Respiratory Mechanics

Question 1

What 2 forces oppose ventilation? How is each measured?

Answer 1

1- Recoil (of wall and lungs); meas by compliance (C=delta V/ P alveoli)

2- Viscous (friction); meas by resistance (R = delta P/ flow)

Question 2

Lung Recoil v. Chest Wall Recoil

Answer 2

Lungs (spring that can only be stretched)

- Lungs always want to deflate so as lungs expand they generate an inward elastic recoil (takes more P to overcome inc elastic forces as vol inc)
- P-V curve does not cross x-axis (P does not go below 0)

Chest wall alone (spring that can be stretched or compressed)

- Takes neg pressure to make chest wall smaller than its equilibrium volume; generates outward recoil pressure
- Takes pos pressure to expand chest beyond equilibrium volume; generates inward recoil pressure
- P-V curve crosses X-axis (can have neg and pos pressures)

Question 3

Functional Reserve Capacity

Answer 3

• when PL = PW or PRS=0 (lungs inward recoil = wall outward recoil)
• resp system at equil

Question 4

Total Lung Capacity

Answer 4

-PL + PW must both be overcome by inspiratory muscles

Question 5

Residual Volume

Answer 5

-when outward recoil of PW is overcome by expiratory muscles

Question 6

What 2 forces contribute to compliance?

Answer 6

Tissue forces (collagen or elastin in lung parenchyma and rib, muscles, cartilage in chest wall)

Surface forces (surfactant - generates surface tension)

Question 7

What 2 factors contribute to resistance of airway?

Answer 7

• R = 8nL/pi r^4
• So less radius = more resistance
• And resistance inc w/ inc length

Question 8

What role does surfactant play? How does it depend on volume?

Answer 8

• Generates surface tension (but less ST than water)
• @ high alveolar vol (high surface area- larger air-liquid interface) … more surface tension generated
• @ lower alveolar vol (lower interface surface area) … less surface tension generate
• *This allows for dec recoil at low volumes so alveoli do not collapse

Question 9

Inspiratory v. Expiratory Muscles

Answer 9

• Inspiratory Muscles
  
  • Diaphragm
  • SCM
  • External Intercostals
• Expiratory Muscles
  
  • Internal Intercostals
  • Abdominal wall muscles

Question 10

2 Roles of Diaphragm

Answer 10

During inspiration it contracts –> flattens and lowers

1 - Inc vol of thorax to dec thorax pressure

2 - At same time it dec abdominal vol to inc abdominal pressure (this expands lower ribs out)

Question 11

What happens to pleural pressure during inspiration? What happens to alveolar pressure during inspiration?

Answer 11

• Pleural surfaces being pulled apart so intrapleural pressure becomes more negative as chest expands (relative to atm press); sep of layers
• Alveolar pressure is negative at first which creates a pressure gradient (b/n mouth and alveoli) to drive air into alveoli; this pressure gradient is used to overcome viscous forces as air moves into alveoli
• *Generated b/c we expand lungs faster than air actually enters alveoli

Question 12

What happens to pressures during passive expiration?

Answer 12

• Only have to overcome viscous forces of airflow going out b/c chest wall and lungs are returning to equilibrium volume (elastic recoil works in favor of outflow of air)
• Flow reaches 0 b/c alveolar pressure both go back to 0
• Volume reaches 0 b/c total elastic recoil goes back to 0

Question 13

What happens to pressures and flow during active expiration?

Answer 13

• Harder you push w/ expiratory muscles –> faster flow rate out
• Max flow depends on starting volume
  - If high volume at start of expiration - more effort = faster flow rate
  - If low volume at start of expiration - plateau at which inc effort no longer inc flow rate (lower Vmax); lower the starting volume, the less difference expiratory effort makes

Question 14

Expiratory Flow Limitation

Answer 14

• During active expiration there is positive pleural pressure (2 pleural layers come together)
• Inc pleural pressure is transmitted into alveoli but also pushes down on airway itself –> narrowing of airway –> when P inside airway = Pleural pressure outside airway (EPP- equal pressure point) –> airway collapses somewhere distal to this EPP
• Pressure gradient driving flow is now from alveoli to EPP NOT from alveoli to mouth opening

Question 15

What is the pressure gradient equal to in active expiration? How is this affected by volume?

Answer 15

• Pressure gradient = lung recoil pressure
• Greater lung volume = more recoil pressure = more pressure gradient
• SO… as volume inc the flow inc b/c more driving pressure out of lungs