2.3 - mechanics of breathing

Question 1

what is compliance?

Answer 1

stretchiness of the lungs

Question 2

what is compliance defined as?

Answer 2

the volume of change per unit pressure change (emphasis on pressure change)

Question 3

when is compliance increased?

Answer 3

during emphysema - alveoli air sacs permanentely stretched

Question 4

how is compliance decreased?

Answer 4

during lung fibrosis - loss of elastin in alveoli wall so alveoli can’t stretch

Question 5

even with constant elasticity of lung structures, what will compliance also depend on?

Answer 5

the starting volume from which it is measure:

specific compliance = volume change per unit pressure change / starting volume of lung

Question 6

where do the elastic properties of the lungs arise from?

Answer 6

1. elastic tissues in the lungs

2. surface TENSION forces of the FLUID lining the alveoli

Question 7

where is surface tension found?

Answer 7

the airways and alveoli of the lungs are lined with a film of fluid which is increased in area as the lungs expand
this increase in area is opposed by ST of the lining fluid (VdW forces)

Question 8

what does an area of a gas-liquid interface always tend to?

Answer 8

a minimum

Question 9

what does the alveolar lining fluid contain?

Answer 9

a surfactant whose effect is to reduce tension forces

Question 10

what is surfactant?

Answer 10

a complex mixture of phospholipid & proteins, with detergent properties

Question 11

where do the hydrophilic and hydrophobic ends of the surfactants lie?

Answer 11

hydrophilic ends in alveoli fluid (facing fluid)

hydrophobic towards gas (away from liquids - phobic)

Question 12

what does the orientation of the surfactants result in?

Answer 12

surfactants float on the surface of the lining fluid, interspersed between the fluid molecules, disrupting interaction between surface molecules (reduce VdW) and so reducing surface tension

Question 13

what does surface tension of the alveolar fluid vary with?

Answer 13

the surface area of the alveolus

Question 14

what happens as the alveolus expand?

Answer 14

its surface area increases & the surfactant molecules are spread further apart, making them less efficient in reducing surface tension
as alveolus expands, surface tension of fluid lining it increases

Question 15

what happens as an alveolus shrinks?

Answer 15

the surfactant molecules come closer together increasing their conc on surface - therefore more efficient in reducing surface tension
effect of surfactant is to reduce surface tension forces greatly as area of the alveolus decreases

Question 16

what is the force required to expand small alveoli?

Answer 16

less than force required to expand large ones

due to less surface area as surfactant as more closely dispersed

Question 17

what is the property of surfactant also to serve?

Answer 17

to stabilise the lungs, by prevent small alveoli collapsing into big ones

Question 18

which law are the alveoli determined by?

Answer 18

law of Laplace:
pressure = (2 x ST) / radius of alveolus
(P=2T/r)
alveoli - like interconnected series of bubbles

Question 19

what happens if T is constant to smaller alveoli?

Answer 19

smaller radius, so higher pressure

higher pressure will empty into larger alveoli with lower pressure , so smaller alveoli will collapse

Question 20

alveoli vary in size, so if surface tension is constant, what would happen?

Answer 20

the alveoli would collapse to form a huge air filled space ‘bullae’

Question 21

what would happen if a bullae formed?

Answer 21

hughly reduce surface are for gas exchange

Question 22

why do bullae not form in lungs?

Answer 22

as alveolus expands, increasing radius
surfactant molecules are spread further apart, making them less efficient, thereby increasing the surface tension ‘T’, so..
as alveolus expands, r & T increases
alveolus shrinks, r & T decreases

Question 23

what does surfactant mean in relation to bullae and lung alveoli?

Answer 23

different sized alveoli can have the same pressure within them
stabilises lungs by preventing small alveoli from collapsing into big ones

Question 24

what are the 3 functions of the surfactant?

Answer 24

1. increase lung compliance by reducing surface tension
2. stabilises the lungs, preventing small alveoli collapsing into big ones
3. prevents the surface tension in alveoli creating a suction force tending to cause transudation fluid (pulling fluids from) pulmonary capillaries

Question 25

what is respiratory distress syndrome?

Answer 25

lung surfactant is absent from alveoli of a foetus younger than about 30 weeks (sometimes absent from full term babies) if born premature, lack surfactant
so ST of alveolar sacs is high - leading to increase tendency of alveoli collapse

Question 26

what are typical signs of respiratory distress?

Answer 26

cyanosis, grunting, intercostal & subcostal recession

Question 27

what is the treatment for RDS?

Answer 27

surfactant replacement via endotracheal tube, and supportive treatment with oxygen and assisted ventilation

Question 28

what else happens aside from work done against the elastic nature of the lungs?

Answer 28

energy must be expended to force air through the airways

Question 29

what is resistance of airway to flow determined by?

Answer 29

Poiseuille’s Law (when flow is laminar) - true of most airways of lungs
resistance = pressure / rate of flow
(radius^4 increase, resistance decrease)

Question 30

when does resistance of a single tube sharply increases?

Answer 30

with falling radius (terminal bronchioles etc.)
however, combined resistance of small airways is normally low because they are connected in parallel over a branching structure

Question 31

where is total resistance to flow higher?

Answer 31

total resistance to flow in downstream branches is less than upstream branch
most of resistance to breathing therefore resides in upper respiratory tract, except when small airways are compressed during forced expiration (resistance increases significantly)

Question 32

what is asthma and COPD characterised by?

Answer 32

airways narrowing

Question 33

what happens in asthma?

Answer 33

inflammatory mediators (histamine?) released as a result of hypersensitivity reaction, cause contraction of bronchial smooth muscles and inflammatory swelling of the mucosa –> leading to small airways

Question 34

what happens in COPD?

Answer 34

the host response to inhaled cigarette smoke and other toxic substances cause chronic inflammation and oxidative injury

Question 35

how are the airways narrowed in COPD?

Answer 35

1. excessive mucus in lumen

2. breakdown of elastin leading to destruction of alveolar walls, causing loss of ‘radial traction’

Question 36

what is radial traction?

Answer 36

the outward tugging action of the alveolar walls on the small bronchioles which holds them open

Question 37

why is there excessive mucus in lumen in COPD?

Answer 37

from increased mucus production and reduced clearing of mucus due to ciliary dysfunction

Question 38

overall, what is work done against in lungs?

Answer 38

work is done against the elastic recoil of lungs and thorax, greatest part of work being against more or less equal to:

1. elastic properties of lung tissues
2. surface tension forces in alveoli

Question 39

how much effect does resistance to flow through airways have?

Answer 39

of little significance to total work load of breathing in healthy subjects, through it can often be affected by disease

Question 40

at rest, the work of breathing consumes how much oxygen consumption?

Answer 40

only 0.1% of total O2, so efficient