Lecture 3.2: Lung Mechanics

Question 1

By what mechanism is air drawn into the lungs?

Answer 1

Air is drawn into the lungs by expanding the volume of the thoracic cavity

Question 2

What is Pleural Fluid?

Answer 2

A thin layer of fluid between visceral and parietal pleura ensures that lungs fill thoracic cavity and change volume as thorax does

Question 3

What happens if the integrity of the pleural seal is broken?

Answer 3

• Lungs will tend to collapse (due to elastic recoil)
• Pneumothorax

Question 4

What is the Mechanism of Breathing In?

Answer 4

• Active
• Mainly by contraction of diaphragm (flattening)
• External intercostal muscles contract

Question 5

What is the Mechanism of Breathing Out?

Answer 5

• Breathing out to resting expiratory level is passive just stop breathing in
• Diaphragm relaxes
• Internal intercostal muscles contract

Question 6

What is the Mechanism of Forced Expiration?

Answer 6

• Requires force
• Exerted by abdominal muscles and internal intercostal muscles
• Then inspiration to resting expiratory level is passive

Question 7

What is Lung Compliance?

Answer 7

• The stretchiness of the lungs is known as compliance
• Volume change per unit pressure change
• Higher compliance means easier to stretch

Question 8

Are lung airways elastic?

Answer 8

• Airways have elastic walls

Question 9

If lung airways are elastic, why are they also stiff?

Answer 9

• Compliance reduced by surface tension of lining fluid

Question 10

What is Surface Tension?

Answer 10

• Interactions between molecules at surface of a liquid
• Makes the surface resist stretching
• Higher the surface tension the harder the lungs are to stretch

Question 11

What effect do Detergents have Surface Tension?

Answer 11

• Reduce surface tension by disrupting interactions between surface molecules
• The Lung has a mixture of detergents

Question 12

What is a Surfactant? What do they do? What in the Lungs produces Surfactants?

Answer 12

• Surfactant (mixture of phospholipids & proteins, with detergent properties)
• Reduces surface tension when lungs are deflated (but not when fully inflated)
• So little breaths are easy
• Big breaths are hard
• Produced by Type 2 Alveolar Cells

Question 13

What is Hysteresis? When is the most lost?

Answer 13

• The energy put into stretching a film of surfactant
• Is not all recovered when the film recoils
• This loss is greatest when tidal volume is maximal
• Another reason why little breaths are best

Question 14

What is Asthma?

Answer 14

• Disease of the airways
• Narrowing of airways causes obstruction
• Makes hard to expel air
• Air retained in lungs (gas trapping)
• Makes lungs bigger thus harder to expand

Question 15

What is Laplace’s Laws Equation?

Answer 15

• Pressure = 2 x surface tension/radius
• P= 2T/R
• Big bubbles have low pressure (R large)
• Little bubbles have high pressure (R small)

Question 16

What is the Law of Bubbles?

Answer 16

• If a big bubble is connected to a small bubble
• Air will flow from high pressure to low
• Small bubble collapses into big
• ‘Big bubbles eat little bubbles’

Question 17

Why does Laplace’s Law not apply to the Lungs?

Answer 17

• Alveoli form an interconnecting set of bubbles
• If Laplace’s law applied big alveoli would eat little ones
• Thus the lungs become a physical impossibility

Question 18

What stops the ‘Bubble Law’ from occurring?

Answer 18

• As alveoli get bigger
• Surface tension in their walls increases because surfactant is less effective
• So pressure stays high
• And stops them eating little alveoli

Question 19

What is Respiratory Distress Syndrome? Who is affected by it?

Answer 19

• Babies born prematurely
• Have too little surfactant
• Lungs very stiff
• Few, large alveoli
• Breathing and gas exchange compromised

Question 20

What is Poiseuille’s Law? What is its relevance?

Answer 20

• Resistance = 8 x viscosity of air x length of tube/ (radius)^4
• Airway resistance is the pressure difference between the alveoli and the mouth
divided by flow rate and is determined by Poiseuille’s Law

Question 21

Where is Airway Resistance highest and lowest in the Lungs?

Answer 21

• Highest resistance in the trachea
• Lowest resistance in the small airways
• So breathing is easy

Question 22

Obstructive vs Restrictive Airway Disease

Answer 22

Obstructive lung diseases include conditions that make it hard to exhale all the air in the lungs
People with restrictive lung disease have difficulty fully expanding their lungs with air