Mechanics of breathing I

Question 1

What do changes in lung volume induce?

Answer 1

→ Changes in alveolar pressure which generate pressure gradients between alveoli & atmosphere, causing air to flow.

Question 2

How does inspiration occur?

Answer 2

→Diaphragm contracts & thoracic cavity expands. →Alveolar pressure decreases

Question 3

How does expiration occur?

Answer 3

→Diaphragm relaxes (and lung recoils).
→Thoracic cavity volume decreases
→ alveolar pressure increases

Question 4

At the end of expiration why is there no movement of air?

Answer 4

→Palveoli = Patmosphere

Question 5

During inspiration what happens in terms of pressure?

Answer 5

→The outer surfaces of the lung are pulled outwards
→ ↑volume
→ ↓alveolar pressure.
→Palveoli < Patmosphere
→Air flows from high (atmosphere) to low (alveoli) pressure.

Question 6

During expiration what happens in terms of pressure?

Answer 6

→Air within the lung is compressed.
↓volume = ↑alveolar pressure.
→Palveoli > Patmosphere
→Air flows from high (alveoli) to low (atmosphere) pressure.

Question 7

What is the pleural cavity?

Answer 7

→fluid filled space between the membranes (pleura) that line the chest wall and each lung.

Question 8

What do pleurae provide?

Answer 8

→a frictionless surface to aid movement of the lungs.

Question 9

What decreases the pressure within the pleural cavity?

Answer 9

→The opposing elastic recoil of the chest wall (outward) and lungs (inward) generates negative pressure within the pleural cavity

Question 10

What does negative pressure do to the pleurae?

Answer 10

→Negative pressure acts to pull the two membranes together

Question 11

What are the criteria for the lung volume to remain constant?

Answer 11

→At the end of expiration, the system is in equilibrium:

→If the force pulling the visceral pleura inwards (lung recoil) = the force pulling the visceral pleura outwards (negative intrapleural pressure)
→lung volume will remain constant.

Question 12

How does the lung expand in relation to interpleural membranes?

Answer 12

→During inspiration, muscular contraction pulls the parietal pleura outwards
→stretching the cavity
→decreasing intrapleural pressure
→As the intrapleural pressure becomes more negative, →the force pulling the two pleurae together increases. →When this force becomes greater than the force generated by the elastic recoil of the lung
→the visceral pleura will be pulled outward
→expanding the lung

Question 13

How does the intrapleural pressure increase during passive expiration?

Answer 13

→During (passive) expiration
→relaxation of contracted respiratory muscles
→reduces the outward force acting on the parietal pleura →reducing the cavity stretch
→ increasing intrapleural pressure

Question 14

How is lung volume decreased in relation to intrapleural pressures?

Answer 14

→When the increased (less negative) intrapleural pressure does not generate sufficient force to overcome the elastic recoil of the lung
→ the visceral pleura will be pulled inward (along with the pleural cavity and parietal pleura)
→decreasing lung volume

Question 15

During forced expiration why is there a more rapid decrease in lung volume?

Answer 15

→During a forced expiration
→Contraction of other respiratory muscles (such as the abdominals and internal intercostals) acts to provide further inward force on the parietal pleura
→ compressing the pleural cavity (further increasing PIP) →forcing an increased and more rapid decline in lung volume.

Question 16

Describe the whole process of inspiration

Answer 16

Respiratory muscles (e.g. diaphragm) contract
↓
Volume of thoracic cavity increases
↓
Intrapleural pressure becomes more negative
↓
Outward force exerted on visceral pleura becomes greater than inward recoil force
↓
Lungs expand, increasing volume
↓
PAlv (alveolar pressure) decreases below PAtm (atmospheric pressure)
↓
Air moves down pressure gradient, through airways into alveoli, expanding the lungs

Question 17

Describe the whole process of expiration

Answer 17

Respiratory muscles (e.g. diaphragm) relax, lungs recoil due to elastic fibres
↓
Volume of thoracic cavity decreases
↓
Intrapleural pressure increases
↓
Lungs compressed*, volume decreases
↓
PAlv increases above PAtm
↓
Air moves down pressure gradient, into atmosphere, deflating lungs

Question 18

When does compression of lungs due to increased intrapleural pressure occur and what happens during quiet breathing?

