Gas Exchange in the Lungs

Question 1

what does efficient gas transfer require

Answer 1

large surface area of contact between air (alveoli) and blood (capillaries)

Question 2

what forms the ideal lung

Answer 2

extensive branching in both bronchial and arterial anatomy
blood vessels branch more than bronchi so have bigger airspaces with smaller vessels

Question 3

what feature of alveoli and capillaries allows for gas transfer

Answer 3

thin walls

Question 4

equilibrium of partial pressure

Answer 4

PP of gas in solution = PP of gas above air

Question 5

what happens to most oxygen

Answer 5

carried by haemoglobin rather than dissolved

Question 6

features of haemoglobin

Answer 6

a tetramer - 2 alpha and 2 beta subunits
each subunit has a haem group - a porphyrin with a central ferrous atom which binds o2
combines loosely with oxygen
combination alters shape and charge

Question 7

allosteric effect

Answer 7

affinity of binding o2 increases with each successively bound o2 molecule

Question 8

what happens once o2 is bound to hb

Answer 8

number of factor can change ability of hb to take up and liberate oxygen

Question 9

what does right shift o2/hb dissociation mean

Answer 9

less affinity for o2
gives up oxygen more readily
hb liberates o2 in tissue

Question 10

what causes a right o2/hb dissociation

Answer 10

increase co2
increase H+
increase temp
increase 2,3-DPG

Question 11

what does a left o2/hb dissociation result in

Answer 11

hb takes up o2 in lung

Question 12

partial pressure of oxygen pO2 (kPa)

Answer 12

gas exchange driven by PP
PP of o2 in alveolus = PP in blood draining alveolus

Question 13

partial pressure when considering lung as complete unit

Answer 13

no apparent equilibrium
PP of o2 in arterial blood lower than PP of alveolus
due to shunting and dead space

Question 14

what is shunting

Answer 14

to move something from one place to another, usually because that thing is not wanted, without considering any unpleasant effects

Question 15

anatomical shunts

Answer 15

small amount of arterial blood doesnt come from lung - thebesian veins
small amount of blood goes through without seeing gas

Question 16

physiological shunts (decrease V) and alveolar dead space (decrease Q)

Answer 16

not all lung units have same ratio of ventilation (V) to blood flow (Q)
V/Q mismatch

Question 17

what accounts for lower pO2 of arterial blood than expected

Answer 17

anatomical shunts
physiological stunts and alveolar dead space

Question 18

physiological dead space

Answer 18

anatomical dead space represents the conducting airways where no gas exchange takes place
alveolar dead space represents areas of insufficient blood supply for gas exchange and is practically non-existant in health young but appears with old age
physiological dead space = anatomical dead space + alveolar dead space

Question 19

ventilation and perfusion ration

Answer 19

V/Q
if ventilation = perfusion then will get perfect gas exchange (shunting aside)
in the lung naturally have V/Q mismatch with less blood and air going to top of the lung

Question 20

‘normal’ v/q mismatch

Answer 20

less airflow and blood flow at the top of the lung but v>q = increased v/q, higher pO2
middle of lung v/q normal
bottom of lung more ventilation and more blood flow but v<q so decreased v/q, lower pO2

Question 21

physiological v/q mismatch

Answer 21

in healthy lungs physiological v/q mismatch generally cancels itself out

Question 22

v/q mismatch in lung disease

Answer 22

mismatch becomes more apparent with disease
lung diseases cause additional mismatch leading to gas exchange problems

Question 23

why do patients become hypoxaemic

Answer 23

hypoventilation
ventilation-perfusion (v/q) mismatch (pathological vs physiological)
combination of both

Question 24

hypoventilation as a cause of low oxygen levels

Answer 24

not enough oxygen being provided for gas exchange

Question 25

what causes hypoventilation

Answer 25

CNS - decreased central respiratory drive
airway - potential difficult airway, obstructive sleep apnoea
cardiovascular - coronary artery disease, congestive heart failure
respiratory - restrictive chest physiology, pulmonary hypertension, hypoxaemia/hypercapnia
others - difficult vascular acccess, difficult positioning

Question 26

failure of ventilatory pump

Answer 26

wont breathe - control failure
- brain failure to command e.g. drug overdose
cant breathe - broken peripheral mechanism
- nerves not working e.g. spinal injury
- muscles not working e.g. muscular dystrophy
- chest cant move e.g. severe scoliosis
- gas cant get in and out e.g. asthma, copd

