4 Compliance Flashcards
What do type 2 alveolar cells produce?
Surfactant, A detergent like fluid
What is the function of surfactant?
Reduces surface tension on alveolar surface membranes
Reduces chance they collapse.
What causes alveolar surface tension?
Occurs in air-water interface
Attraction between water molecules
Attraction between surface molecules balances out with a net inward pull
How does surfactant affect compliance?
It increases compliance (distensibility) effectively making breathing easier
Where is surfactant most effective?
On small alveoli
Surfactant molecules get closer & more concentrated
Why is pressure greater in smaller alveoli?
P = (2 x surface tension)/radius
Smaller alveoli have smaller radius & same surface tension
When is surfactant produced in embryo?
Starts ~25 weeks
Complete by ~36 weeks
What surfactant related syndrome is often suffered by premature babies?
IRDS
Infant Respiratory Distress Syndrome
What stimulates surfactant production?
Thyroid hormones
Cortisol
Both increase towards end of pregnancy
What is compliance?
How much the lung volume changes for any given change in Ip pressure
What does high compliance mean?
The lung will have a large volume increase for a small intrapleural pressure decrease
How does compliance relate to elasticity & stretchability?
Compliance is a measure of stretchability (how easy the lung is to fill with air)
It doesnt tell you anthing about elasticity (how easily the air is pushed out)
Law of Laplace?
P = 2T/r
Inwardly directed pressure of alveoli
What kind of alveoli collapse most easily?
Smaller alveoli
Inwardly directed pressure is largest
Surfactant combats this
What hapens in IRDS?
No surfactant
Very high inward pressure in alveoli
So lots of small alveoli collapse between breaths
Makes it very hard to breath
What alveoli are most efficient?
Small alveoli
Have largest surface area to volume ratio
How does age affect compliance?
Compliance decreases with age
How do we determine compliance from graphically?
Draw a graph of ip against lung volume with a curve for inspiration and one for expiration.
A straight line between them will give the value for compliance from its slope;
Why is a curve for inspiration less steep at the start?
At start of inspiration tissue inertia, elasticity and surface tension must be overcome.
Why is the curve for expiration less steep at the start?
Compressing the chest to push out air also compresses the airways and narrows them
Once chest is quite compressed, pressure is high enough to overcome compression and expiration curve steepens
What makes expiration passive
elastic recoil of lungs
What is emphysema?
Tissue loses elasticity
Expiration requires more effort and a larger increase in ip pressure to push out all the air
What is fibrosis?
excess inert fibrous CT means inspiration requires more effort (decreases compliance)
How does compliance vary throughout a lung?
Greatest compliance at base of lung
Least compliance at apex
