CPR 46-47 - Respiratory Muscles and Breathing Cycle; Mechanics of Lung and Chest

Question 1

What are the accessory muscles of inspiration? What do they do upon contraction?

Answer 1

External intercostals - pull ribs up and out

Scalene - helps stabilize first rib

Sternomastoid - helps stabilize first rib

Question 2

What are the accessory muscles of expiration? What do they do upon contraction?

Answer 2

Internal intercostals

Rectus abdominis

Obliques

Transversus abdominus

All serve to pull ribcage downwards and inwards

Question 3

What creates the pressure required for tidal expiration?

Answer 3

Elastic recoil of lung and chest wall. Mostly lung though

Question 4

Describe the positioning of the internal and external intercostal muscles.

Answer 4

Question 5

How is chest wall recoil pressure calculated?

Answer 5

P_cwr = Pip - P_atm

Question 6

What is P_ip normally and what does it fall to during normal tidal inspiration?

Answer 6

Normally -5 atm

Falls to -8 atm during tidal inspiration

Question 7

Does tidal inspiration or expiration take longer? Why?

Answer 7

Expiration

During inspiration, radial traction from alveolar expansion pulls the airways open wider. This does not occur during expiration which means there is more airway resistance during expiration.

Question 8

Draw these four graphs for one tidal inspiration and expiration:

Lung Volume vs Time

P_ip vs Time

Flow vs Time

P_A vs Time

Answer 8

Question 9

Explain the concept of hysteresis and how it applies to the lungs.

Answer 9

Hysteresis refers to the dependance of the state of a system on that system’s past. This applies to lung compliance because the lungs are more compliant upon deflation (expiration) then they are on inflation (inspiration). This occurs because to inflate the lungs have to pull apart the alveoli (fighting surface tension).

Question 10

Why does the surface of a liquid act as an elastic sheet?

Answer 10

Surface Tension

Molecules below the surface are attracted to each other from all angles. Molecules at the surface are only attracted to molecules below the surface or also at the surface. As a result, the forces holding surface molecules together is unbalanced and this creates the elastic tension.

Question 11

How is the force of surface tension measured?

Answer 11

Force per unit length or energy per unit area

Question 12

What is LaPlace’s Law and why is this important to respiratory physiology?

Answer 12

Surface tension tends to form a sphere with a pressure inside. LaPlace’s Law is an equation for calculating that pressure - P = 4T/r (for a bubble) or P = 2T/r (for alveoli). This concept is important because without surfactant small alveoli would collapase to inflate large alveoli.

Question 13

T/F - Lung surfactant has hydrophobic and hydrophilic components.

Answer 13

True

Question 14

Describe the composition of lung surfactant. Mention what percentage each component makes up and the purpose of each component.

Answer 14

• 90% Phospholipids to interrupt surface tension
  
  • 60% phosphatidylcholine (DPPC)
  • 7-15% phosphatidylglycerol
  • 25-33% other phospholipids
• 10% Protein
  
  • SP-A & SP-D for lung defense
  • SP-B & SP-C to facilitate surfactant monolayer formation

Question 15

Describe how the surface tension across the alveoli changes throughout inspiration and expiration. Explain why these changes occur.

Answer 15

Overall surface tension is higher during expiration than inspiration. This is because at the beginning of inspiration the surfactant molecules are much more concentrated. The opposite is true at the beginning of expiration. Refer to image.

Question 16

What common lung diseases increase/decrease lung compliance?

Answer 16

Emphysema/COPD increase lung compliance

Pulmonary fibrosis and respiratory distress syndrome decrease lung compliance

Question 17

Draw the chest wall and lung recoil graphs. Mark where each force is at residual volume, FRC, and TLC.

Answer 17

Question 18

What sets the FRC?

Answer 18

FRC is the lung volume at which the lung elastic recoil is equal to the chest wall elastic recoil (no muscular input)

Question 19

Describe how and why lung dynamic compliance changes throughout the breathing cycle. Why is this an important concept?

Answer 19

Airway resistance decreases the flow rate of air into or out of the lung. Because of this, when breathing frequency increases the expiratory muscles begin contracting before the lungs have filled with air. This causes a decrease in dynamic compliance and a reduction in tidal volume. This effect is negligible in a healthy person but significant in patients with increased airway resistance (ie-asthma).