Compliance Lecture 11 Flashcards
Describe FRC, intrapularal pressure and distending pressure
the volume of air in the lung at the end of normal expiration (2.5-2.7L) - it is the equilbrium volume of the respiratory system,
FRC is determined by the balance between theinward elastic recoil of the lung and the outward recoil of the thoraic cage.
WHen the lungs and chest wall pull apart, it creates a negative intrapleural pressure.
Intrapleural pressure is the pressure which allows us to link the chest wall and the lungs. Movements of the chest wall allow the genaration of lung distending pressure.
As you make the intrapleural pressure lower, you will increase the lung distending pressure
Lung distneding pressure = aveolar pressure - intrapleural pressure.
The degree of lung expansion is proportional to the lung distending pressure, so generating a more negative pleural pressure leads to a greater lung volume, represented by compliance
Distending pressure is always postive, more positivve during inspiration
Compliance=
Change in volume/change in pressure
Compliance is the ability of the lung to stretch- the opposite of elastance.
INcreased compliance means that a small change in lung distending pressure wll lead to a large change in lung volume. Very difficult to maintain volume and the lung will collapse very readily.
Decreased compliance means that a larger than normal change in pressure is needed to inflate the lungs (really hard to breath)
Impact of gravity on Ppl, compliance
At rest, more air flow goes to the base as the alveoli can expand more from FRC than the ones at the apex due to gravity pulling lungs away more from apex than at the base.
At apex of lungs, the lungs are pulling away from chest wall with quite a lot of force whilst at the base. It isnt due to being sat on the diaphragm. Therefore at apex of lung we will have a more negative intrepeural pressure than the base.
Lung distanding pressure is greater at apex of lung than the base.
Ventiliation increases as we go from apex to base of lung
What are the determinants of lung compliance?
Stretchability of tissue (elastic connective tissue forces - more elastic =more stretch), therefore thickening lung tissure < compliance
Surface tension at air-water interfaces within alveoli (accounts for over half lung elastic recoil
Describe surface tension in the alveoli
Thin layer of interstitial fluid lining walls of alveoli.
Sruface tension generated by the fluid lining in the alveoli generates a collapsing force and increases the work of breathing. (have to overcome surface tension in order to breathe)
Arises because if the attractive forces between water molecules being greater than those between liquid and gas Is actually 10% the theoretical value
Surfactant released by type 2 pneumocyte reduces surface tension and stops alveoli collapsing
Keeps the lungs ‘dry’- prevents surface tension sucking fluid out of the capillaries into the alveoli
Stops small alveoli collapsing into buffer alveoli because of its area-dependent effect- more efficient at reducing surface tension I smaller alveoli
Surfactant will break down H bonds that have the collapsing force so alveoli cant collapse.
In smaller alveoli, you will have a higher concentration of surfactant and it wll have a greater surface tension lowering effect than large alveoli.
If there was no surfactnant, there would be high elestic recoil, decreased compliance, alveolar collapse and hypoxia/hypercapnia. Need lots of enegery to breathe without surfactant.
Functions of surfactant
Lowers surface tension of the water layer at alveolar surface, increasing complaince, making lungs easier to expand.
Bubble forms a sphere as small as possible, surface tension tends to shrink the bubble.
P = 2T/r P = pressure inside alveoli/bubble T- surface tension force r = radius (Laplaces law)
Keeps lungs dry - prevent surface tension from sucking fluid out of capillaries into the alveollus
Aids alveolar stabality - lack of surfactant results in collape of small alveoli
Smaller radius alveoli have smaller surface area, greater density of surfactant, more effective at reducing surface tension in smaller alveoli and during deflation of the lung (expiration)
In the pressure of surfactant, there can be equal pressures in alveoli of different radius
Chest wall compliance
Residual volume = minimal volume you can get chest wall to
As compression pressure is reduced, the chest wall volume starts to increase.
At 0 distending pressure , thi the natural volume of the chest wall. to expand chest wall furter, you need to put in postive distending pressure.
With a pneumothorax distending pressure is lost as air eneters the pleural cvity, so the lung collapses an the chest wall expands.
Equilborium volume between the chest wall compliance and lung complance sets the FRC
What is the intrapleural pressure?
Lungs tend to recoil inwards and the chest wall tends to move out creating a negative intrapleural pressure of 0.5kPa
So the lung distending pressure at rest is 0.5kPa which prevents the lung from collapsing
