Mechanics of breathing

Question 1

How does air reach gas exchange surfaces?

Answer 1

Passing through series of increasingly narrow and numerous airways
So impaired airway function= insufficient ventilation

Question 2

What is airway resistance?

Answer 2

An opposing force that acts to reduce flow of gas through airway
- Occurs when air comes into contact with airway surface

Question 3

State the 3 major variables that affects airway resistance?

Answer 3

Cross-sectional area of the airway lumen
Pattern of airflow
Airway patency (ability of a person to breathe)

Question 4

State what rate of airflow is dependent on?

Answer 4

• Pressure gradient
• Level of airway resistance

Question 5

What is the equation for airflow?

Answer 5

Ohm’s law
Airflow (V) = Change in Pressure (P)/ Resistance

Question 6

Why can increasing the pressure to increase flow be limited?

Answer 6

Respiratory system has specific physical limitations

• Effort/force required to do so may not be able to be generated
• Or Airway might be completely obstructed.

Question 7

What is the simplified Hagen-Poiseuille equation?

Answer 7

Resistance (R) is proportional to 1/r^4.
r= radius (cross-sectional)

R = 8nl / πr4 (Overall equation) FC

Question 8

What happens as the airway radius decreases?

Answer 8

The resistance increases and airflow decreases dramatically

Question 9

State events which can reduce size of airway lumen?

Answer 9

Contraction of airway smooth muscle
Excessive mucus secretion
Oedema/swelling of the airway tissue
Damage to the integrity of airway structure (loss of patency)

Question 10

How does pattern of airflow affect resistance?

Answer 10

Turbulent flow will lead to higher airway resistance than laminar flow

Healthy airways have laminar flow that has minimal resistance

Question 11

How is turbulent flow achieved?

Answer 11

High velocities of airflow occur due to either

• Forced breathing manoeuvres
• Sudden decrease in luminal area e.g. obstructured airways

Generates vibration

Question 12

What is generated as a result of the vibration from turbulent airflow?

Answer 12

Wheezing sound produced in patients with obstructured airways

Question 13

What is airway patency?

Answer 13

“Refers to state of the airway being open or unobstructed
- Loss of patency= closing/obstruction”

Question 14

Describe airway patency in healthy individuals?

Answer 14

“Elastin in surrounding alveoli provides radial traction to splint (support) bronchioles against positive Palv

radial traction as elastic springs that anchor the airway lumen to the lung interstitium (parenchyma) hence helping to keep the airway patent when insp/exp occurs.”

Question 15

What can reduce airway patency?

Answer 15

Pressure differentials between interpleural space and airways during forced expirations

Question 16

What can occur if COPD occurs to airway patency?

Answer 16

Degradation of airway structure
Intrapleural pressure becomes postiive, collapsing force is exerted onto airways
Without radial traction, bronchioles collapse
Airway obstruction occur
Loss of airway patency

Question 17

What is transpulmonary pressure?

Answer 17

Transpulmonary pres. (Ptp) = Alveolar pres. (Palv) – Intrapleural pres. (Pip)
- It determines the level of force acting to expand or compress the lungs

Question 18

What is lung compliance?

Answer 18

The relationship between the level of expansive force (a particular change in transpulmonary pressure) applied to the lung and the resulting change in lung volume
- Essentially describes how easy a lung can expand/distended

More compliance = less force/pressure required to produce volume change

Question 19

State equation for lung compliance?

Answer 19

Compliance (C_L)= ΔVolume/Δ (transpulmonary) Pressure

Transpulmonary pressure = It determines the level of force acting to expand or compress the lungs

Question 20

Describe features of higher lung compliance?

Answer 20

Higher lump compliance -> Less elastic recoil -> Less force required to inflate -> Greater volume change per pressure change (greater gradient on volume-pressure curve)
- Looser/easier to inflate lung

Question 21

Describe features of lower lung compliance?

Answer 21

Lower lump compliance -> more elastic recoil -> more force required to inflate -> Lower volume change per pressure change (Lower gradient on volume-pressure curve)
- So stiffer/harder to inflate lung

Question 22

What is lung compliance determined by?

Answer 22

Structure of the lung tissue

Question 23

On a graph of lung volume vs transpulmonary pressure, what represents
compliance?

Answer 23

Gradient of the curve

Question 24

For curve of lung vol. vs transpulmonary pressure, what is used to represent compliance
for static compliance and dynamic compliance

Answer 24

Static: Measurements taken when airflow=0
- Steepest part of curve is sued

Dynamic: Measurements taken when airflow is occuring
- Gradient between end tidal inspiratory and end tidal expiratory

Question 25

State factors or diseases that would affect the chest wall mechanics for lung compliance?

Answer 25

Scoliosis, Muscular dystrophy, obesity - All lead to lowered lung compliance Chest wall naturally recoils outwards so increases compliance as role

Question 26

State factors or diseases that would affect the alveolar surface tension for lung compliance?

