Lung Ventilation & Perfusion Flashcards
What is ventilation?
The movement of gases into and out of the lungs
Where does ventilation occur and how is it measured?
Ventilation of the lungs occurs through the airways
It is measured as a change in volume per unit time
What is perfusion?
The blood flow through any organ, such as the lungs
Where does perfusion occur and how is it measured?
It occurs through blood vessels
It is measured as the flow of blood per unit time
Why do the basal regions of the lungs have better ventilation than apical regions?
Before inspiration, there is already a large quantity of air in the apical alveoli
During inspiration, basal alveoli can expand more and take in more air than apical alveoli
How much bigger are apical alveoli than basal alveoli?
Apical alveoli are around 4 times larger than basal alveoli in the upright position
This is due to the effect of gravity
What are the 2 types of circulation in the lungs?
Functional supply and structural supply
What is functional supply involved in?
Oxygenation of venous blood
It is the main blood supply to the lungs
What is the role of pulmonary arteries and pulmonary veins?
Pulmonary arteries take deoxygenated blood from the right ventricle to the lungs
Pulmonary veins carry oxygenated blood back to the heart through the left atrium and ventricle
What is the role of structural supply?
Supplies nutrition to the lung tissue
Also known as bronchial circulation and accounts for only 2% of cardiac output
What blood vessels are involved in structural supply?
The bronchial artery comes from the thoracic aorta
The bronchial vein comes from the superior vena cava
How much blood do the lungs receive?
The full cardiac output of 3-5 L/min
How does pulmonary circulation differ to systemic circulation?
The blood is at a much lower pressure
Why is blood pumped by the left ventricle into systemic circulation at a much higher force?
Left ventricle has a much higher muscle mass and is around 3x thicker than the right ventricle
How much lower is pressure in the pulmonary blood vessels than systemic circulation and why?
pressure in the pulmonary vessels is 5-6 x lower than systemic vessels
systemic vessels have more elastic tissue and smooth muscle which provides more resistance to the flow of blood
Where are extra-alveolar vessels found?
they run through the lung parenchyma
How does the pulmonary artery split as it enters the lungs?
The pulmonary artery splits into the right and left branches as it enters the hilum
As it leaves the hilum it successively splits into smaller branches - extra-alveolar vessels
Where are alveolar vessels found and what is their role?
They are the capillaries which surround the alveolus
This creates a large surface area for efficient gas exchange at a fast rate
How long does the whole process of gas exchange take?
3/4 of a second
How many alveoli are in the lungs and how many capillaries supply each alveolus?
300 million alveoli in the lungs and 1,000 capillaries per alveolus
What generations of airway divisions make up the conducting and respiratory portions?
The first 16 generations of airway divisions make up the conducting portion
The last 7 generations of airway divisions make up the respiratory portion
In which blood vessels does gas exchange start?
Small arterioles
The blood vessels begin to form capillary beds beyond the terminal bronchioles which start with smaller arterioles
What is the combined surface area of the capillary network forming a ‘sheet’ of blood?
50 - 150 metres squared
What does emphysema result in?
widespread destruction and dilatation of the distal airway
due to the loss of elastic recoil and destruction of the lung parenchyma
what are the symptoms of emphysema?
poor gas exchange and hypoxia
What is hypoxia?
Hypoxia is a condition in which a region of the body is deprived of adequate oxygen supply at the tissue level
What is hydrostatic pressure and where is it greatest?
the force exerted by the weight of a fluid due to gravity
it is greater at the bottom
How can pulmonary circulation be considered in terms of hydrostatic pressure?
A column of blood exerting different hydrostatic pressures in different areas of the lungs
This occurs in relation to the position of the right ventricle
What is the diameter of the extra-alveolar vessels dependent on?
Diameter is affected by lung volume
This is due to the pull of the lung parenchyma
What is the diameter of the alveolar vessels dependent on?
it is dependent on many factors
What is Starling’s resistor comprised of?
upstream and downstream pipes connected by a soft, flexible pipe that is responsive to pressure changes
How is flow through the collapsible segment in Starling’s resistor determined?
It is determined by the difference of two out of the three largest pressures
Pus, Pds, Pout
What is upstream pressure equivalent to?
arterial pressure
hydrostatic pressure of the blood at the arterial end of the capillary
What is the downstream pressure equivalent to?
venous pressure
the hydrostatic pressure of the blood at the venous end of the capillary
What is P(out) equivalent to?
alveolar pressure
this is the gaseous pressure inside the alveoli
What is zone 1 of the lungs?
alveolar dead space
what is the perfusion and ventilation like in zone 1 of the lungs?
There is no perfusion under any circumstances
There is good ventilation
What is gas exchange like in zone 1?
No gas exchange occurs due to poor blood flow
Where is the alveolar dead space found in the lung?
It is very small and comprises the apices of the lungs in healthy people
The apex of the lung is 15cm above the right ventricle
How is blood flow determined in zone 1 of the lungs and why is there no blood flow?
Blood flow is determined by the difference in alveolar and arterial pressure
There is no blood flow as P(alveolar)>P(arterial)>P(venous)
What is zone 2 of the lungs and why?
