Why do the top of the lungs.....

[Brian1968]

Dr Deco once bubbled...
Dear Readers:


The lung is often described as acting in a manner similar to the toy referred to as a “Slinky.” This little coil, if held in two separated hands, will be stretched the greatest at the top and will bunch together at the bottom. In a similar fashion, the lung will also have alveolar space greatest at the top and alveoli will be collapsed at the bottom (above the diaphragm). More blood will flow through the upper portion of the lung tissue and less through the bottom. There will not be a ventilation/perfusion mismatch.

This arrangement will be true whether the individual is standing on dry land or in the water. When immersed, the blood will be shifted from the legs towards the thoracic cavity.

Dr Deco :doctor:

I'm writing this essentially in two ways. Professional to professional and professional to laypeople. So I apologize in advance to everyone.

In normal gravity under normal atmospheric pressure:

The Slinky analogy is very appropriate in that ventilation is greatest at the upper part of the lung and poorest in the more gravity dependant (closest to the ground) portion of the lung. Perfusion (blood flow) is also unevenly distributed throughout the lung with the greatest blood flow tending to the gravity dependant regions This is the same principle which causes pooling of blood in the legs.

For the sake of simplicity the lung is theoretically divided in to three sections (West's Lung Zones). The upper portion of the lung being well ventilated but little perfused (i.e Dead Space Ventilation: V>Q), the middle portion essentially equally ventilated and perfused (no mismatch, V=Q), and the lowest portion well perfused but not so well ventilated (Right to Left Shunting, V<Q). In reality of course there are no clearly defined zones like that but rather a gradual change from one to the other through the lungs. In the end it pretty much all averages out such that very little V/Q mismatch is evident unless there is pathology present. This holds true regardless of body position. As the person changes positions so too do the zones.

I'm thinking in a diving environment that there would be an increase in the V>Q and V=Q regions albeit probably small and one offsetting the other.

While there has been some challenging of Dr. West's model, all I can say is that in the ICU setting we use that model and put it to practice daily, especially in those with severe pulmonary pathologies and it does make a difference.

An interesting discussion. I have a theory on what the answer to the original question may be but I wonder first what GUE has to say about it and what deco gases are being used.

Respectfully submitted,

Brian1968
Registered Respiratory Therapist

 

[The Iceni]

Uncle Pug once bubbled...
a better way to phrase the question would be:

"Do the top of the lungs collapse during deco?"

The answer is, "If they do, this can be no more than on the surface" . . .

100 days a year once bubbled...
So Doc,without getting into the hor.vs.vert. deco thing it seems you are saying for deco purposes it has no perceptible effect on lung orientation?That the human lung is not the limiting factor,but the ability of gases to dissolve out of tissue into the blood is?

I have not seen the video so cannot comment on what it attempts to show but (simplified for clarity)

• The rib cage acts like a sealed box with the gas pressure throughout equal to that at the regulator so all alveoli are held normally patent by this pressure (in the lungs) acting against the chest wall.
  
• Gas transfer mainly depends on this pressure (and the pulmonary circulation which is equal to the entire cardiac output).
  
• Gravity will affect the pulmonary bed in exactly the same way, whether immersed or not.
  
  [*]Regardless of the orientation this holds true in any case except in a zero gravity situation.
  

Brian touched on VQ ratios, but these are only really of significance in the presence of pathology such as emphysema, pulmonary embolus or asphyxia.

On immersion, there is a mass increase in the volume of the pulmonary bed with a certain responding reduction in alveolar volume (V<Q) but this is hardly significant or one could not breath underwater using scuba;- oxygen and carbon dioxide exchange takes place with remarkable efficiency, as does nitrogen offgassing, provided, of course, an offgassing diffusion pressure exists.

At risk of introducing confusion a very long thread was devoted to what's best for offgassing. I contributed by tuppence worth on page 5!

100 days a year once bubbled
Thanx Doc,I'll still deco horizontally due to fact I'm usually weighted and trimmed to stay horizontal anyway.

That's the point, surely? :doctor:

 

[Brian1968]

Dr Paul Thomas once bubbled...
Brian touched on VQ ratios, but these are only really of significance in the presence of pathology such as emphysema, pulmonary embolus or asphyxia.

I had to look back on my post to make sure I'd covered my butt and said that too. Whew! I did.

On immersion, there is a mass increase in the volume of the pulmonary bed with a certain responding reduction in alveolar volume (V<Q).

My thinking was that while alveolar pressure must be a lot higher than normal , pulmonary arterial and capillary pressures would not and so there would be an increase in V>Q.

This has got me thinking and curious and wanting to know why the *increase* in Q?

 

[The Iceni]

Brian1968 once bubbled...
My thinking was that while alveolar pressure must be a lot higher than normal , pulmonary arterial and capillary pressures would not and so there would be an increase in V>Q.

This has got me thinking and curious and wanting to know why the *increase* in Q?

Hi Brian,

There was a lot of heated discussion on the thread I referenced but can I just clarify the physics /physiology (again grossly simplified and very basic)?

Divers cannot use a snorkel below about one foot of water because the muscles of the chest wall and diaphragm are just not strong enough to expand the chest against the pressure produced by water at that depth, when the lungs are in direct communication with the surface and so contain air at surface pressure. The lungs will be effectively "squashed flat."

The principle behind the "aqualung" is it raises the pressure in the lungs (via the mouth) to that of ambient pressure so that this pressure differential is eliminated and the lungs are capable of expanding. (Of course, this high pressure has the adverse effect of forcing the inert gasses found in the breathing medium into solution with all the subsequent problems of DCI.)

There is a further complication and that is the pulmonary vascular bed. As you know this is wholely passive and the volume of blood in the lungs entirely depends on venous return and cardiac output being equal. This is one more reason the deep snorkeler cannot breath since his alveoli are eventually completely compressed by blood forcing out all the air if he does not eventually hold his breath. Put simply, blood from the venous return fills the spaces within the chest no longer filled by air.

The freediver does hold his breath and as the ambient pressure increases with depth the fixed mass of gas held in the lungs is compressed and its volume reduced by Boyle's Law. The pulmonary vascular bed passively increases in volume on the descent and then returns to normal during the ascent.

Of course, in a free diver ventilation is zero during the whole dive but a similar phenomenon does occur with scuba. Immersion reduces venous pooling in the legs normally caused by gravity and this temporarily increases venous return to the heart, reduces the work of the heart and increases cardiac output. Various hormones and reflexes then come into play to adjust cardiac output back to normal (including one causing a diuresis) but some of this added venous return remains in the pulmonary vascular bed the volume of which is increased, although the cardiac output, venous return, and pulmonary perfusion "Q" remain in balance. I apologise if I may have inadvertently mislead here.

Furthemore respiration is of course under reflex control compensating for any temporay VQ mismatch that there may be.

The important message here is as Dr Deco has already posted,

The lungs are NOT the limiting factor in offgassing.