Mechanics of Breathing - Trachte

Question 1

How does inspiration occur primarily?

Answer 1

By contraction of the diaphragm

(phrenic nerve → diaphragm & intercostal contraction)

Question 2

How does expiration occur primarily?

Answer 2

Passive process → elastic recoil of lung

Question 3

If the phrenic nerve is paralyzed, the diaphragm moves up with inspiration… Why?

Answer 3

Negative pressure in the intrathoracic cavity → pulls diaphragm up

(subatmospheric pressure)

Question 4

Why is it important that the lungs have elastic properties?

Answer 4

• Inflate when pressure around the lung becomes subatmospheric
  
  • the pressure differential req’d to inflate the lung is greater than that req’d to deflate it
• Lung volume never goes to 0
  
  • always some air trapped in the lung
  • pressure to exhale collapses some small airways
• Lung collapses after it has been stretched by inspiration
  
  • reduces work of exhalation
  • surface tension of alveoli contributes to elastic behavior

Question 5

What is compliance? In the lung?

Answer 5

• Compliance = volume change per pressure change
  
  • measures distensibility of an organ, compartment or vessel
• Normally its 200 mL/cm H₂O for the lung

Question 6

What will reduce the compliance of the lung?

Answer 6

Fibrotic disease

(take smaller breaths because the lungs will not comply with large air intake)

Question 7

What will increase the compliance of the lungs?

Answer 7

Emphysema

(frequently take larger breaths to reduce work of breathing)

Question 8

What is lung surfactant critical?

Answer 8

• Decreases surface tension
  
  • prevents smaller alveoli from emptying into larger alveoli (smaller alveoli has lower pressure)
  • allows smaller alveoli to accomodate same pressure
• Surfactant provides a physical barrier preventing water molecules from interacting at smaller volumes

Question 9

What happens to intrathoracic pressure during expiration?

Answer 9

• Intrathoracic pressure increases
  
  • Causes collapse of airways
    
    • only occurs at low lung volumes in young healthy people
    • can occur at functional residual capacity in older folks and in respiratory disease
      
      • results in air trapping (emphysema)

Question 10

What effect to the chest wall and lungs have on the chest volume?

Answer 10

• Chest wall:
  
  • resting chest wall wants to expand
  • due to elastic recoil
  • opposes lungs
• Lungs:
  
  • resting lungs want collapse
  • due to elastic recoil
  • opposes chest wall force
• They normally oppose each other → keeping the lungs somewhat expanded and the chest wall somewhat compressed

Question 11

The work required to breath has to overcome what components?

(Hint: 3 components)

Answer 11

• Elastic elements of the lung
• Elastic elements of the chest wall
• Airway resistance

Question 12

What is airway resistance primarily determined by?

Answer 12

RADIUS of the airways

(decreased radius → increased resistance)

Question 13

How do you determine laminar flow? Resistance?

Answer 13

Flow = resistance / flow

Flow = alveolar pressure – pleural pressure

Resistance = Pressure / flow

Question 14

What is the pressure differential during inspiration?

Answer 14

atmospheric pressure – alveolar pressure

Question 15

Which type of airway is the major site of resistance?

Answer 15

Medium-sized bronchi

Question 16

When does airway resistance decline? Increase?

Answer 16

• Declines at larger lung volumes
  
  • negative pressure pulls airways open, as does radial traction of tissue
• Declines with sympathetic stimulation
• Increases at smaller lung volumes and during expiration
• Increases with parasympathetic nerve stimulation via acetylcholine

Question 17

How does parasympathetic nervous stimulation cause increased airway resistance?

Answer 17

Acetylcholine → muscarinic receptors → IP3 →

Ca²⁺ → smooth muscle contraction in lungs → increased airway resistance

Question 18

How does sympathetic nervous stimulation caused decreased airway resistance?

Answer 18

Epinephrine → Beta-2 receptors → cAMP → smooth muscle dilation → decreased airway resistance

(this is why we treat anaphylaxis with Epi pens)

Question 19

How does emphysema effect the work of breathing?

Answer 19

• Reduced lung recoil → reduced driving pressure
• Reduced radial traction on airways → easier to compress

Question 20

What is the Forced Expiratory Volume (FEV₁)?

Answer 20

• Volume of expired in 1 second after a maximal inhalation
  
  • normally will approximate 80% of forced vital capacity

Question 21

How does lung disease alter the FEV₁ and FEV₁:FVC ratios?

Answer 21

• Chronic obstructive lung diseases
  
  • low FEV₁
  • low FEV₁:FVC ratios
• Restrictive lung diseases (pulmonary fibrosis)
  
  • low FEV₁
  • low FVC
  • normal FEV₁:FVC ratios
    
    • both decrease at proportional values

Question 22

What does the FEV_25%-FEV_75% measure?

Answer 22

Average flow over the middle portion of a forced expiration.

Question 23

How is work defined? Work of respiration?

Answer 23

• Work = force x distance
• For respiration → involves pressure-volume relationships
  
  • Total work of breathing = effort to overcome elastic (collapsing) forces + work of overcoming airway/tissue resistance
    
    • the stiffer the lungs (less compliant) → the more work that must be done to inflate the lungs

Question 24

How does the work of breathing compare in Obstructive Disease vs. Restrictive Diseases?

Answer 24

• Obstructive
  
  • work to overcome flow resistance is increased
  • elastic work of breathing remains unchanged
• Restrictive
  
  • increase of elastic work of breathing
  • work to overcome flow resistance is normal