Surfactant and Resistance Flashcards
what is the elastic properties of the lung & thoracic cage?
If we consider a point when no air flow is occurring:
Distending pressure is equal but opposite to Intrapleural pressure
As when there is no air flow alveolar pressure = 0 (relative to barometric pressure)
Therefore, Distending pressure = Pin (0) - Pout (intrapleural pressure) = - Intrapleural pressure (Ppl)
E.g., Ppl = -0.5 🡪 Distending pressure = 0 - -0.5 = 0.5
Distending pressure is generated by the elastic recoil forces of the lungs and chest wall (both looking to move towards equilibrium volumes so are exerting recoil pressure)
Lung/chest wall volume is determined by the distending pressure that acts across the structure
The Compliance of the structure determines volume for any given distending pressure
what does compliance of the chest wall depend on?
The compliance of the chest wall depends on rigidity and shape of the thoracic cage
Therefore if we distort the rigidity of the thoracic cage or change its shape –> change in chest wall compliance
Most efficient breathing occurs in an upright position, any other change in shape of thoracic cage results in a decrease in chest compliance.
E.g. A decrease in chest wall compliance is caused by arthritis spondylitis, kyphoscoliosis and by spasticity/rigidity of thoracic or abdominal musculature
There are lots of factors that can cause decrease chest wall compliance however there are no diseases associated with an increase in chest wall compliance.
what would difference in lung compliance result in?
A more compliant lung will have greater volume at any distending pressure than a less compliant lung
what is one of the things lung compliance is determined by?
Lung compliance is determined by a number of things:
The elastic properties of lung tissues determine compliance (e.g. collagen, elastic fibres)
When they reach their elastic limit at higher lung volumes the compliance curve flattens out
what is another thing lung compliance is determined by?
explain graph
Surface tension forces due to the air-liquid interface
Dashed line with positive gradient –> compliance around FRC
Line 1 = fluid filled lung
Line 2 = air filled lung
Fluid filled lung: lung surrounded with liquid and being filled with liquid
A Fluid filled lung has much greater compliance than air filled lung - due to removal of air liquid interface - no surface tension
Surface tension: the force found at air-liquid interface trying to collapse the lung
60-75% of the elastic recoil of the lung is caused by surface tension effects
Shaded pink area (includes area under stripes): The work required to inflate an air-filled lung, i.e., work required to both stretch the elastic properties of the lung and work against the surface tension.
Stripey area: work required simply to stretch elastic properties of lung. There is no air liquid interface so no surface tension.
what is surface tension and how does it arise?
Surface tension: the force found at air-liquid interface trying to collapse the lung
How does it arise:
Throughout the liquid water molecules attracted to each other in all directions (shown by small black arrows). Therefore, they are held together.
At the air-liquid interface there are no water molecules in the air to attract water molecules at surface
Therefore the net attractive force of water molecules at surface is inward –> pulling liquid interface inward –> Forms tension (‘skin’)
what causes the alveoli to be pulled inwards?
Lung alveoli are fluid-lined, spherical bubbles (NOT flat surface like a pond!)
Water molecules at the surface of a bubble (i.e. at the air-liquid interface) pull on each other and thus create a tangential component (T) which generates wall tension which pulls the bubble inwards, collapsing the bubble (red arrows)
This generates an internal positive pressure (P) in the gas phase of the bubble.
Therefore, a bubble, with a certain diameter exists at equilibrium (not fully collapsed) when the tangential collapsing force is to equal and opposite to internal positive pressure (P) being generated in the gas phase.
what is Laplace’s Law (for spherical vessels)?
Pressure inside the bubble is = 2 x wall tension / radius
Therefore if surface tension is constant (which is usually is for the same strucutres) then as radius decreases (bubble gets smaller, collapsing), internal pressure within bubble increases to counteract and keep it inflated
what is alveolar Interdependence?
common misconception?
Different alveoli of different sizes can coexist, due to alveolar inter-dependence
Alveolar interdependence: Each alveolus is surrounded by other alveoli - tendency of one to collapse prevented by tendency of surrounding others not to collapse
Misconception: The reason why different alveoli can co-exist is because smaller bubble have larger pressure, but within lung this is not significant for why bubbles can co-exist
Think of alveoli as ‘frothy foam’ (share wall) and not a bunch of grapes (separate walls):
why do calculations using an air-interstitial fluid interface show a very high surface tension?
calculations using an air-interstitial fluid interface show a very high surface tension
This reduces compliance of lungs significantly, making breathing difficult, if not impossible
But breathing is not normally impossible so it can’t just be normal air-interstitial fluid interface in the lungs.
There must be something there that is acting to increase compliance considerably.
