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24
Ventilation-Perfusion Relationships
73
Pharmacology
Undergraduate 1
12/19/2016

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Term
Define ventilation
Definition
air supply
alveolar ventialtion = 4.2Lmin-1
Term
Define perfusion
Definition
blood supply
Term
Describe the pressure in pulmonary circulation
Definition
remarkably low
Term
MAP in the pulmonary artery is...
Definition
only 15mmHg
pressure drop across the circuit is only 10mmHg
Term
How do pulmonary capillaries compare to systemic capillaries regarding pressure?
Definition
pulmonary < capillary pressure
pulmonary are much more susceptible to collapse if external pressures are high
Term
What is the pressure acting on pulmonary capillaries?
Definition
alveolar pressure
Term
How does the pulmonary circulation decrease its resistance as pressure increases?
Definition
recruitment and distension
Term
Describe recruitment
Definition
as pressure rises, previously 'closed' capillaries open and being to conduct blood (adding resistance units in parallel decreases overall resistance)
Term
Describe distension
Definition
as pressure rises, capillary lumen radius increases (shape changes from oval to circular), which decreases resistance to flow (Poiseuille's Law)
Term
What is the direction of blood flow in the lungs?
Definition
apex (top) to base (bottom)
flow decreases linearly from bottom to top
Term
What does blood flow in the lungs depend on? (3)
Definition
1. hydrostatic pressure (blood pressure within the capillary)
2. arteriovenous pressure difference (difference between the blood entering the capillary and the blood leaving the capillary)
3. alveolar pressure (external pressure acting upon the capillary)
Term
What are the zones of the lungs?
Definition
The zones of the lung divide the lung into 3 vertical regions, based upon the relationship between the pressure in the alveoli (PA), in the arteries (Pa) and the in the veins (Pv)
Term
Define ventilation
Definition
air supply
alveolar ventialtion = 4.2Lmin-1
Term
Define perfusion
Definition
blood supply
Term
Describe the pressure in pulmonary circulation
Definition
remarkably low
Term
MAP in the pulmonary artery is...
Definition
only 15mmHg
pressure drop across the circuit is only 10mmHg
Term
How do pulmonary capillaries compare to systemic capillaries regarding pressure?
Definition
pulmonary < capillary pressure
pulmonary are much more susceptible to collapse if external pressures are high
Term
What is the pressure acting on pulmonary capillaries?
Definition
alveolar pressure
Term
How does the pulmonary circulation decrease its resistance as pressure increases?
Definition
recruitment and distension
Term
Describe recruitment
Definition
as pressure rises, previously 'closed' capillaries open and being to conduct blood (adding resistance units in parallel decreases overall resistance)
Term
Describe distension
Definition
as pressure rises, capillary lumen radius increases (shape changes from oval to circular), which decreases resistance to flow (Poiseuille's Law)
Term
What is the direction of blood flow in the lungs?
Definition
apex (top) to base (bottom)
Term
What does blood flow in the lungs depend on? (3)
Definition
1. hydrostatic pressure
2. arteriovenous pressure difference
3. alveolar pressure
Term
What are the zones of the lungs?
Definition
The zones of the lung divide the lung into 3 vertical regions, based upon the relationship between the pressure in the alveoli (PA), in the arteries (Pa) and the in the veins (Pv)
Term
How can the inequality of blood flow within the upright human lung be explained?
Definition
by the hydrostatic differences within the blood vessels: a gradient of around 23mmHg exists between apex and base
Term
Where is the most amount of blood flow in the lungs?
Definition
in the capillaries at the base of the lung as the capillaries have higher pressure at the bottom compared to the top (so there will be more capillary recruitment and distension at the base)
Term
How does this change in the supine position?
Definition
base to apex differences disappear
apical part of the lung blood flow increases and asal zone remains unchanged
Term
How does this change in exercise?
Definition
blood flow increases throughout the lung and regional variation is reduced
Term
Describe zone 1
Definition
apex
PA>pa>Pv
capillaries squashed flat; no flow (alveolar dead space)
Term
Describe zone 2
Definition
mid-lung
Pa>pA>Pv
flow determined by the difference between Pa and PA
Term
Describe zone 3
Definition
base
Pa>Pv>PA
Flow determined by the arteriovenous pressure difference
Term
Which zone doesn't exist under normal conditions? why?
Definition
zone 1
in normal health pulmonary arterial pressure (Pa) exceeds alveolar pressure (PA) in all parts of the lung
Term
When is zone 1 present?
Definition
is arterial pressure is reduced (e.g. during a haemorrhage)
blood vessels can become completely collapsed by alveolar pressure and blood does not flow through these regions.
they become alveolar dead space.
Term
How does ventilation differ in the regions of the lung?
Definition
there are regional differences in ventilation
per unit volume, ventilation is greater at the base of the lung than at the apex
Term
What causes the ventilation differences in the upright lung?
Definition
the weight of lung and the effects of gravity
intra pleural pressure is increased at the base of the lung compared to the apex
Term
Compare the alveoli at the base to the apex, at rest
Definition
alveoli at the base will be less expanded than the alveoli at the apex
Term
How do the alveoli begin to change upon respiration?
Definition
greater change in volume in the alveoli at the base (partly because it has more scope to expand)
Term
Describe and explain the differences in surfactant in the alveoli and the effects it has
Definition
in an expanded alveolus, the concentration of surfactant is significantly less than in a smaller alveolus. Therefore, less surfactant = increased surface tension = decreased compliance = less of a volume change
Term
What is the ventilation-perfusion ratio?
Definition
the ratio between alveolar ventilation (VA) and pulmonary blood flow (Q)

