Mechanics of breathing 1

Question 1

Describe the ‘active process’ of inspiration vs the ‘passive’ expiration

Answer 1

• inspiration is an active process - which occurs due to the contraction of the inspiratory muscles
• expiration is a passive process that occurs due to the elastic recoil of the lungs

Question 2

What occurs as a result of the contraction of the inspiratory muscles?

Answer 2

• contraction expands the thoracic cavity which leads to a decreased intrapleural pressure
• the drop in intrapleural pressure** helps the lungs to expand**
• the expansion of the lungs** decreases intrapulmonary pressure to sub-atm level **to which air from the atmosphere is sucked into the lungs

Question 3

What is Boyle’s law?

Answer 3

• the pressure (P) exerted by a constant number of gas molecules in an enclosed space is** inversely proportional** to the volume (V) of the container

Question 4

How does air move in bulk flow in terms of pressure

Answer 4

air moves in bulk flow from areas of high pressure to one of low pressure

Question 5

What is flow (F) equal to?

Answer 5

• the pressure difference between 2 points (delta P) and inversely propotional to the resistance

Question 6

What is the equation for flow (F)?

Answer 6

F= delta P/R

ie F = Palveolar - Patm/R

Question 7

What is the pleura?

Answer 7

• The pleura is a 2 layered membrane - the parietal and the visceral pleura
• it functions to reduce any friction between the rib cage, lung and chest cavity

Question 8

What are the main muscles associated with inspiration?

Answer 8

1. Diaphragm
2. External intercostal muscle

Question 9

What are the main muscles involved in expiration?

Answer 9

1. Internal intercostal
2. pectoral muscle
3. abdominal muscles

Question 10

What happens to the abdominal contents when the diaphragm contracts?

Answer 10

• the abdominal contents are forced downwards and forwards

Question 11

What happens to the ribs when the diaphragm contracts?

Answer 11

the rib margins are lifted and move outwards

Question 12

Where are the external intercostal muscles present?

Answer 12

they are present between ribs in a forward and downward direction

Question 13

What 2 effects does contraction of external intercostal muscles?

Answer 13

• bucket-handle effect : increases the transverse diameter of the thoracic cavity
• pump-handle effect: increases the vertical diameter of the thoracic cage

Question 14

What are important ‘accessory’ muscles for inspiration?

Answer 14

• scalene muscles - muscles attaching cervical vertebrae to first and second rib
• pectoralis major - muscles attaching thoracic cage to upper limb

Question 15

What is the function of the scalene muscle during inspiration?

Answer 15

• elevates 1st and 2nd rib’s

Question 16

Describe the sequence of active inspiration

Answer 16

1. inspiratory muscles contract
2. lung volume increases
3. alveolar pressure** decreases **
4. alveolar pressure is less than atm pressure
   5.** air flows into** the lungs

Question 17

Describe the basic sequence of expiration (passive)

Answer 17

1. relaxation of inspiratory muscles
2. lung volume decreases
3. alveolar pressure increases
4. alveolar pressure is greater than atm pressure
5. air flows out of the lung

Question 18

What are 3 pressures of the thoracic cavity

Answer 18

1. **intrapleural **pressure
2. transpulmonary pressure
3. alveolar pressure

Question 19

what is intrapleural pressure?

Answer 19

• the pressure in the pleural space
• ie the space between the lungs and the chest wall (the virtual space between the visceral and parietal pleura)

Question 20

what is the normal value (during quiet breathing) of intrapleural pressure ?

Answer 20

-2.5 to -4mm of Hg

Question 21

what is transpulmonary pressure?

Answer 21

• the measurement is the pressure difference between the alveolar space and the intrapleural space

Question 22

what is the function of the transpulmonary pressure?

Answer 22

keeps the lungs inflated and prevents the lungs from collapsing

Question 23

what is the alveolar pressure?

Answer 23

• the pressure inside the alveoli
• the change in alveolar pressure moves air in and out of the lungs

Question 24

what happens to the alveolar pressure during inspiration?

Answer 24

• alveolar pressure decreases which sucks air into the lungs

Question 25

what happens to the alveolar pressure during expiration?

Answer 25

the alveolar pressure increases, removing air from the lungs

Question 26

what is the normal value for alveolar pressure during inspiration & expiration?

Answer 26

Inspiration :-1mmHg expiration: +1mmHg