There is in fact a chemical in the interstitial fluid called surfactant (surface acting substance)
what is surfactant?
Surfactant: detergent-like substance that acts to greatly reduce surface tension in alveoli
Surface tension of pure interstitial fluid = ~70 mN/m
Surface tension of interstitial fluid with lung surfactant: can drop lower than 2 mN/m (typically ~10x reduction)
So, surfactant increases compliance by lowering surface tension.
what is surfactant made up of?
Surfactant made up of:
35-40% dipalmitoyl phosphatidylcholine (DPPC), a phospholipid
DPPC is the most important phospholipid for reducing surface tension.
30-45% other phospholipids
5-10% protein (SP-A, B, C + D)
Cholesterols (neutral lipids) + trace amounts of other substances
what is surfactant secreted by?
structure of surfactant molecule?
Surfactant secreted by alveolar type II epithelial cells
Structure of a surfactant molecule: glycerol back bone with phosphate and choline residues on one side and palmitate residues (tails) on other side
Palmitate is oily and therefore hydrophobic - when placing surfactant into liquid (alveolar fluid), palmitate residues must stick out of water away from liquid
While hydrophilic elements (charged choline and phosphate) will orientate and place into liquid
Therefore surfactant will always line air-liquid interface due to hydrophobic/hydrophilic components.
how does surfactant prevent surface tension?
Interstitial fluid has high surface tension
Lung-surfactant has detergent effect, decreases alveoli surface tension. This increases lung compliance and therefore reduced the work of breathing
Mechanism of surfactant: Surfactant prevents water molecules from getting to air-liquid interface => prevent such a high surface tension.
what is the area dependent effect of surface tension being altered?
Lung-surfactant can alter its surface tension lowering effect depending on the surface area (red sloped line)
So, in a smaller area you have a greater reduction in surface tension,
Mechanism: Smaller the radius (lower SA) the greater the density of DPPC therefore greater surface tension lowering effect. This is because at a higher density less water molecules can get to air-liquid interface
what is hysteresis of pressure-volume curve for lung?
Hysteresis of pressure-volume curve for lung: if you inflate the lung and measure pressure and volume the curve moves along a different trajectory than when you deflate lung. Particularly if going from Residual volume → TLC and back.
This is due to area-depending effect of lung surfactant.
graph:
Arrow forward = breathing in, Arrow backwards = breathing out.
NB: Saline Inflation (when no air-fluid interface is present) there is no area dependent effect of lung surfactant so no hysteresis
what would a typical lung compliance curve look like?
Dot at bottom = residual volume
In order to increase the volume of the lungs from dot 1 to dot 2 we need to work to stretch the lung.
If you ask patient to breathe to certain lung volume and then hold their breath (no air movement) we can measure change in distending pressure - indication of work required
Green arrows show work requires to stretch lungs
However, if we do this measurement when air is moving (second curve) instead of two static components, in order to go from dot 1 to dot 2 we require greater work (respiratory muscles have had to work harder)
2nd green line represents extra work required to move air.
This extra work is required to overcome dynamic resistance that is seen when air is moved.
So the work of breathing is into two components
Work required to overcome:
1. Work required to stretch elastic components of lung
2. Extra work required to move air and overcome dynamic resistance
Resistance made up of:
- Airway resistance 80-90% total
This is resistance to flow caused by air molecules rubbing alongside air ways causing friction
Most of work is therefore required to move air against these resistances
- Viscous resistance 10-20% total
Resistance to flow caused by lung tissue friction (movement of lungs themselves)
how do you calculate resistance through airways?
Grey rectangle = airways
ΔP = Barometric pressure vs Alveolar pressure
Air Flow for given delta P dependent on resistance:
what is Poiseuille’s equation?
L = length of tube
n = viscosity of fluid
8 + π are constants
Length (L) is variable, but in terms of airways, pretty constant from breath to breath
Viscosity effectively stays constant
Can alter artificially: giving different gas to breathe e.g. Heliox: helium and oxygen mixture which has lower viscosity so easier to breathe - used in patients who have difficulty in breathing due to increased resistance.
Therefore, you can break down this equation to:
Airway resistance: proportional 1/r4
If you double the radius it will decrease the resistance by 16 (24)
Power relationship - small changes in radius, make large changes to resistance
Therefore, airways diameter will impact on resistance and therefore work of breathing considerably
give an example of different effects on resistance?
Breathing through a hosepipe vs breathing through a straw of the same length
Clearly the resistance is greater in the straw as it is much smaller in diameter (cross sectional area) .. but now imaging bundling 200 straws together to make a big tube and breathing through them. They will have a big total surface area and a much smaller resistance now than the hose even though each one is of higher resistance.