Answer 18

→Compression of the lungs due to increased intrapleural pressure only occurs during forced expiration.

→In quiet breathing, elastic recoil is sufficient to decrease lung volume.

Question 19

What is the PiP in a healthy lung?

Answer 19

99.5kPa

Question 20

What is the PiP in open pneumothorax?

Answer 20

100kPa

Question 21

What is the PiP in closed pneumothorax?

Answer 21

100kPa

Question 22

What happens if there is entry of air into the pleural cavity?

Answer 22

→loss of negative intrapleural pressure

→ collapse of lung tissue

Question 23

What happens if either pleural membrane is ruptured?

Answer 23

If either pleural membrane is ruptured
→ the pressure gradient between the pleural cavity and surrounding environment will cause air to enter (pneumothorax)
→until intrapleural pressure = atmospheric pressure.

Question 24

How does entry of air into the pleural cavity prevent the lungs from expanding properly?

Answer 24

→Entry of air = ↑ pleural cavity volume (at the expense of the lung volume, which decreases).
→This then acts to reduce intrapleural pressure changes generated during inspiration
→preventing the lungs from expanding properly.

Question 25

What does transpulmonary pressure reflect and what is it equivalent to?

Answer 25

→reflects the difference between PAlv and PIP

→and is equivalent to the level of force being exerted to change lung volume

Question 26

What are alveoli lined with and why?

Answer 26

→Alveoli are lined with fluid to enable gas exchange (the gas molecules dissolve into water before diffusing

Question 27

What does the water-air interface formed by the fluid lining on the wall of the alveolar airspace form and what is within this?

Answer 27

→The water-air interface creates a bubble.

→ Surface tension arises due to the hydrogen bonds between water molecules combining to exert an overall collapsing force toward the centre of the bubble.

Question 28

What does the collapsing force in the bubble generate?

Answer 28

→The collapsing forces generates pressure.
→The amount within a specific bubble is described by the Law of Laplace
→P = 2T/r

Question 29

What is the relationship between the size of the alveoli and the pressure generated?

Answer 29

→The smaller the alveoli, the larger the pressure generated.

Question 30

What is a major problem that can arise from different sized alveoli?

Answer 30

→Pressure gradients would be created between different sized alveoli
→resulting in smaller alveoli emptying into larger ones

Question 31

How is alveolar surface tension reduced?

Answer 31

→Alveolar surface tension is reduced by the presence of pulmonary surfactant.

Question 32

What is pulmonary surfactant secreted by?

Answer 32

→Type II pneumocytes

Question 33

What does pulmonary surfactant do?

Answer 33

→acts to equalise pressure & volume across varying alveoli
→As alveoli expand, the concentration of surfactant molecules decreases
→ increasing the surface tension.
→Larger alveoli tend to collapse into smaller ones
→helping the consistent inflation of the lungs.

Question 34

What is neonatal respiratory distress syndrome caused by?

Answer 34

→insufficient production of pulmonary surfactant

Question 35

How does respiratory distress syndrome lead to pulmonary vasoconstriction, endothelial damage, acidosis & pulmonary + cerebral haemorrhage? (flow chart)

Answer 35

Premature birth, maternal diabetes, congenital developmental issues
↓
Insufficient surfactant production
↓
Stiff (low compliance) lungs, alveolar collapse
↓
Respiratory failure
↓
Hypoxia
↓
Pulmonary vasoconstriction, endothelial damage, acidosis, pulmonary + cerebral haemorrhage.

Question 36

How can respiratory distress syndrome be treated and by what?

Answer 36

→Artificial surfactant supplementation of infant

→Maternal glucocorticoid supplementation.