the ratio that o2 is being added to the lung and the rate at which it is diffusing in the blood
Term
Describe the ventilation-perfusion ratio at the base of the lung
Definition
pulmonary blood flow > the ratio between alveolar ventilation
Q > VA
ratio is 0.8
Term
How does the ventilation-perfusion ratio change as you move up the lung?
Definition
both Q and VA decrease
Q decreases at a faster rate
ratio increases towards 1 (and exceeds 1 towards the top of the lung)
Term
What is the result of a mismatched ventilation-perfusion ratio?
Definition
impairment of both o2 and co2 transfer
Term
What does VA:Q ratio play a key role in determining?
Definition
PaO2
PaCO2
Term
In a normal lung, VA:Q is around 1, under these circumstances, what are PaO2 and PaCO2?
Definition
PaO2: 100mmHg
PaCO2: 40mmHg
Term
What are the effects of decreased ventilation due to airway obstruction?
Definition
VA:Q decreases
decreased PaO2
increased PaCO2
Term
What are the effects of decreased perfusion due to capillary damage?
Definition
VA:Q increases
increased PaO2
decreased PaCO2

however, increased PaO2 doesn't lead to a significant rise in the amount of o2 transported, as haemoglobin is almost fully saturated at normal PaO2 levels
Term
How does the body try to match the VA:Q?
Definition
ventilation-perfusion coupling
local auto regulatory mechanisms continuously respond to alveolar conditions
Term
What causes PAO2 to fall and PACO2 to rise? what is the response in pulmonary arterioles?
Definition
reduced alveolar ventilation; excessive perfusion

pulmonary arteriolar constriction which reduces perfusion and redirects blood to better ventilated areas
Term
What causes PAO2 to rise and PACO2 to fall? what is the response in pulmonary arterioles?
Definition
inadequate perfusion

pulmonary arterioles dilate which increases capillary blood flow and increase o2 uptake
Term
What is the response in the systemic capillaries?
Definition
the opposite
e.g. increased PAO2 causes systemic arteriole to constrict
Term
What are the effects of high PACO2?
Definition
bronchodilator to increase elimination of co2
Term
What are the effects of low PACO2?
Definition
bronchoconstriction to redirect airflow to better perfused area
Term
Define hypoxaemia
Definition
abnormally low level of o2 in the blood

PaO2 is abnormally low (either as a result of a low PAO2 or a normal PAO2 but a significant difference between PAO2 and PaO2)
Term
Define hypoventilation
Definition
ventilation is inadequate to perform needed gas exchange -> leads to hypoxaemia
Term
Describe the 'ideal lung'
Definition
no difference between PAO2 and PaO2:
- diffusion across the respiratory membrane proceeds to equilibrium in all lung units
- all the blood passing through the pulmonary circulation is exposed to alveolar air
Term
What are the 4 types of hypoxaemia
Definition
1. hypoventilation
2. diffusion limitation
3. shunt
4. ventilation perfusion inequality
Term
What can cause hypoxaemia
Definition
- anything that affects the rate or volume of air entering lungs (ventilation)
-anything that affects the transfer of air from the lungs to the blood (perfusion)
- cardiovascular shunts
Term
What is PAO2 determined by?
Definition
balance between
1. rate of removal of o2 by the blood (set by metabolic demands of respiring tissue)
2. rate of replenishment of o2 by alveolar ventilation
Term
What happens if rate of replenishment of O2 falls and rate of removal stays the same?
Definition
PAO2 and PaO2 fall
PACO2 and PaCO2 rise

hypoventilation
Term
What causes hypoventilation?
Definition
- drugs that depress central respiratory drive (e.g. opiates, barbiturates)
- damage to chest wall
- paralysis of respiratory muscles
Term
Does diffusion ever achieve equilibrium in a healthy lung?
Definition
no, but very close (very small difference)
Term
When does a larger difference occur?
Definition
1. during exercise (due to reduced time available for diffusion to reach equilibrium)
2. result of thickening of the blood-gas barrier (e.g. asbestosis)
Term
Explain what is meant by a ventilation-perfusion mismatch
Definition
a disruption in the ventilation-perfusion equilibrium
Term
What causes a ventilation-perfusion mismatch
Definition
- oxygen entering the lungs typically diffuses across the alveolar-capillary membrane into blood
- this does not occur when the alveolus is insufficiently ventilated
- blood exiting alveolus is hypoxaemic
- blood mixes with well ventilated alveolus blood and the mix has a lower oxygen partial pressure than alveolar air
- alveolar-arterial difference develops
Term
What is shunting?
Definition
- shunting refers to blood that bypasses the pulmonary circulation, meaning that the blood does not receive o2 from the alveoli
Term
Where can a shunt be?
Definition
within the heart or lungs
cannot be corrected by administering o2 alone
Term
What causes a shunt?
Definition
- abnormal vascular connection between a small pulmonary artery and vein (pulmonary arteriovenous fistula)
- cardiac atrial septal defect exists, allowing venous blood to mix with arterial blood
Term
What are the effects when the whole lung is exposed to lowered PAO2, like in high altitude?
Definition
HAPE
Term
Describe the mechanism of HAPE
Definition
1. uniform constriction of all the pulmonary arterioles = increased pulmonary TPR and rise in pulmonary MAP and increased RV work
2. some arterioles constrict more than others
3. this gives rise to hydrostatic edema
Term
What is hydrostatic edema?
Definition
fluid leaks out into and around the surrounding alveoli
Term
What does hydrostatic edema initiate?
Definition
an inflammatory response which increases pulmonary capillary permeability and makes the situation worse
Term
What is the best treatment for HAPE?
Definition
descent
or supplemental o2 and hyperbaric bag
Term
Which drug can be used to treat HAPE?
Definition
nifedipine
blocks Ca2+ channels in vascular smooth muscle
reduces the extent of the pulmonary hypoxic vasoconstrictor response to hypoxia
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