Airway function and mechanics

Question 1

Ventilation

Answer 1

Movement of air in and out of lungs to allow replenishment of O2 and removal of CO2

Question 2

The Respiratory Cycle
- Inspiration
- Expiration

Answer 2

INSPIRATION (ACTIVE)
1. Stimulation of diaphragm by phrenic nerve and IC nerves
2. Increases thorax volume to generate negative intra-pulmonary pressure (less than atm pressure)
3. Allows inhalation of air into lungs

EXPIRATION (PASSIVE)
1. Normally passive: once inspiratory muscles relax, elastic recoil of lungs generate +ve intra-pulmonary pressure
2. Pushes air out

** Active expiration can occur during exercise / disease

Question 3

Work of Breathing (WOB)

Answer 3

Energy required to actively inspire by overcoming resistive (friction through bronchi) and elastic forces of lungs

Question 4

Airway Resistance: Turbulent vs Laminar Flow

Answer 4

Turbulent Flow
- Larger upper airway
- Used clinically to listen to breath sounds

Laminar Flow
- Silent, small airways (bronchioles)
- Smooth and silent: only heard in disease (COPD and asthma) states

Question 5

Site of Maximum Airway Resistance

Answer 5

• Segmental bronchi
• Decreases as breathe progresses along bronchial tree due to increasing SA (more branches)

Question 6

How does negative intra-pulmonary pressure impact ventilation?

• Intrapulmonary pressure
• Atmospheric Pressure
• Intra-alveolar pressure

Answer 6

INTRAPULMONARY PRESSURE:
- Space b/w chest wall and lung
- Always lower than atmospheric and intra-alveolar pressure

INTRA-ALVEOLAR PRESSURE
- Always equalises to atmospheric pressure due to connection by bronchus

NEGATIVE IP PRESSURE:
- Competed by elastic recoil of lungs away from thoracic wall
- Outward pull is greater than inward recoil

Question 7

How does Pulmonary Pressure Gradient drive Ventilation?

Answer 7

INSPIRATION: Intra-alveolar pressure < Atmospheric Pressure

= pressure gradient
= air travels down pressure gradient from high in atm to low in alveolar

EXPIRATION: Intra-alveolar pressure > atm

= air travels from high in alveolar to low in atm

Question 8

Lung Volumes
- TV
- IRV
- ERV
- RV

Answer 8

Tidal Volume
- Amount of air inhaled and exhaled during normal resting breath

Inspiratory Reserve Volume
- Additional air that can be inhaled after a normal breath

Expiratory Reserve Volume
- Additional air that can be exhaled after a normal breath

Residual Volume
- Volume of air remaining in lungs after maximal exhalation

Question 9

Lung Capacities
- Inspiratory capacity
- Vital capacity
- Functional residual capacity
- Total lung capacity

Answer 9

IC
- Total amount of air that can be inhaled (TV + IRV)

VC
- Max amount of air that can be exhaled after full inhalation (TV + ERV + IRV)

FRC
- Amount of air remaining in lungs after normal exhalation (RV + IRV + ERV + RV)

TLC
- Total volume of air that lungs can hold (all lung volumes)

Question 10

Lung Pressures
- Intra-alevolar pressure
- Intrapleural pressure
- Atm pressure
- Transpulmonary pressure

Answer 10

Intra-alveolar pressure
- Pressure within alveoli of lungs
- Always equalises to atm pressure

Intrapleural pressure
- Pressure within pleural cavity
- Normally -ve to intra-alveolar and atm pressure

Atm Pressure
- Pressure of air outside body

Transpulmonary pressure
- Difference b/w intrapleural and intra-alveolar pressures
- Higher = larger lungs

Question 11

Inspiration vs Expiration Pressure Gradient Summary

Answer 11

Inspiration: IA < Patm

Expiration: IA > Patm

End of inspiration and expiration: IA = Patm (equalised)

Question 12

Consequences of Increased Respiratory Muscle Effort (Increased WOB)

Answer 12

• Recruitment of accessory muscles of inspiration (Scalenes, SCM)
• Increased O2 consumption by respiratory muscles = reduced blood O2 and increased CO2
• Risk of respiratory muscle fatigue (severe cases)