Week 21: (A) Breathing I - Static Forces Flashcards
What are the 2 physical forces that influence breathing?
Static and dynamic
What is static force?
Mechanical properties of the lung that influence gas flow but which are independent of volume change
What are the 3 types of static force?
Elasticity
Compliance
Surface tension
What is Dynamic force?
Mechanical properties affecting flow of air into and out of lung as volume changes with time
What are the 3 types of dynamic force?
Force
resistance
turbulence
What force will be focused on in this lecture?
static force
What is the pleural sac?
The thin space between 2 pleura of the lungs.
Filled with liquid
A pleura is a serous membrane which folds back onto itself to form a two-layered membranous pleural sac
What is barometric pressure?
measure of air pressure in a given area
pressure is the force of air on pressing down on something
What is the function of the pleural sac in the lungs?
links elastic forces in the chest wall and lung
What is elastic recoil?
Elastin in alveoli acts as an
inward “collapsing” force
What 2 factors contribute to the sub-atmospheric
intrapleural pressure?
elastic recoil of the alveoli
and the opposing elastic recoil of the ribs as an outward expanding force
What happens to alveolar pressure during an inhale?
inspire, alveolar P decreases below atmospheric (needs to happen to draw gas in)
equalises back to 0 cmH20 and the end of insp
What happens to pleural pressure during an inhale?
goes from -5 and decreases to the end on an inhale
gets more negative as the chest cavity increases in vol.
What is pleural pressure?
pressure surrounding the lung tether the lung movement to the rib cage
What happens to pleural pressure as lung vol increases?
Ppl decreases
from ~-5 start of inhale to ~-8 at the end
What pressure is Ppl always during quiet breathing?
sub-atmospheric
What aids the alveolar pressure to move in a negative direction?
Ppl being negative ~-5 cmH2O (pressure)
What drives expiration?
elastic forces, drive lung collapse/ return of lung to inspired state.
Elastic retraction enables alveolar pressure go above atmospheric
What happens to Alveolar pressure during exhalation?
increases so the pressure in the lungs increases to overcome the forces of the upper airways and then returns to 0 at the end of exhalation
What happens to Ppl during an exhalation?
increase from around -8 cmH2O to -5
What is the equation for compliance?
change in Volume L (lung) /
change in Ptp (transpulmonary pressure)
What is the compliance equation asking us?
How much work (pressure) lung vol do we need to do to achieve a change in lung volume.
What is the effect of a lung with high compliance on ability to distend?
a high compliance balloon is easily inflated as its elasticity is low
What is the effect of a lung with low compliance on ability to distend?
a low compliance balloon straight out of the packet is difficult to inflate because its elasticity is high
What is the relationship between compliance and lung elasticity?
Compliance varies inversely with lung elasticity
What does a high density of elastic fibres mean for compliance?
High density of elastic fibres means low compliance as ‘balloon’ will be harder to blow as the inward force is strong
What does an increase in compliance mean for lung volume?
increase the change in lung volume for the same pressure
What is the effect of COPD and emphysema have on the level of compliance?
Increases compliance
their lung volume is larger with the same transpulmonary pressure applied. due to loss of elastin
1.0/2.5 = 0.4L/cmH2O
What are the features relating to elasticity of COPD and emphysema?
decrease in elastic fibres
alveoli tend to collapse inwards of one another
–> breakdown of the connective tissue matrix that normally holds the lung open
What is the normal compliance of the lung?
0.2L/cmH20
What is the effect of having idiopathic pulmonary fibrosis on the compliance of the lungs?
stiffen lung
lower compliance
0.1L/cmH2O
lower lung volume for the same pressure
What are the 2 factors which produce elastic recoil?
elastin connective fibres
alveolar surface tension
What is elastin connective fibres?
alveoli meshed in elastin connective fibre tissues matrix which runs around the alveoli and gives them their flexibility
What causes surface tension?
