Any physicists out there?

Originally Posted by yorkie_chris

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$$....

No really, this is serious.

Originally Posted by the_lone_wolf

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...

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?

Originally Posted by Dr Rich

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]

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.

Originally Posted by Dr Rich

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

*Standard lecturer response faced with veteriniary students.

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

Originally Posted by K

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.

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

[yorkie_chris]

Originally Posted by K

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?

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.

[the_lone_wolf]

Originally Posted by Dr Rich

...medical stuff...

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

Originally Posted by Dr Rich

Mind you whatever you do know some bright spark will bring up something odd....like the liver of the ocelot or something.

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

Originally Posted by K

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

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'

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'

[the_lone_wolf]

Originally Posted by Dr Rich

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'

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

[yorkie_chris]

Originally Posted by the_lone_wolf

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

Your telling me, had the exam on thursday.