Mechanics of breathing Flashcards
What is the relationship between the lungs and the chest wall?
The lungs are surrounded by the pleural sac.
Outside membrane is the parietal pleura - in contact with the chest wall.
The inside membrane is the visceral pleura - in contact with the lung tissue.
The pleural cavity contains pleural fluid.
How are the lungs inflated?
The visceral and parietal pleura are in intimate contact.
The pleural space contains fluid that cannot expand.
When the thorax moves, the lungs move as a consequence of the fluid linkage.
What are the requirements of the respiratory muscles to breathe?
The lungs inflate as a consequence of the chest wall moving.
The muscles must:
Stretch the elastic components of the respiratory system - lungs and chest walls.
Overcome resistance to flow - airways and lung tissues.
What are the pressures in the respiratory system?
Alveolar
Intrapleural
All pressures are relative to barometric pressure (PB).
What is barometric pressure?
The pressure outside the chest wall (PB).
What is intrapleural pressure?
The pressure between pleural membranes (Ppl).
What is alveolar pressure?
The pressure in the alveolus (Palv).
What are the forces at rest?
At rest, opposing forces act on the lungs and chest wall.
Elastic recoil of the chest wall tries to pull the chest wall outward.
Elastic recoil of the lung creates an inward pull.
What is functional residual capacity?
At rest, at the end of expiration, the lung contains a volume called functional residual capacity (FRC).
This remains even after expiratory reserve volume - the air that is forcibly expired.
What is Expiratory reserve volume?
The air that can be forcibly expired.
What is lung distending pressure?
The difference in pressure outside the lung and inside the lung.
The lung distending pressure at FRC
= Pin - Pout
= Palv - Ppl
= 0 - -0.5kPa
= +0.5kPa which is a positive pressure difference.
This prevents the lungs from collapsing, by distending the lung.
What is chest wall distending pressure?
At FRC = Pin - Pout
= Ppl - PB
=-0.5 - 0
= -0.5kPa
Negative pressure difference - prevents the chest from springing outward, pulls chest in.
The lung is now at rest.
What are the forces at FRC?
The distending pressures across lung and chest wall are equal and opposite.
The positive distending pressure prevents the lung collapsing.
The negative distending pressure prevents the chest wall springing out.
What happens to FRC in pathology?
If the natural elasticity of the lung changes, where the lung comes to rest changes, so FRC changes.
In emphysema, there is little elasticity, so the chest springs out - barrel chest.
In interstitial fibrosis - the lungs become very elastic and hard to inflate.
How does air enter the lungs?
Air flows from high pressure to low pressure.
Air moves in and out of the lungs by making the alveolar pressure less than barometric pressure then more than barometric pressure.
In inspiration, the alveolar pressure is made less than barometric.
How does alveolar pressure change?
The change in alveolar pressure happens because of changes in lung volume when the thorax expands.
What is Boyle’s law?
The pressure exerted by a constant number of gas molecules in a container is inversely proportional to the volume of the container.
P ∝ 1/V
What is the diaphragm?
The major inspiratory muscle, and the only muscle used in quiet breathing, it lengthens the thorax.
At rest, the diaphragm is relaxed.
In inspiration, the diaphragm contracts and thoracic volume increases.
In expiration, the diaphragm relaxes and thoracic volume decreases.
What are accessory inspiratory muscles?
Used to move the thorax more, during exercise, coughing or vomiting.
The external intercostal muscles - moves the ribs and sternum up.
Scalene
Sternocleidomastoid - pulls the thorax up.
What are the accessory expiratory muscles?
During forced breathing, moves the thorax down faster.
Internal intercostals - contract and pull the ribs in.
External and internal obliques.
Rectus abdominus.
What is the process of inspiration?
Thorax expands
More stretch on interpleural space - so pressure becomes more negative
Lung distending pressure increases.
Lungs expand
Alveolar pressure becomes less than barometric pressure, so air flows in.
What is the pressure difference in inspiration?
There is not a massive pressure difference, Palv only needs to be -0.1kPa less than PB for air to enter.
So there is not massive resistances.
What is the process of expiration?
Thorax returns to original size.
Interpleural pressure returns to resting value.
Lung distending pressure returns to resting value.
Lungs recoil - squeezes air in alveolus.
Alveolar pressure increases above barometric pressure.
Air flows out.
