L3 Mechanisms of Breathing

Question 1

What is the recoil pressure of the lung?

Answer 1

Recoil pressure of the lungs = alveolar pressure - intrapleural pressure.

The alveolar pressure is in continuity with the atmospheric pressure when the epiglottis is open. The tendency of the lung is to collapse as it is maintains above its natural volume by articulating to the chest wall. It standsopenduring breathing, allowing air into the larynx.

Question 2

What is the chest wall recoil pressure?

Answer 2

Chest wall recoil pressure = chest wall pressure - barometric pressure.

The chest wall has a natural tendency to expand, it has an opposite recoil to the lungs. Barometric pressure is always 0 cm of water.

Question 3

When its the compliance of the lung lowest?

Answer 3

Where there is low lung volumes there is a big change in pressure to start expanding the chest, stretching the chest i.e. initially there is a low compliance. This is the same with all elastic structures.

Question 4

What diseases are associated with change in lung compliance?

Answer 4

• Pulmonary fibrosis leads to reduced lung compliance
• Circumferential burns to reduced lung compliance
• Emphysema - An obstructive condition causing the elastic tissue of the lung to be destroyed. As a result, the lung is more reality expanding leading to an increase in compliance of the lung tissue.
• Kyphoscoliosis to reduced lung compliance

Question 5

How does compliance of the alveoli vary in the lungs?

Answer 5

Due to gravity different alveoli, according to their position in the lung, have a different compliance. At the bottom of the compliance curve, it is quite steep and levels off at the top. In a healthy individual the alveoli, even those at the bottom, are open. When you apply equal pressure across the lung, you get a bigger change in pressure in the basal alveoli compared to that at the top of the lungs. This is as basal alveoli have a higher compliance. Under normal physiological conditions, the alveoli at the base of the lungs are better ventilator than those at the apex of the lungs. This is important in matching and ventilation and perfusion of the lungs.

Question 6

What is the closing capacity? When does this effect normal breathing?

Answer 6

Outside the range of 8-40 years, some of the basal alveoli close. The closing capacity is the volume within the chest cavity in which the alveoli start to close. Normally the closing capacity is less than the FRC. At reduced lung compliance, FRC is lower. If the FRC falls lower than closing capacity, then within normal tidal breathing there is closure of some of the alveoli and so ventilation is less efficient.

Question 7

What is the law of Laplace?

Answer 7

P = 2t/r
In a sphere, pressure is proportional to the surface tension and the inverse of the radius.
The Law of Laplace essentially states that the tension within the wall of a sphere filled to a particular pressure depends on the thickness of the sphere. Consequently, even at a constant pressure, the tension within a filled sphere can be decreased simply by increasing the thickness of the sphere’s wall.

Question 8

What is surface tension?

Answer 8

Surface tension is a force on the membrane on an air fluid interface that is trying to compress down. The liquids have a tendency to form a sphere to minimise surface tension and so potential energy is minimised.

Question 9

Why does the air in a small alveoli connected to a large alveoli move towards the larger alveoli?

Answer 9

There is a layer of surfactant on the alveoli. This is used to reduce surface tension and so allows the pressure in the alveoli to equilibrate so that the gas exchange can remain efficient.

Question 10

What is the function of Type I alveolar cells?

Answer 10

To allow gas exchange

Question 11

What is the function of Type II alveolar cells?

Answer 11

Create surfactant

Question 12

What is surfactant composed of?

Answer 12

10% Protein

90% Phospholipids

Question 13

What is atelectasis?

Answer 13

Atelectasis is collapse of lung tissue with loss of volume.

Question 14

What is the function of surfactant?

Answer 14

• Increase pulmonary compliance
• Prevent atelectasis
• Aids alveolar recruitment
• Minimises alveolar fluid

Question 15

What is Infant Respiratory Distress Syndrome?

Answer 15

Infantile respiratory distress syndrome (IRDS), is a syndrome in premature infants caused by developmental insufficiency of pulmonary surfactant production and structural immaturity in the lungs. Surfactant is only produced after week 28. If born before this, there is high surface tension leading to collapse of the small airways and so the baby cannot ventilate well.

Question 16

What is a pleural effusion?

Answer 16

Pleural effusion, also called water on the lung, is an excessive buildup of fluid in the space between your lungs and chest cavity.

Question 17

What is cause of hysteresis in the compliance curve?

Answer 17

• Reduced compliance once the lung has started to inflate compared to when you started
  
  • Airway calibre - as you expand the lung, the diameter of the bronchioles also increases. There is therefore less resistance to the flow of air.

Question 18

Where does laminar flow occur in the lungs?

Answer 18

Laminar flow - air molecules are going in an organised path- flowing smoothing. There is laminal flow in the more distal branches of the alveolar tree. Beyond the 15-20th branch, air flow is laminar.
In the upper airways which are wider, and the flow of air is quicker, there is more turbulent flow.

Question 19

What is FVC?

Answer 19

Forced vital capacity - is the total amount of air exhaled during the FEV test. The volume of air that can be forcibly expelled from the lung from the maximum inspiration to the maximum expiration. This is volume dependent.

Question 20

What is FEV1?

Answer 20

The forced expiratory volume 1 - measures how much air a person can exhale during a forced breath. The amount of air exhaled may be measured during the first (FEV1)

Question 21

What can the FEV1/FVC tell us?

Answer 21

A normal value is around 0.7-0.8. The ratio of FEV1/FVC can be used to distinguished between obstructive and restrictive conditions.
If obstructive, you struggle to breath out and so your FEV1 is lower and so the ratio is smaller. If restrictive, the individual struggles to inhale and so the maximum air inhaled is less and so the ratio is equal to or greater than 0.7 - 0.8.
Ratio < 0.7 ≅ obstructive
Ratio > 0.7 ≅ restrictive

Question 22

What is PEFR?

Answer 22

A peak flow meter is used to measure the peak expiratory flow rate (PEFR).
This is a convenient way so measuring airway obstruction.
Patients can use a peak flow meter to monitor their asthma or COPD at home.
Wide diurnal variations occur in PEFR, with the highest readings in the evening and the lowest in the early hours of the morning. In asthma there is at least 20% variability however in COPD, there is little to no variability in peak flow.

Question 23

How can you distinguish between asthma and COPD using Salbutamol?

Answer 23

Measurements of FEV1 and PEFR made before and after inhalation of a bronchodilator (e.g. the β-adrenoceptor agonist salbutamol) can be used to distinguish between asthma and chronic obstructive lung disease. In asthma, the airways constriction is reversible so that the FEV1 and PEFR would be restored to normal after salbutamol.
In COPD, the airways constriction is irreversible, or nearly irreversible. There would be <15%, or <200 mL/s, improvement in FEV1 and PEFR after salbutamol.

Question 24

What is dynamic airway collapse?

Answer 24

When you forcefully breath out, the transpulmonary pressure, puts pressure on the airways causing it to collapse reducing the diameter of the airways and increases the resistance to flow and limits the maximum flow.

Excessive dynamic airway collapse (EDAC) is defined as the pathological collapse and narrowing of the airway lumen by 50% or more of the sagittal diameter which occurs as a result of laxity of the posterior wall membrane with intact cartilage.