Mechanisms of breathing

Question 1

what is lung compliance

Answer 1

lungs stretch on inhalation 
static measure (active respiration not occurring)

Question 2

what happens during breathing

Answer 2

Lungs stretch on inhalation and recoil on expiration
Balanced by chest wall tendency to recoil in opposite direction
At end of quiet expiration, pressures balance

Question 3

what muscles are involved in inspiration

Answer 3

Active process driven by inspiratory muscles 
Diaphragm (75% change in vol)
External intercostals (contract up and forwards like a bucket handle)
Accessory muscles (scalene and sternomastoids)

Question 4

how does inspiration occur

Answer 4

Contract inspiratory muscles to increase intrathoracic volume
Causes decrease in intrapleural pressure (-2.5-6 mmhg)
Lungs pulled into more expanded position and airway pressure negative (air moves in)
At end pressures are equal, recoil of chest wall and lungs occurs

Question 5

what is transpulmonary pressure

Answer 5

Chest wall exerts distending pressure on the pleural space which is transmitted to alveoli to increase its volume, lower pressure and generate airflow inwards.
Distending pressure is called transpulmonary pressure
Chest wall expansion done by muscles

Question 6

how is compliance measured

Answer 6

Vol change per unit pressure change

Question 7

how is compliance different to resistance

Answer 7

function of elastic resistance, dynamic accounts for airflow resistance

Question 8

what is the balance point

Answer 8

when lung and chest are in equilibrium after exhaling (functional residual capacity)

Question 9

what is the endpoint of compliance

Answer 9

total lung capacity (TLC) and residual volume (RV)

Question 10

what does a pressure volume curve show about compliance

Answer 10

depends on inflation

lung volume increases, become less compliant

Question 11

do compliance curves differ for inspiration and expiration

Answer 11

Compliance curves are different for inspiration and expiration (hysteresis, frictional resistance changes)

Question 12

how compliant is the normal lung

Answer 12

compliance is right, good compliance for low work of inhalation and good retention of elasticity of alveolar units for effective inhalation

Question 13

how does interstitial fibrosis effect compliance

Answer 13

compliance decreased, alveolar walls are stiff and scarred

Question 14

how does emphysema effect lung compliance

Answer 14

compliance increased due to loss of alveolar interdependence

Question 15

how does a compliance curve change for diseased lungs

Answer 15

Stepper lung compliance curve for emphysema and shallower for fibrosis

Question 16

what happens during exhalation

Answer 16

Distending pressure is released
Built up potential in form of elastic recoil (passive relation of alveoli, decrease in alveolar volume, increase in Palv leading to outward air flow)

Question 17

when does active exhalation occur

Answer 17

expiratory respiratory muscles are engaged but short of exercise and disease (usually don’t need these)
Loss of elastic recoil impairs this

Question 18

what is surface tension

Answer 18

Cohesive forces between molecules
Molecules on surface have no molecules above them
Results in stronger attractive forces to nearest neighbours

Question 19

how does surface tension affect liquids

Answer 19

In liquids, SA small as possible due to ST pressures. Can lead to alveolar collapse especially with emphysema and age

Question 20

what is surfactant

Answer 20

Surfactant (type 2 alveolar cells) reduces surface tension (major component is phosphatidylcholine)
Hydrophilic and hydrophobic ends repel each other and interfere with liquid molecule attraction to lower surface tension

Question 21

is a smaller or larger radius more likely to collapse

Answer 21

Higher pressure from smaller radius so more likely to collapse

Question 22

why is surfactant important

Answer 22

Increases lung compliance (lung forces are reduced)
Promotes alveolar stability
Prevents alveolar collapse
Surface tension tends to suck fluid from capillaries into alveoli (reduced surface tension reduces hydrostatic pressure in tissue outside capillaries and keeps lungs dry

Question 23

what is airway resistance

Answer 23

Force needed to inflate greater than elastic recoil
Airway resistance originates from friction between air and mucosa
Is pressure difference between alveoli and mouth divided by flow rate

Question 24

how is pulmonary resistance calculated

Answer 24

Pulmonary resistance = tissue + airway

Question 25

what is a small airway

Answer 25

less than 2mm

Question 26

what are different types of airflow

Answer 26

Laminar smooth flow, resistance generated is proportional to radius
Turbulent flow is irregular, chaotic with eddie currents. Good for heat transfer but resistance is high.
Reynold’s number helps to predict laminar flow converting to turbulent

Question 27

why is radius most important to flow

Answer 27

Resistance is inversely proportional to the 4th power of radius, flow is inversely proportional to viscosity of fluid

Question 28

why is resistance not higher in smaller airways

Answer 28

ohm’s law parallel circuits (all add up to total resistance)

Question 29

what factors affect airway resistance

Answer 29

Inflammation
Mucus
Bronchodilators 
Steroids
Gas density (heliox, diving)

Question 30

how much work is required to breathe

Answer 30

required to stretch elastic tissues of chest wall and lungs, moving inelastic tissues and air through tubes. Amount of energy/O2 needed by respiratory muscles to produce enough ventilation and respiration to meet metabolic needs of the body.

Question 31

how does elastic work compare to non elastic work

Answer 31

Elastic work
Decreased elasticity in restrictive diseases
Greater WOB

Non elastic work
Obstructive diseases lead to greater WOB to overcome increased airway resistance