Week 3 - Lung Compliance Flashcards
What units are used to measure trans-pulmonary pressure?
cm H2O
By definition, what is atmosphere’s pressure?
0 cm H2O
At the beginning of a normal tidal volume breath, what is the pressure found in the alveoli?
0 cm H2O
At the beginning of a normal tidal volume breath, what is the pressure found in the pleural space?
-5 cm H2O
What is trans-pulmonary pressure?
The pressure difference between what is in the alveoli and what is in the pleural space.
What is the equation for trans-pulmonary pressure?
P(TP) = P(ALV) - P(PL) The quantity is with respect to being in the alveoli
If pressure in the alveoli is 0 and there is a negative pressure found in the pleural space, what will happen?
The lungs will expand.
What is pneumothorax?
When a breach of the thoracic cavity causes air from the atmosphere to move to the pleural space which forces the lung to collapse. In other words, because there is no trans-pulmonary pressure, there is no force keeping the lungs distended out.
What is lung compliance?
A measure of how easily the lung can be stretched out or inflated. Therefore, compliance is similar to distensibility.
What is the equation for compliance?
C = dV /dP
where dV = change in volume
and dP = change in pressure
Describe normal compliance
Normal value is around 0.2 liters of air per cm H2O and compliance decreases as one moves up the curve.
Describe low lung compliance
There is a relatively small change in lung volume for a given change in Pressure (dV/dP). Also, if one has a low lung compliance, it means that they have a very high recoil.
Describe high lung compliance
There is a relatively large change in lung volume for a given change in Pressure (dV/dP). Also, if one has a high lung compliance, it means that they have a very low recoil.
In what instance would you find someone with lungs that have low compliance?
someone with pulmonary fibrosis
In what instance would you find someone with lungs that have high compliance?
someone with emphysema.
Compliance is ___________ proportional to lung recoil.
inversely
So what actually happens during inspiration? (7 steps)
1) The diaphragm contracts down
2) The volume of the pleural space increase
3) The pressure in the pleural space becomes more negative (-7.5 cm H2O…originally -5 cm H2O)
4) The trans-pleural pressure difference becomes greater (TP = Palv - Ppl)
5) The greater trans-pleural pressure pushes the lung out which opens airways and alveoli
6) The increase in alveolar volume causes the relative pressure in the alveoli to drop (becomes sub-atmospheric)
7) Air then moves from the atmosphere (0 cm
H20) into the alveoli (-1 cm H2O)
So what actually happens during expiration?
1) The diaphragm relaxes and moves back up towards the thoracic cavity
2) The pleural space decreases in volume
3) The pleural pressure becomes less negative therefore causing a decrease in PTP
4) The decrease in Transpulmonary pressure lessens the driving force to hold the lungs open
5) The lung collapses back inward compressing the alveoli
6) The compression of the alveoli causes the relative pressure in the alveoli to increase
7) Air moves from an area of high pressure (alveoli) to an area of low pressure
(atmosphere)
During maximal quiet inspiration, what is the tidal volume?
500ml
Pleural pressure during quiet inspiration?
-7.5 cmH2O
What muscle is essential to create trans-pleural pressure differences?
the diaphragm.
What keeps the lungs from completely collapsing?
The lungs are held open due to the opposite forces within the thoracic cavity. Lungs want to collapse, but the chest cavity wants to expand so this keeps the lungs held open.
When the inward force of the lungs equals the outward force of the chest cavity, what point in respiration are you at?
FRC
