Mechanics of breathing Flashcards
Surface tension in alveoli
The cohesion forces between water molecules on the alveolar surface- pulling water molecules together.
The greater the surface tension, this increases the tenancy for alveoli to collapse.
Transpulmonary pressure
The pressure difference between the pleural pressure and alveolar pressure.
Ptp= Palv-Ppl
This pressure is generated by inspiratory muscles.
Force of the diaphragm on the lungs
The larger the radius of the diaphragm, the smaller the force is generated.
In patients with a flattened diaphragm, this makes expiration more difficult- as seen in emphysema patients.
Lung surfactant
A phospholipoprotein molecule produced by type II pneumocytes.
Decreases surface tension in alveolar by repelling water molecules, thus increasing pulmonary compliance.
Prevents atelectasis and also allows alveoli to open with a lower amount of pressure.
Compliance of the lung
The relationship between changes in lung volumes and transpulmonary pressure.
The greater the change in volume, the greater the compliance.
The compliance of the lung is never negative due to its elastic recoil. The pressure will always exceed that of the chest wall
Compliance of the chest wall
The relationship between the changes in lung volumes and transpulmonary pressure.
An increase in lung volume due to an increase in transpulmonary pressure = higher chest wall compliance.
Chest wall compliance is negative at first as it draws outwards. It then becomes positive when it recoils inwards.
Lung and chest wall compliance
Calculated by the addition of chest wall and lung compliance.
When the chest wall compliance is equal to lung compliance, the pressure is 0- the functional residual capacity, there is no air flow.
Functional residual capacity
The amount of air left in the lungs after quiet exhalation.
Residual volume
The amount of air remaining in the lungs after forced exhalation
Total lung capacity
The amount of air in the lungs after the maximum inhalation.
TLC= Vital capacity (VC) + residual volume
Vital capacity
The amount of air expelled from the lungs after maximum inspiration.
VC= Tidal volume + Inspiratory reserve volume + expiratory reserve volume
How lung disease affects compliance
Increase in compliance in seen in emphysema due to excessive distension. HOWEVER, they lack elastic recoil so it is difficult to expel air.
Decreased compliance is seen in the addition of fibrous tissue- fibrosis.
Difficult to inspire large volumes of air.
Hysteresis loop in ventilation
Compliance curve comparing expiration and inspiration.
Lung volume at any given pressure at exhalation is higher than that during inspiration- indicating that there is more compliance during exhalation.
However compliance is increased when the alveoli is filled with saline and there is loss of hysteresis due to reduced surface tension.
Infant respiratory distress syndrome
Occurs in premature infants, less than 30 weeks old, who are unable to produce adequate amounts of lung surfactant.
This causes the air-spaces in the alveoli to collapse and have difficulty expanding due to increased surface tension.
Alveolar interdependance
The alveoli are interconnected, forming a network like a honeycomb.
If an alveolus starts to collapse, the surrounding alveoli are stretched which generates elastic recoil.
Recoil then exerts expanding forces on the collapsing alveolus to open it.
