Week 2 - Mechanics Of Breathing Flashcards
What is compliance?
The sketchiness of the lungs, volume change per unit of pressure, higher compliance means easier to stretch
What factors affect compliance?
Supine/prone position, laparoscopic surgical interventions, severe restrictive pathologies, chronic restrictive pathologies, hydrothorax (pleural effusion), pneumothorax, high standing of diaphragm
Why might certain things affect compliance?
Reduced AP motion, increased intrabdominal pressure, fibrosis/myasthenia gravis, asbestosis/motor neurone disease, congestive heart failure, physical trauma/diseased lung, obesity/pregnancy
What causes elastic recoil of lungs?
Elastic tissue of airways
Surface tension of fluid lining alveoli
How is compliance reduced?
By surface tension of lining fluid
What is surface tension?
Water molecules attract each other
Interface of air and fluid makes the surface resistant to stretching
How does surface tension affect stretchiness of lung?
Makes it harder for the lung to stretch
Force is required to stretch alveoli mainly because its used to stretch the lining fluid
How is surface tension in the lungs reduced?
Surfactant disrupts interactions between surface molecules - breaks up hydrogen bonds
This makes it easier to inflate
What is surfactant made of?
90% phospholipids
10% protein
What is the important protein in surfactant?
Surfactant protein A
What cell type produces surfactant?
Type 2 alveolar cells (pneumocyte)
Why are little breaths easy but big breaths (forced inspiration) hard?
Surfactant reduces surface tension when lungs are deflated but less so when fully inflated so this makes it harder to stretch the lungs
What is Laplace’s law for calculating pressure?
Pressure = (2 x surface tension) / radius
How does the lung keep its structure so the big alveoli don’t ‘eat’ the little ones?
As alveoli coated with surfactant get bigger the surface tension in their walls increases so pressure increases proportionally
What is surfactants role in maintains pressure in alveoli?
Equalised pressure between different sizes of alveoli so they don’t collapse into each other
What is infant respiratory distress syndrome?
Babies born prematurely (less than 36 weeks) have too little surfactant which makes the lungs very stiff and they have few large alveoli
This means breathing and gas exchange is compromised
What is resistance like in the lungs?
Have many very small airways so individual resistance is high but the viscosity of air is very low and there are loads in parallel so this compensates for the increased resistance as the air passes down
Where does the highest resistance occur in the lungs?
In the trachea and bronchi
Where is the lowest resistance in the lungs?
In the small airways
What happens when the lung is compressed?
Raised intrathoracic pressure means small airways are narrowed and resistance increases dramatically and air is trapped in alveoli
Why is breathing out more difficult in diseases such as asthma and bronchitis?
Inspiration is ok as airways can expand and lower resistance but expiration is a problem as resistance increases much earlier so breathing out can become very difficult