Answer 26

NRDS (neonatal respiratory distress syndrome) - Decreases Lung compliance Alvolar surface tension decreases compliance

Question 27

State factors or diseases that would affect the elastin fibres for lung compliance?

Answer 27

Fibrosis- lower lung compliance COPD (emphysema)- Higher lung compliance Elastin fibres decrease compliance (resists expansion of lungs)

Question 28

State the effect emphysema and fibrosis have on the lungs?

Answer 28

Emphysema (COPD) - Degradation of elastin fibres - Lungs are less stiff - More compliant (but reduced recoil) Fibrosis: - Scarring and deposition of structural fibres e.g. collagen - Stiffer lung - Less compliant

Question 29

What are alveoli lined with and what is the purpose of this?

Answer 29

Lined with fluid to enable gas exchange. The gas molecules dissolve into water before diffusing.

Question 30

How is surface tension generated within the alveoli?

Answer 30

1. The water-air interface formed by the fluid lining on the wall of an alveolar airspace essentially creates a bubble. 2. Within the bubble, surface tension arises due to the hydrogen bonds between water molecules. 3. Combining to exert an overall collapsing force toward the centre of the bubble. - A sufficient force must be generated to resist the collpasing pressure, Otherwise the bubble will collapse

Question 31

What is the result of the collapsing force of the bubble in an alveoli?

Answer 31

The collapsing force produced at the water-fluid interface generates pressure within a bubble

Question 32

How can the amount of pressure in a specific bubble of an alveoli be calculated?

Answer 32

Law of Laplace.

Question 33

What does the Law of Laplace for the alveoli describe?

Answer 33

Described the relationship between: 1. Collapsing pressure. 2. The radius of the bubble. 3. The surface tension (which varies depending on the nature of the fluid)

Question 34

Equation for the Law of Laplace of the alveoli

Answer 34

``` P = 2T/r P = Pressure T = Surface Tension (tendency of liquid surfaces to shrink into the minimum surface area possible) r = Radius of Bubble ```

Question 35

What is true if surface tension from the law of laplace remains constant?

Answer 35

P ∝1/r The smaller the alveoli (smaller bubble), the larger the pressure generated.

Question 36

What is the consequence of different sized bubbles of alveoli connected to each other

Answer 36

Pressure gradients would be created between different sized alveoli, resulting in smaller alveoli emptying into larger ones. Occurs due to more collapsing pressure in smaller than larger bubble and pressure gradients move from high to low. This would make inflation of the lungs extremely difficult.

Question 37

How is alveolar surface tension reduced?

Answer 37

Reduced by the presence of pulmonary surfactant.

Question 38

What is pulmonary surfactant secreted from?

Answer 38

Secreted by Type II Pneumocytes (alveolar cells)

Question 39

State the two roles of pulmonayr surfactants?

Answer 39

- Reduces alveolar surface tension - Equalizes pressure and volume in alveoli of varying size

Question 40

How does pulmonary surfactant prevent smaller alveoli from collapsing?

Answer 40

Surfactant molecules are amphipathic - So naturally position themselves at the air-liquid interface Presence of surfactants disrupt H-bonds between water molecules Reduces surface tension and collapsing pressure generated

Question 41

How does pulmonary surfactant equalise pressure and volume in varying alveoli?

Answer 41

1. During inflation of the lungs, as alveoli expand. 2. The concentration of surfactant molecules decreases. - Same number of molecules within an increased surface area 3. Increasing surface tension. 4. Overall collapsing pressure increases as alveolar size increases - This generates pressure gradients within the lung - Now, larger (more inflated) alveoli tend to collapse into smaller ones, helping consistent inflation of the lungs.

Question 42

What does surface tension in alveoli reduce?

Answer 42

Surface tension produced at the air-liquid interface reduced hydrostatic pressure in the alveolar tissue Fluid is pulled out of the surrounding capillaries and into the alveoli and interstital tissue.

Question 43

What does pulmonary surfactant help prevent?

Answer 43

Alveolar oedema - It reduced surface tension so excessive fluid isn't taken up from the capillary

Question 44

How can alveolar oedema occur when there is insufficient surfactant?

Answer 44

High surface tension Leads to collapsing force produced Decrease in hydrostatic pressure in tissue surrounding capilalry Fluid pulled from capillary into alveolus Alveolar oedema occurs

Question 45

How is neonatal respiratory distress syndrome caused by/

Answer 45

insufficient production of pulmonary surfactant

Question 46

Outline the steps of Neonatal Respiratory Distress Syndrome:

Answer 46

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

Question 47

What are the two ways in which insufficient surfactant production is overcome?

Answer 47

Maternal glucocorticoid supplementation: stimulate the production of surfactants via maturation of type 2 pneumocytes Artificial surfactant supplementation of infant.