L5- pressure and compliance

Question 1

What is meant by the following terms:

• TLC
  VC
• RV
• IC
• FRC
• IRV
• VT
• ERV

Answer 1

• TLC- total lung capacity- 6L- volume that the lungs hold when they are inflated to maximum
• VC- vital capacity- 4.8L maximum volume that can be exhaled forced inspiration
• RV- residual volume- 1.2L- volume remaining in the lungs after forced expiration
• IC- inspiratory capicity
• FRC- 2.7 L- Functional residual capacity- volume of air in the lungs at the end of passive expiration
• IRV- inspiratory reserve volume- extra volume that can be breathed in above tidal volume to meet increase in metabolic need
• VT- tidal volume- volume that we breath in normally
• ERV- expiratory respiratory volume- extra volume that can be breathed out below tidal volume

Question 2

What are the factors that affect lung volume?

Answer 2

Correlaton between lung volume and:

• Height (taller > smaller)
• Sex ( males > females)
• Age
• Race ( European>Asian)
• Respiratory Disease

Question 3

How does air flow into and out of the lung occur?

Answer 3

• Air flows from high pressure to low pressure
• for air to move into and out of our lungs there must be a pressure difference between P_B and P_A
• Given that we cannot (normally) change barometric pressure (P_B), then air flow into/out of lungs MUST occur as a consequence of changes in alveolar pressure, P_A
• According to Boyle’s law
• P₁ x V₁ = P₂ x V₂
• Increase V_A → P_A decreases, air in
• Decrease V_A → P_A increases, air in

Question 4

How is thoracic volume altered?

Answer 4

Passive breathing

• Inspiration
  
  • Diaphragm and external intercostal muscles contract
• Expiraton
  
  • Diaphragm and external intercostal muscles relax
  • elastic recoil

Forced breathing

• Forced inspiration
  
  • Diaphragm and external intercostal muscles contract
  • Accessory muscles- scalenes, SCM, pectoralis major and minor, serratus anterior and latissimus dorsi
• Forced expiration
  
  • Abdominal muscles contract
  • internal and innermost intercostal muscle contract

Question 5

Why are respiratory muscles needed?

Answer 5

• stretch elastic components of respiratory system
• overcome resistance to flow

Question 6

How are the lungs and the throrac cage coupled? i.e. how does the work of respiratory muscles acting on the thoracic volume get transmitted into a change in lung volume

Answer 6

• Lung thorax coupling is determined by the elastic properties of the lungs and thoracic cage
• At FRC, respiratory muscles are relaxed
  
  • the outward and inward recoils are equal and opposite- lungs want to recoil inwards, throacic cage wants to recoil outwards
  • this creates a negative subatmospheric intrapleural pressure but the respiratory system is at equilibrium
  • the visceral and paritetal pleura push one against the other

Question 7

What happens to airflow, alveolar pressure and intrapleural pressure during a normal breathing cycle?

Answer 7

• Airflow profile follows P_A
• P_A is negative in inspiration and postive in expiration
• P_pl becomes more negative in inspration and less negative in expiration
• P_A is always > P_pl
• In quiet breathing, P_pl is always negative
• Airflow ceases when P_A is zero

Question 8

What happens to intrapleural pressure in high ventilation and forced expiration?

Answer 8

At high ventilation, P_pl and P_A changes are increased

P_pl can be +ve in forced expiration (eg +8kPa in coughing/sneezing)

Question 9

What happens to recoil pressure during a breathing cycle?

Answer 9

• Lung recoil pressure is greatest at the start of expiration

Question 10

How does a loss of recoil due to emphysema affect intrapleural pressure and what is the clinical significance?

Answer 10

• Positive intrapeural pressure can’t be created
• lead to a difficulty coughing
• infection- unable to clear mucus from airways