're asking if the trouble breathing through a tube is related to the tube's volume, length, diameter or combination of the three?

if so i'd say the increase in the amount of deadspace is only dependent on the volume of the tube, however you should consider that air is a gas and will mix freely, the air in the deadspace will not the same air that was in the deadspace the last time you took a breath, and that some CO2 rich air that you exhaled will move along the tube and out the end, to be replaced with fresh air, obviously two tubes of equal volume, but very different shapes, one short and wide, one long and narrow, will behave differently. the short fat tube, despite having the same volume, will provide a much greater area at the end for stale air to escape and fresh air to be brought in.

i think the critical factor is the volume of the tube compared to the volume of your breath, consider if you will three tubes of equal diameter but different lengths such that one is 10% of the volume of one breath, one is exactly equal to the volume of one breath and the last is 10x the volume of a breath. the first one will mean that when you exhale, 90% of what was in your lungs will be replaced with fresh air (ignoring re-inhalation of your exhaled air) - the second tube means that when you exhale your breath only just reaches the end of the tube and you only have an area the size of the cross section of the tube for old air to be exchanged for new air. in the last one, you only have the same area in which the air can exchange, but whatever exchanges then has to pass along the pipe to be ultimately exchanged with the open air.

the problem with breathing through narrow tubes comes from the fact that the rate of volume of air carried through a tube is the area of the tube cross section, multiplied by the speed of the fluid travelling through it. to inhale 500ml of air in 2 seconds for example would require a flow rate of 250ml/s. now if you remember the area of the cross section is dependent on the square of the diameter, so in order to sustain the same volume flow when you reduce the diamter by a factor of two you must increase the speed at which the fluid moves by a factor of 4, a third the diameter (say 3cm to 1cm) means that you have to increase the speed the air flows through the tube by nine times... i would be inclined to say that the flow rate is independent of the length of the tube, within reason, i suspect as you increase the length of the tube by factors of a hundred or more the surface friction would come into play, this would be proportional to the internal surface area of the tube, which is in turn proportional to the diameter, not the diameter squared, so it wouldn't shrink exponentially as the tube diameter decreases


either that or i completely missed the boat on what you were aiming for, if that's the case then ask me a question and i'll try and answer it:)

Re: Any physicists out there?
 

Well, they shouldn't have eaten it in the first place. Duh.:rolleyes:

Re: Any physicists out there?
 

Quite.

Thanks lone_wolf I am happy that my view that its is more complicated than some lead students to believe is true.
In other words 'its complex' :lol:

I had to pick up some respiratory teaching last week which is off topic for me so I havent been satisfied with some of the points being made, so good.

You can come and visit sometime like Mac99 did after he helped me out with 'stripey lines' :lol:

Re: Any physicists out there?
 

tbh, fluid dynamics is one of the toughest subjects in physics, very interesting but as you say- "complex":D

Re: Any physicists out there?
 

Your telling me, had the exam on thursday.