What is surfactant?
Detergent like fluid that reduces surface tension on alveolar surface membrane
What cells produce surfactant?
Type 2 alveolar cells
Describe briefly surface tension
Due to the attraction between water molecules causing an air-water interface
Does surface tension cause a net force and movement forwards or backwards?
Forwards
What does surfactant reduce? (3)
Tendency for alveoli to collapse
Tendency for alveoli to recoil
Attraction between air and water
What does surfactant increase? (1)
Lung compliance => distensibility
Does surfactant make breathing easier or more difficult?
Easier
Is surfactant more effective in small or large alveoli? Why?
Small
Surfactant molecules come closer together and are more concentrated
When does surfactant start in gestation and when is it complete?
About 25 weeks
About 36 weeks
What is surfactant production stimulated by which are produced at the later stages of pregnancy?
Cortisol
Thyroid hormone
If a baby is premature what may they suffer from and how can this be treated?
Infant respiratory distress syndrome (IRDS)
Synthetic surfactant
What is seen when saline is used to demonstrate lung inflation in utero?
Less change in pressure is required as no need to overcome surface tension
Compliance
Change in volume relative to change in pressure
Does compliance represent stretchability or elasticity?
Stretchability
Stretchability
How easy to stretch open the lungs
Elasticity
How easy it is to get the air out of the lungs
High compliance
Large increase in lung volume for small decrease in Pip
Low compliance
Small increase in lung volume for a large decrease in Pip
What 2 factors can change Compliance?
Disease states
Age
Is more pressure needed to reach a particular lung volume during inspiration or maintain this same volume during expiration?
Inspiration
Why does inspiration require a larger pressure change?
Expiration is passive as the effort from inspiration is recovered as elastic recoil and at the start of inspiration the chest cavity is constricted leading to a greater resistance
Emphysema
Loss of elastic tissue so expiration takes a lot of effort
Pip becomes positive so PA = -ve
Fibrosis
Inert fibrous tissue means effort of inspiration increases
Decreased compliance
Is the greatest compliance at the base or apex of the lung?
Base - due to gravity
Volume change is greater for a given change in pressure
Does alveolar pressure incline or decline with height from base to apex?
Decline eg most at base
Why is the compliance highest at the base?
Alveoli are compressed at the base by the weight of the lung and diaphragm below
Small change in Pip leads to a large change in volume
Top alveoli are already quite full
Obstructive lung disease - brief description and example
Obstruction of flow especially in expiration eg asthma and an increases airway resistance
Restrictive lung disease - brief description and example
Restriction of lung expansion eg fibrosis with a loss of compliance
COPD
Chronic bronchitis - inflamed bronchi
emphysema - destroyed alveoli
80 M word wide with about 1% in UK with 10% over 75s
Restrictive lung disease
IRDS, pneumothorax and oedema which is a fluid build up around alveoli
Spirometry
Technique used to measure lung function
Name the 2 types of spirometry
Static and dynamic
Describe the differences between the 2 kinds of spirometry
Static only considers the volume exhaled
Dynamic takes into account the time taken to exhale a certain volume in 1 second
Which type of spirometry is more useful?
Dynamic
What can be directly measured by spirometry?
Tidal volume, vital capacity, inspiratory capacity, inspiratory reserve volume and expiratory reserve volume
What is the reason why not everything can be measured by spirometry
Anything containing residual volume cannot which also includes functional residual capacity and total lung capacity
What is the difference between volume and capacity
Capacity are 2 or more volumes added together
What does the ratio FEV1/FVC mean?
Forced expiratory volume in 1 second
What is the normal FEV1/FVC?
80%
Describe the FEV1/FVC ratio in obstructive lung disease
Both volumes fall but FEV1 falls more so the ratio is reduced
Describe the FEV1/FVC ratio in restrictive lung disease
Both ratios fall so same ratio or even increases
Why is the FEV1/FVC ratio limited?
Not always an indicator of health as in restrictive lung disease there is a severe compromise of function although the ratio does not change much from normal or will even increase
Forced expiratory flow rate
The average expired flow over the middle of an FVC
Compare FEF25-75 to FEV1
They both correlate but changes are more striking
Greater “normal” range