- Joined
- May 28, 2007
- Messages
- 680
- Reaction score
- 3
Please xplain LUNG & CHESTWALL graph on page 477 of FA 2009
Thank YOu
Thank YOu
FA 2009-- page # 477
- Thread starter MDpride
- Start date
- Joined
- Sep 27, 2008
- Messages
- 3,015
- Reaction score
- 2,223
man, that is a confusing graph. costanzo has a better explanation of this concept. the main point is that the lungs tend to collapse and the chest wall tends to spring out. these two forces are balanced at FRC, thus the system is at rest. this also explains why expiration is passive under normal conditions
- Joined
- Apr 16, 2008
- Messages
- 1,560
- Reaction score
- 1,298
- Joined
- Jan 11, 2010
- Messages
- 127
- Reaction score
- 1
check out pg 123 in BRS physio as that graph can be used to explain the "barrel-chest" in emphysema as the systems seeks a higher FRC.
Look at the lung curve first. You can see it is at an airway pressure of 0 cm water at a volume of 0. Basically this means that the lungs "want" to be at a volume of 0, and at this volume there is no force intrinsic in the lungs trying to expand or contract. So if you took the lungs out of the chest, they collapse to 0 volume and you can consider them in equilibrium.
Look at the chest wall curve. You can see it crosses the 0 cm water airway pressure vertical line at approximately 50% of vital capacity. You can think of this point as the desired "home" of the chest wall. In other words, the chest wall "wants" to be at approximately 50% vital capacity and at that point there are no longer forces intrinsic to it that are working to either expand or collapse the chest wall.
Now, look back at the lung curve, you can see as you expand the lungs to higher vital capacities the airway pressure becomes more and more positive. What this means is that as the lungs expand further, the collapsing force intrinsic to the lungs increases. So when you expand the chest wall and the lungs, the further you expand, the stronger the lungs pull inward on the chest wall trying to collapse it.
Now, back to chest wall. You can see at low volumes (below 50% VC) the airway pressure is negative. At these low volumes the lungs want to expand and they are pulling on the lungs to try and expand the chest wall outward. As the volumes go above 50% VC the pressure becomes positive and at these higher volumes the chest wall, like the lungs, wants to collapse inward (albeit with less force than the lungs).
This brings us to the system curve, simply the sum of the other two curves. So at low volumes, the lungs have a fairly weak force trying to collapse the lungs downward (weak because it's closer to where the lungs want to be [i.e., 0 volume]) and the chest wall is exerting a large force trying to expand the chest wall, so the system has a net negative airway pressure. As you increase volume, the lungs pull down harder and the chest wall pulls out less, and at FRC, the inward pull of the lungs matches the outward pull of the chest wall and you've reached equilibrium. Hence at this point, your chest wall and lungs are in balance and they will remain at FRC unless something changes. If you increase even more and add more volume, the whole system wants to collapse downward because after 50% VC both lungs and chest wall are trying to collapse downward. Hence, when you breathe in, the elastic forces of the lungs/chest wall do the work of expiration for you and it requires no energy.
Look at the chest wall curve. You can see it crosses the 0 cm water airway pressure vertical line at approximately 50% of vital capacity. You can think of this point as the desired "home" of the chest wall. In other words, the chest wall "wants" to be at approximately 50% vital capacity and at that point there are no longer forces intrinsic to it that are working to either expand or collapse the chest wall.
Now, look back at the lung curve, you can see as you expand the lungs to higher vital capacities the airway pressure becomes more and more positive. What this means is that as the lungs expand further, the collapsing force intrinsic to the lungs increases. So when you expand the chest wall and the lungs, the further you expand, the stronger the lungs pull inward on the chest wall trying to collapse it.
Now, back to chest wall. You can see at low volumes (below 50% VC) the airway pressure is negative. At these low volumes the lungs want to expand and they are pulling on the lungs to try and expand the chest wall outward. As the volumes go above 50% VC the pressure becomes positive and at these higher volumes the chest wall, like the lungs, wants to collapse inward (albeit with less force than the lungs).
This brings us to the system curve, simply the sum of the other two curves. So at low volumes, the lungs have a fairly weak force trying to collapse the lungs downward (weak because it's closer to where the lungs want to be [i.e., 0 volume]) and the chest wall is exerting a large force trying to expand the chest wall, so the system has a net negative airway pressure. As you increase volume, the lungs pull down harder and the chest wall pulls out less, and at FRC, the inward pull of the lungs matches the outward pull of the chest wall and you've reached equilibrium. Hence at this point, your chest wall and lungs are in balance and they will remain at FRC unless something changes. If you increase even more and add more volume, the whole system wants to collapse downward because after 50% VC both lungs and chest wall are trying to collapse downward. Hence, when you breathe in, the elastic forces of the lungs/chest wall do the work of expiration for you and it requires no energy.
