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)
there is extensive branching in both bronchial and arterial anatomy to achieve this
blood vessels branch more that bronchi so have bigger airspaces with smaller vessels

Question 2

how is oxygen transported

Answer 2

majority of oxygen is carried by haemoglobin, a small fraction is dissolved in the blood but this is not a very effective method of transportation

Question 3

what occurs at equilibrium

Answer 3

partial pressure of gas in a solution equals partial pressure of gas above liquid

Question 4

what makes up haemoglobin

Answer 4

2 alpha and 2 beta subunits, each subunit has a haem group which binds oxygen, this alters its shape and charge

Question 5

what is a haem group

Answer 5

a porphyrin with a central ferrous atom

Question 6

describe oxygen/haem dissociation

Answer 6

the affinity of binding O2 increases with each successively bound O2 molecule (allosteric affect)
a number of factors can change the ability of haem to take up and liberate O2
goal is to take up O2 in lungs and liberate O2 at tissues

Question 7

oxygen haemoglobin dissociation curve

Answer 7

a right shift in the curve decreases haems affinity for oxygen
a right shift is caused by increased CO2, increased H+, increased temp, increased 2,3 DPG

Question 8

what is gas exchange driven by

Answer 8

partial pressure
partial pressure of O2 in the alveolus equals the partial pressure in the blood draining the alveolus

Question 9

partial pressure in the lungs as whole

Answer 9

there is no apparent equilibrium
the partial pressure of oxygen in arterial blood is lower than the alveolus
this is due to shunting and dead space

Question 10

describe anatomical shunts

Answer 10

a small amount of arterial blood doesn’t come from the lungs (thebesian veins)
a small amount of blood goes through without seeing gas (bronchial circulation)

Question 11

describe physiological shunts

Answer 11

where there is a V/Q mismatch
anatomical dead spaces represent the conducting airways where no gas exchange takes place
alveolar dead space represents areas of insufficient blood supply for gas exchange - practically non-existent in healthy young but appears with age and disease
physiological dead space is both of these combined

Question 12

describe the V/Q ratio

Answer 12

if ventilation = perfusion then we will get perfect gas exchange (shunting aside)
at the top of the lungs there is a natural V/Q mismatch with less blood and air going to the top of the lung

Question 13

describe V/Q mismatch in the lungs

Answer 13

top of the lung ventilation is greater than perfusion
middle of lung ventilation is roughly equal to perfusion
bottom of lung ventilation is less than perfusion
in healthy lungs the physiological V/Q mismatch generally cancels itself out, in disease it becomes more apparent, lung diseases also cause additional V/Q mismatch leading to gas exchange problems

Question 14

why do patiints become hypoxaemic

Answer 14

hypoventilation
V/Q mismatch (pathological vs physiological)
or both combined

Question 15

describe hypoventilation

Answer 15

not enough oxygen provided for adequate gas exchange
common causes are respiratory diseases, airway obstruction, opiate toxicity

Question 16

describe failure of the ventilatory pump

Answer 16

wont breathe - control failure, brain fails to command, often caused by drug overdose
cant breathe - broken peripheral mechanism, caused by nerves not working (spinal injury), muscles not working (muscular dystrophy), chest cant move (severe scoliosis) and gas cant get in and out (asthma/COPD)

Question 17

hypoventilation and the effect on CO2

Answer 17

during normal ventilation CO2 diffuses out of blood into alveolus following a partial pressure gradient
CO2 is mostly involved in blood, if there is lower ventilation then CO2 accumulates in the alveolar space meaning less can be removed from the blood

Question 18

describe V/Q mismatch

Answer 18

not enough oxygen encountering blood to allow adequate gas exchange

Question 19

what causes V/Q mismatch

Answer 19

conditions that thicken the alveolar wall or narrow and block small airways
lung infection such as pneumonia
bronchial narrowing such as asthma and COPD (although they can progress to hypoventilation and type 2 resp failure)
interstitial lung disease
acute lung injury

Question 20

V/Q mismatch in pneumonia

Answer 20

hypoxaemia as blood does not come in contact with adequate O2
CO2 will also increase but this does not impact overall CO2 levels in blood

Question 21

pathology of pneumonia

Answer 21

causes inflammation and damage in the small airways and alveoli
narrows airways and causes fluid build up in alveoli

Question 22

what happens to arterial O2 in V/Q mismatch

Answer 22

blood leaving areas of low V/Q ratio have low PaO2 and high PaCO2
high PaCO2stimulates ventilation
‘extra’ ventilation goes to areas of normal lung and areas with high V/Q ratio
but extra ventilation cant push O2 content much higher than normal
blood from both areas mix but cannot overcome the low O2 level

Question 23

what happens to arterial CO2 in V/Q mismatch

Answer 23

blood leaving areas of low V/Q ratio have 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 mixes so overall CO2 is normal

Question 24

how do we treat hypoxia in V/Q mismatch

Answer 24

high flow O2

Question 25

causes of V/Q mismatch due to perfusion problems

Answer 25

pulmonary embolism
can range from small pe causing no problems with gas exchange to massive pe with hypoxia
emboli effectively cause areas of dead space where there is ventilation but not perfusion causing hypoxia
massive emboli can cause circulatory failure and death

Question 26

diagnosing respiratory failure

Answer 26

if PaO2 is low patient is respiratory failure
if PaCO2 high then respiratory failure is type 2

Question 27

describe type 1 respiratory failure

Answer 27

decrease in PO2
normal PCO2
common causes in hospital - pneumonia, pe, acute severe asthma and COPD
due to VQ mismatch as main problem

Question 28

type 2 respiratory failure

Answer 28

decrease in PO2
increase in PCO2
common causes in hospital - opiate toxicity, severe COPD (can be acyte or toxic) acute severe asthma and pulmonary oedema in left ventricular failure
due to hypoventilation as main feature

Question 29

treatment for type 1 respiratory failure

Answer 29

give oxygen
this is a sort term life saving measure
the fundamental problem is inadequate gas exchange
improve gas exchange by treating the underlying cause
in some cases mechanical ventilation is required

Question 30

treatment for type 2 resp failure

Answer 30

give oxygen (controlled in COPD patients with chronic resp failure)
treat the underlying cause to reverse hypoventilation
support ventilation (non-invasive or invasive)

Question 31

describe O2 masks

Answer 31

variable performance - they are cheap, exact inspired O2 concentration unknown
fixed function - constant know inspired concentration
reservoir mask - high inspired concentration of O2
venturi mask - aim to supply oxygen at a flow rate faster than the patient can breathe
CPAP mask or helmet - provides positive pressure as well as high flow O2, increases alveolar SA and improves VQ mismatch
nasal high flow - reduces anatomical dead space, gives close to 100% O2, comfortable