Physiology - Lung Mechanics

Question 1

What does ventilation do?

Answer 1

Delivers the oxygen and removes the carbon dioxide that is exchanged across the alveolar capillary membrane.

Question 2

In order for ventilation to occur, what needs to be overcome?

Answer 2

Elastic recoil in lung (and chest wall) to change volume 
Airways resistance (friction) to get airflow 
= Work of Breathing

Question 3

What is the Work of Breathing

Answer 3

Need to overcome the elastic recoil in the lungs and the airways resistance

Question 4

State Boyle’s Law.

Answer 4

At constant temperature, the volume of a gas varies inversely with absolute pressure.
P1V1 = P2V2

Question 5

What is the primary principle of ventilation?

Answer 5

Air moves from an area of high pressure to an area of lower pressure.

Question 6

Describe what happens in inspiration.

Answer 6

Diaphragm and external intercostal muscles contract
Increase in thoracic volume
Lungs expand, increase volume (decrease pressure)
Air moves into lungs

Question 7

Describe what happens in expiration.

Answer 7

Inspiratory muscles relax 
Diaphragm moves upwards 
Decrease in thoracic volume and hence lung volume 
Increase alveolar pressure
Air moves out of the lungs

Question 8

Why is the intrapleural pressure sub atmospheric?

Answer 8

Lung has tendency to recoil inwards while the chest wall tends to expand outwards - they pull away from each other.

Question 9

Explain what drives expiration.

Answer 9

Expiration driven by elastic recoil of resp system

Question 10

What happens in forced expiration?

Answer 10

Use accessory muscles to augment expiration (generate more pressure)

Question 11

State what is shown in an Xray showing pneumothorax.

Answer 11

Usually caused by trauma
Trachea gets displaced
Intrapleural space is disrupted and lung is unventilated

Question 12

What does intubation do?

Answer 12

Creates a pressure difference. Increase the outside pressure causing air to move into the lungs

Question 13

Name the two factors involved in the work of breathing.

Answer 13

Elastic Recoil/Compliance

Airways Resistance

Question 14

What is lung compliance?

Answer 14

Measure of the elastic properties (stiffness) of the lung

Question 15

What causes compliance to vary?

Answer 15

Lung volume - as the lung is stretched, it gets harder to stretch
Disease - fibrosis/loss of elastic tissue
Age - lungs are more compliant
Alveolar surface tension

Question 16

What graph do we look at for compliance?

Answer 16

Change in volume/change in pressure graph

Question 17

What happens to the compliance in a lung with fibrosis?

Answer 17

Lung is non-compliant, very stiff.

Thus for a given change in pressure, the change in volume is less.

Question 18

What happens to the compliance in a lung with emphysema?

Answer 18

Destruction of the elastic tissue makes the lung more compliant.
Thus for a given change in pressure, the change in volume is more than normal.

Question 19

Draw the compliance curve for a normal, fibrosed and emphysematic lung

Answer 19

xxx

Question 20

What is surface tension and what does it contribute to?

Answer 20

Fluid lining the alveoli exerts surface tension which causes them to contract/resist expansion.
Need to overcome this force to expand the lung.
Component of compliance

Question 21

What can reduce surface tension?

Answer 21

Surfactant

Question 22

What is surfactant?

Answer 22

Phospholipid produced by alveolar type II cells

Question 23

How does surfactant lower the surface tension?

Answer 23

Reduces attractive forces of hydrogen bonding by becoming interspersed between the water molecules.
Surface tension is therefore reduced and lungs become more compliant.

Question 24

What occurs as a result of a lack of surfactant?

Answer 24

Stiff lungs e.g. in neonates or Adult Respiratory Distress Syndrome

Question 25

Describe airflow in normal quiet breathing.

Answer 25

Airflow is mainly laminar flow.

Question 26

Why are bronchoconstriction and bronchodilation important elements in airway resistance?

Answer 26

Poiseuille’s Law shows that resistance is inversely proportional to the radius.
1/2 r = 16 x R

Question 27

Where is the main area of airway resistance?

Answer 27

Bronchi

Question 28

Where does most of the resistance to airflow arise?

Answer 28

Upper airway and the first 6 generations of the lower airway

Question 29

What factors determine airway resistance?

Answer 29

Bronchoconstriction (e.g. vagal parasympathetic fibres, local chemical mediators - histamine, leukotrienes)
Bronchodilation (e.g. activation of B2 receptors via adrenalin/salbutamol)
Dynamic compression of airways (breathe out = squash the airways)

Question 30

Explain what dynamic compression of airways is.

Answer 30

Some airways are very collapsible.
Increased intrathoracic pressure can sometimes lead to airways collapse which increases airway resistance and limits airflow.
Limits airflow in normal subjects during a forced expiration.
Breathe out and squash the airways

Question 31

In what conditions can dynamic compression of the airways occur?

Answer 31

Loss of elastic tissue (emphysema) - lung will collapse more readily
Old lungs - increase in airway resistance