3 Lung Mechanics and Ventilation

Question 1

Name the muscles that contract to increase interthoracic volume:

Answer 1

• External intercostal
• Diaphragm

Question 2

The lung tissue has a natural elastic recoil, meaning that it has a tendency to want to collapse in. So what keeps the lungs against the chest wall?

Answer 2

Pleural fluid found between visceral and parietal pleura: seal between lung and thoracic wall

• Lungs expand w./ thoracic cavity*
• Surface tension between pleural surfaces*

Question 3

What is the resting expiratory level?

(Tendency to always want to return to this state)

Answer 3

• State of equilibrium
• Point before inspiration, just before expiration

Forces= equal and opposite

1. Lung recoil- in and up
2. Chest wall- out
3. Diaphragm- down

Question 4

Why is the pressure in the intrapleural space always negative (relative to atmospheric pressure)?

Answer 4

• Elastic recoil of lungs- inward
• Chest wall- outward

Question 5

What happens if the pleural seal is broken?

Answer 5

Air drawn in to intrapleural space due to negative pressure inside

Collapses lung

Question 6

Define tidal volume:

Answer 6

Volume of air entering and leaving lungs in single breath

Question 7

Forced inspiration and expiration uses the lung reserve volume but requires the helps of accessory muscles. When might these muscles be used?

Answer 7

• Exercise
• Diseases affecting lungs

Question 8

Name the accessory muscles for inspiration:

Answer 8

• Sternocleidomastoid
• Scalene muscles
• Serratus anterior
• Pectoralis minor

Question 9

Name the accessory muscles for expiration (no longer passive):

Answer 9

• Internal intercostals
• Abdominal wall muscles

Question 10

Compliance of the lung is determined by:

• Elastic tissue of lung
• Surface tension forces of fluid lining alveoli

What is compliance?

Answer 10

Stretchiness of lung

Volume change per unit pressure change

Question 11

What is the function of surfactant? (secreted by type 2 pneumocytes in lungs)

Answer 11

1. Reduces surface tension acting on alveoli (ie increases compliance)
   
   1. Surface tension limits expansion of alveoli- decrease compliance
2. Has detergent properties
3. Stabilises lungs- prevent small alveoli collapsing into bigger ones

Question 12

Why does surface tension increase as alveoli size increases?

Answer 12

Surfactant more effective at disrupting surface tension when molecules= closer together

ie smaller alveoli

Question 13

How does surfactant prevent small alveoli collapsing into big alveoli?

Answer 13

Allows different sized alveoli to have same pressure inside them

Question 14

What is respiratory distress syndrome?

Answer 14

• Condition seen in premature babies
• Surfactant= absent until fetus> 25 weeks
• Lack of surfactant= Respiratory distress syndrome

Question 15

Why is the parallel arrangement of small airways in the lungs significant?

Answer 15

Compensates for increase in individual resistance in small airways

Question 16

Why will a residual volume remain even after forced expriration?

Answer 16

We cannot empty our lungs completely

Question 17

Find each of the following on the graph:

• Inspiratory capacity
• Functional residual capacity
• Vital capacity
• Total lung volume

Answer 17

• Inspiratory capacity: inspiratory reserve + tidal volumes
• Functional residual capacity: vol of air in lungs at end of quiet expiration
• Vital capacity: Inspiratory capacity + expiratory reserve volume
• Total lung volume: Vital capacity + reserve volume

Question 18

With relation to the respiratory system, what is ‘anatomical dead space’?

Answer 18

Conducting airways- where no gas exchange occurs

…therefore only part of tidal volume used for gas exchange

Question 19

What is ‘alveolar dead space’?

Answer 19

Air in alveoli- not perfused / damaged alveoli- no gas exchange

Question 20

What is physiological dead space?

Answer 20

Alveolar dead space + anatomical dead space

Question 21

Fill in the x values in the following equations:

• Tidal vol = Anatomical dead space + X*
• Total pulmonary ventilation = X x respiratory rate*

X = (Tidal volume - alveolar dead space) x respiratory rate

Answer 21

• Tidal vol = Anatomical dead space + alveolar ventilation*
• Total pulmonary ventilation = Tidal volume x respiratory rate*

Alveolar ventilation = (Tidal volume - alveolar dead space) x respiratory rate

Question 22

What muscles are used during quiet expiration?

Answer 22

No muscles used

Passive and due to elastic recoil

Question 23

As the alveolus expands, does the surface tension increase or decrease?

Answer 23

Increases as surfactant molecules= further apart

(So less force required to expand smaller alveoli)

Question 24

Apart from the energy expended on muscle contraction, what other work must be done on inspiration (2)?

Answer 24

• Force air through airways
• Work against elastic nature of lungs

Question 25

Where does the majority of the resistance against air flow reside in the respiratory tracts?

Answer 25

Upper respiratory tract

Due to parallel arrangement of the small airways

Except when small airways= compressed during forced expiration