Question 27

hypoventilation and co2

Answer 27

oxygen levels decrease in hypoventilation
normal ventilation - co2 diffuses out blood into alveolus following partial pressure gradient
co2 mostly dissolved in blood rather than bound to haemoglobin
lower ventilation - co2 accumulates in alveolar space meaning less can be removed from blood

Question 28

v/q mismatch as cause of low oxygen

Answer 28

not enough oxygen encountering blood to allow adequate gas exchange

Question 29

what causes v/q mismatch

Answer 29

conditions thickening alveolar wall or narrow and block small airways
lung infections e.g. pneumonia
bronchial narrowing such as asthma and copd - can progress to hypoventilation and type 2 resp failure
interstitial lung disease
acute lung injury

Question 30

v/q mismatch in pneumonia

Answer 30

pneumonia causes inflammation and damage in small airways and alveoli
hypoxaemia is because blood does not come into contact with adequate o2
co2 will also decrease but doesnt impact overall co2 levels in blood

Question 31

what happens to arterial o2 in v/q mismatch

Answer 31

blood leaving areas of low v/q ratio has low PaO2 and high PaCO2
high PaCO2 stimulates ventilation
extra ventilation goes to areas of normal lung and areas with high v/q ratio
extra ventilation cant push o2 content much higher than normal
blood from both areas mix but cant overcome low o2 level

Question 32

what happens to arterial co2 in v/q mismatch

Answer 32

blood leaving areas of low v/q ratio has low PaO2 and high PaCO2
high PaCO2 stimulates ventilation
extra ventilation goes to areas of normal lung and areas with high v/q ratio so get blood with low co2
blood from both areas mix so overall co2 is normal

Question 33

v/q mismatch due to perfusion problems

Answer 33

pulmonary embolism
can range from small PTE causing no problems with gas exchange to massive PE with hypoxia
emboli effectively cause areas of dead space with ventilation but no perfusion causing hypoxia
massive emboli can cause circulatory failure and death

Question 34

respiratory failure - PaO2 lower than expected

Answer 34

v/q mismatch or hypoventilation

Question 35

diagnosing resp failure

Answer 35

PaO2 is low - patient has resp failure
if PaCO2 high - type 2 resp failure - ventilatory failure (hypoventilation)
if PaCO2 not high - type 2 resp failure - v/q mismatch

Question 36

type 1 resp failure

Answer 36

decrease in po2
normal pco2
common causes in hospital - pneumonia, PE, acute severe asthma, copd
due to vq mismatch as main problem

Question 37

type 2 resp failure

Answer 37

decrease in po2
increase in pco2
common causes in hospital - opiate toxicity, severe copd (acute or chronic), acute sever asthma, pulmonary oedema in acute left ventricular failure
due to hypoventilation as main feature

Question 38

type 1 resp failure treatment

Answer 38

give oxygen - short term life saving measure
fundamental problem inadequate gas exchange
improve gas exchange by treating underlying cause
some cases - mechanical ventilation required

Question 39

type 2 resp failure treatment

Answer 39

give oxygen - controlled in copd patients with chronic resp failure
treat underlying cause to reverse hypoventilation e.g. bronchodilators for acute asthma or opiate antagonists for overdose
support ventilation - either non-invasive or invasive

Question 40

oxygen therapy - masks

Answer 40

variable performance - cheap, exact inspired o2 concentration not known
fixed function - constant, known inspired concentration
reservoir mask - high inspired o2 concentration

Question 41

pre mixed gases - venturi mask

Answer 41

venturi principle uses negative pressure zone
velocity increase - increased kinetic energy
uses specifically designed plastic jet system to deliver oxygen

Question 42

controlled oxygen therapy - venturi mask

Answer 42

aims to supply oxygen at faster rate than patient can breathe

Question 43

reservoir masks

Answer 43

supplies maximum amount of oxygen
stores during expiration
delivers during inspiration

Question 44

nasal high flow oxygen

Answer 44

reduces anatomical dead space
gives close to 100% oxygen
comfortable

Question 45

cpap

Answer 45

either a mask or a helmet
provides positive pressure as well as high flow oxygen
increases alveolar surface area and improves vq mismatch

Question 46

invasive ventilation

Answer 46

required for severe resp failure not responding to o2 therapy
not suitable for all patients
provided in intensive therapy units

Question 47

non invasive ventilation for type 2 resp failure

Answer 47

common treatment for copd exacerbations with type 2 resp failure
tight fitting mask, no need to sedate and intubate
increases ventilation efficiency
also useful in neuromuscular disease and thoracic wall disease