Lung compliance

Question 1

what is lung compliance?

Answer 1

compliance is the distensibility of the lungs

Question 2

how are compliance and elasticity related?

Answer 2

compliance is the reciprocal of elasticity

Question 3

if something is more elastic is it more/less compliant?

Answer 3

less
N.B. think of this in terms of stretching a thick and thin rubber band, the thick rubber bands will be harder to stretch outwards but will have a larger elastic recoil

Question 4

if something is very elastic is it more/less compliant?

Answer 4

more
N.B. think of this in terms of stretching a thick and thin rubber band, the thick rubber bands will be harder to stretch outwards but will have a larger elastic recoil

Question 5

the more elastic something is the easier/harder it is to stretch, and the more/less the elastic recoil?

Answer 5

harder

more

Question 6

how is compliance defined?

Answer 6

compliance is defined as a change in volume produced by a change in pressure
i.e.
compliance = change in volume/change in pressure

Question 7

how can we measure intrapleural pressure? why can we do this and what can this be used for?

Answer 7

using a small balloon inserted through the nose into the oesophagus 30-35cm down (to standardise for changes in intrapleural pressure). the oesophagus is thin walled and exposed to the pleural pressure, under static conditions can use this pressure as representative of trans-pulmonary pressure (the pressure across the alveoli) to determine lung compliance.

Question 8

how do we measure changes in volume (how much air is coming in and out of the lungs) to calculate compliance?

Answer 8

spirometry

Question 9

what is trans-pulmonary/lung recoil pressure equal to?

Answer 9

the difference between the alveolar pressure and the pleural pressure (same as the thoracic pressure)
i.e.
trans-pulmonary pressure = alveolar pressure - pleural pressure

Question 10

what is the average pleural pressure under static conditions?

Answer 10

-0.5 kPa

Question 11

how can we work out trans-pulmonary/lung recoil pressure?

Answer 11

we know that trans-pulmonary pressure = alveolar pressure - pleural pressure. under static conditions the alveolar pressure is equal to the atmospheric pressure i.e. 0, pleural pressure in on average -0.5 kPa and so trans-pulmonary pressure is on average 0.5 kPa

Question 12

name three things that can disrupt chest wall compliance and the changes these have

Answer 12

1. flail chest - multiple rib fractures, resulting in increased compliance of chest wall and paradoxical breathing (chest moves opposite direction to normal as ribs are broken, allowing lungs to recoil inwards)
2. kyphoscoliosis - angulation of the spine anterior-posterior (kyphosis) and lateral (scoliosis) in orientation. rib distortion and respiratory impairment as the chest becomes less compliant - stiffer and more difficult to inflate.
3. neuromuscular disease - stiffening of connective tissue as a result of chronically reduced chest wall motion due to respiratory muscle weakness/reduced nerve supply. decreased chest wall compliance therefore contributes to respiratory dysfunction

Question 13

name the component of lung tissue that is responsible for ~50% of lung recoil and what else is it responsible for?

Answer 13

elastin

along with recoil, elastin is also responsible for stretch

Question 14

when elastin is stretch to double its length, how much does this increase the volume of the lungs by?

Answer 14

increases volume by 8 times

i.e. 1cm –> 2 cm = 8 cm3

Question 15

what limits the maximal expansion of the lungs and why?

Answer 15

collagen - exists in coils/spirals that can extend but not stretch.

Question 16

what is destructed in emphysema and what affect does this have?

Answer 16

elastin

becomes easier to stretch the lungs but much more difficult to breathe out

Question 17

along with stretching elastic fibres, what other force do you need to overcome to breath in, where is this force present and what proportion of energy is required to overcomes this?

Answer 17

surface tension in the alveoli, around half of energy used to breath in is used to overcome this

Question 18

which part of the lungs has the largest surface area exposed to the environment?

Answer 18

the alveoli

Question 19

what affect does the layer of liquid on the air-liquid interface of the alveoli have?

Answer 19

liquid has stronger Van der Waal forces than gas, thus surface tension pulls alveoli in on themselves

Question 20

what affect does surface tension have on the alveoli?

Answer 20

makes them wants to collapse

Question 21

name two properties that help hold airways open?

Answer 21

1. parenchymal tethering - fibres that hold the airways open
2. surfactant

Question 22

how would the inflation curve of the lungs change without surfactant and why?

Answer 22

it would be move significantly to the right as you would need to generate a lot more pressure to get a change in volume, as surfactant acts as a detergent that disrupts surface tension

Question 23

where is surfactant made, stored and released?

Answer 23

surfactant is produced by type II alveolar cells where it is stored in lamellar bodies before exocytosis on to the alveolar surface

Question 24

which cells in the alveoli are involved in gas exchange?

Answer 24

type I alveolar cells

Question 25

what is surfactant, how many constituents does it have and what is the main type of constituents (give an example)?

Answer 25

it is a detergent, made up of 47 constituents, mainly lecithins such as dipalmitoylphosphatidylcholine (DPPC) - a detergent

Question 26

along with molecules that act as detergents, give some examples of other molecules in surfactant (3)

Answer 26

molecules that help get surfactant to the surface to do its job, molecules that help surfactant disperse across the surface and some immunoglobulins

Question 27

describe the structure of a detergent molecule

Answer 27

polar/hydrophilic head with a non-polar/hydrophobic tail

Question 28

in the absence of surfactant alveoli are more prone to ….. and they are ….. to inflate

Answer 28

collapse | harder

Question 29

what is the role of surfactant?

Answer 29

to reduce surface tension within the alveoli, to reduce the pressure generation needed to inflate lungs. also by reducing surface tension it prevents alveoli from collapsing.

Question 30

what forces does surfactant interfere with in order to reduce surface tension?

Answer 30

Van der Waal's forces

Question 31

when alveoli expand and increase in surface area, surfactant is diluted, what affect does this have?

Answer 31

increases surface tension and acts as a break on rapidly expanding alveoli

Question 32

why is initially easier to inflate an alveoli?

Answer 32

surfactant concentration is at it maximal and so have relatively low surface tension

Question 33

for normal inflation to occur in the lungs what pressure needs to be generated and why? and how and why does this change in the absence of surfactant?

Answer 33

a pressure of -0.5 kPa needs to be generated to overcome surface tension and tissue forces. in the absence of surfactant, surface tension is increased and so a much higher pressure of -3 pKa is needed to be produced for inflation to occur, increased work is needed to generate this pressure

Question 34

50% of energy used in inhalation is used for … (2)

Answer 34

overcoming elastic tissue | overcoming surface tension

Question 35

what is the most common cause of lung problems in premature babies and what causes it?

Answer 35

infant respiratory distress syndrome (IRDS) caused by insufficient production of surfactant, due to underdeveloped type II alveolar cells, leads to increased surface tension and alveolar collapse (atelectasis), much more energy needed to generate pressure to overcome this, leads to fatigue

Question 36

when do we start producing surfactant?

Answer 36

around 24 weeks, start having sufficient surfactant around 34 weeks