Recruitment zone
More blood vessels are being recruited and there is recruitment of more alveolar units
What is perfusion like in zone 2?
Perfusion is sporadic
Why is arterial pressure greater in zone 2 than 1?
This is due to the increase in hydrostatic pressure which increases as you move towards the basal surface of the lung
How is blood flow determined in zone 2 and why does more blood flow through zone 2 than 1?
determined by the difference in arterial and alveolar pressures
more blood flows through the system as P(arterial)>P(alveolar)>P(venous)
What is zone 3 of the lungs?
Distension zone which is below the level of the right ventricle
How does pressure change in zone 3 of the lungs?
As it is below the level of the right ventricle, hydrostatic forces raise alveolar and venous pressure above alveolar
What is the state of the alveoli in zone 3?
They are collapsed and do not contain much air
How is blood flow through zone 3 determined?
P(arterial)>P(venous)>P(alveolar) so blood flow is determined by the difference in arterial and venous pressures
What is blood flow like in zone 3 of the lungs?
Blood flow is continuous through this zone
Efficient gas exchange occurs as the alveoli fill with oxygen during inspiration
In which lung diseases is there a pathological expansion of zone 1?
How does it affect blood flow?
emphysema, pneumonia and acute respiratory distress syndrome
it interferes with blood flow in zone 2
What is the definition of dead space?
the volume of air that is inhaled and does not partake in gas exchange
Where does the air that does not partake in gas exchange reside?
it either remains in the conducting airways or it reaches alveoli that are not/poorly perfused
How does haemorrhage affect hydrostatic pressures?
Loss of blood results in a large decrease in blood pressure
This is characterised by a drop in arterial pressure
Why does gas exchange not occur efficiently in haemorrhage?
the alveoli are open but the arterioles are shut
there is lots of air but no blood to perform the gas exchange
What happens to patients who suffer from haemorrhage?
They become hypoxic as the blood is not being sufficiently oxygenated
How is positive pressure ventilation characterised?
A rise in alveolar pressure
What is positive pressure ventilation?
If the patient is not breathing there is no negative pressure in the chest
A ventilation unit may be used to force air into the lungs by positive pressure
What is used to describe the ventilation perfusion mismatch?
the ventilation-perfusion ratio (V/Q)
Under what circumstances would the V/Q ratio be equal to 1?
If the amount of ventilation (V) to an area of the lung is equal to the amount of perfusion (Q)
How does V/Q ratio vary in different parts of the lung?
V/Q ratio varies from 3.3 at the apex of the lung to 0.6 at the base of the lung
What is the normal average V/Q ratio?
0.8
What is the V/Q ratio value in the dead space of the lung?
V/Q is infinite
perfusion is zero and anything divided by zero is equal to infinity
How is ventilation and perfusion in the dead space of the lungs?
There is good ventilation as the alveoli are open
there is no/very poor perfusion
what is anatomic dead space?
what is the average volume?
the amount of air that remains in the conducting airways
150 ml
Why does the air in the anatomic dead space remain deoxygenated?
The anatomic dead space fills with inspired air at the end of each inspiration but this air is exhaled unchanged
What is alveolar dead space?
the amount of air contained in unperfused or poorly perfused alveoli
What is physiological dead space?
the volume of air in each breath that does not participate in gas exchange
anatomical + alveolar = physiological
What is a pulmonary embolism?
A blood clot from a deep vein of the leg/pelvis or a fat emboli
What does the pulmonary embolism do?
It travels to the lungs and blocks the large or intermediate pulmonary arteries
What are the symptoms of pulmonary embolism?
Severe hypoxia due to a lack of blood supply in one region of the lung
How does a pulmonary embolism affect alveolar dead space?
It enlarges alveolar dead space as a particular region of the the lung becomes poorly perfused
What is a shunt?
Blood is shunted from the right to the left side of the heart without any oxygenation
It bypasses the lungs or fails to get oxygenated
Where does a shunt occur in the lungs and why?
Base of the lung due to the alveoli being collapsed
How is perfusion and ventilation at the shunt?
Good perfusion but there is no ventilation
No air available for gas exchange
What is the V/Q value at a shunt?
V/Q is 0 or very low
What are the 3 main reasons for a shunt?
Pneumothorax, bronchial veins and age-related changes
With increased age, areas of the lung change to have a low V/Q ration
How can pneumonia lead to formation of a shunt?
Inflammatory fluid can fill up an area of the lungs and lead to consolidation
All the alveoli in a certain area are filled with fluid
There is sufficient blood flow but insufficient ventilation for gas exchange
How is a pneumothorax brought about?
Usually, inspiration creates a negative pressure
Pressure inside the chest wall becomes equal to atmospheric pressure
The lung cannot withstand this so collapses
Why is a pneumothorax an example of a shunt?
The lung still has a good blood supply but it contains no air
Gas exchange cannot occur
What is the V/Q value in other alveolar units and why?
There is good ventilation and perfusion
V/Q is closer to 1 and there is efficient gas exchange