-Caused by the thin layer in the thin layer in the alveoli of fluid that lines the inner surface
produced by the alveoli
-Type II epithelial cells
-Rich in pulmonary surfactant
What is alveolar fluid lining lined with?
surfactant
What structures line the alveolus?
type I alveolar cell
type II alveolar cell
alveolar fluid lining
What are the features of water molecules on the surface?
The water molecules on the surface of water don’t have all the interactions that those on the bulk have.
Can only interact in 2D rather than 3D
No intermolecular force to pull the molecule upwards
Gives water a small but coherent stiffness
What happens when a force is places on top of the surface molecules?
When the surface molecules dive into bulk fluid, the molecules that remain at the surface develop opposing force (surface tension)
either move down to bulk or remain on the top with opposing forces
How do the water molecules surface tension effect the alveolus?
As they are in the alveolar fluid lining they act outwardly and laterally on the walls of the alveolus, act inwardly to collapse the alveolus into itself.
What are 3 effects of surface tension on the alveoli shape/size?
-Resist stretch (greater surface tension, less compliance)
-Tends to become smaller
-Tends to recoil after stretch- contributes to elastic recoil
pressure
What is transpulmonary pressure?
the difference between the alveolar pressure and the intrapleural pressure in the pleural cavity
What needs to happens for alveoli to expand to allow gas in?
Input of energy needed to overcome surface tension
(intercostals/diaphragm)
need a transpulmonary pressure high enough to overcome the elastic recoil of the lung
What is the elastic recoil of the lungs?
elastic fibres surrounding the alveoli and the surface tension
What happens after the the lungs overcome the apposed pressure, what happens next?
Less energy required to
inflate lung over this phase
What happens for the lung to exhale?
elastic and surface tension recoil.
lung collapses under it sown elastic recoil using its own surface tension. little to no work.
What is hysteresis?
difference in kinetics between inhalation and expiration
What has higher tension: small or large bubbles?
small diameter bubbles
What size bubble has more chance of collapsing and why?
small: Variation in alveolar size/volume would cause small alveoli to collapse into larger ones – not good!
due to higher pressure, the inward force has a larger effect on the surface tension.
What is the equation for pressure in a bubble?
P= 2T/r
Bubbles are connected by an airway, what is the effect if a small bubble collapses?
Tendency for small bubble to collapse inward to a larger one, usually causing lung collapse. can cause an embolism in the heart or brain
How can we prevent this high surface tension (collapse)?
Pulmonary surfactant stabilises alveolar structure
by reducing surface tension
How does surfactant reduce surface tension?
decreases density of water molecules at air-water interface.
it has hydrophobic tails which pulls the surfactant molecule upward, resultant vector is minimal (reduce force/ St)
What is surfactant and what is it secreted by?
composed of phosphatidyl choline
secreted by alveolar type II cells
What are the benefits to pulmonary surfactant, how does it aid breathing?
1) Prevents collapse of alveoli during lung expansion and contraction 2) Reduces pressure required to inflate lungs
What effects the effect of surface tension?
alveolar surface area
How does surfactant effect stability of the alveoli?
- As r falls, surfactant molecules crowded together, surface tension reduced. Smaller alveolus stabilised
- Alveoli also stabilised by mechanical interactions between neighbouring alveoli. Prevent alveolar collapse.
When a lung inflates rapidly how is surface tension increased?
Alveoli r increases, decreases the density of surfactant, the liquid-gas tension has a more dominant presence and elastic recoil comes into play.
puts a ‘brake’ on expansion and alveoli recoil
(opposite for a small alveoli, surfactant are dominant, move outward)
What part of the alveoli increases during inspiration of the lungs?
airway surface liquid.
How is it proven that the lungs need surfactant to reduce work?
-Washed alveoli with saline (no surfactant), more pressure requires to reach the same level of lung air volume compared to a alveoli with surfactant
-Working against elasticity + surface tension.
lung rapidly collapses due to the high surface tension
