VQ Flashcards
What is the effect of gravity on pleural pressure?
There is a negative pleural pressure due to the pleural membranes trying to pull apart so the lungs can get smaller.
Gravity makes the pleural pressure more the further down.
What is the pleural pressure of the lungs?
At heart level, pleural pressure is -0.5kPa.
At the top, there is more retraction due to gravity, so the pleural pressure is more negative.
At the base of the lungs, there is still retraction, but it is less as the lungs are resting on the base, so there is a less negative pleural pressure.
Therefore pleural pressure is graded.
What is the effect of gravity on pressure and volume on a compliance curve?
Volume on y axis as % of individual alveolar volume.
At the base, as inspire, the distending pressure increases, and the volume increases.
At the top, the starting pleural pressure is higher, but as inspire the change in pressure is the same.
The volume change is not a great because it is less compliant at larger volumes.
What does the compliance curve look like?
See picture.
What is Boyle’s law?
When there is a bigger volume change, the pressure will decrease more.
So the alveolar pressure will be greater in the base of the lungs compared to the top, as there is a lower volume change.
This will affect flow.
How is flow affected by the pressures in the lungs?
There is a bigger pressure gradient for flow from the top to the bottom of the lungs.
Air flow is ventilation, so there will be more ventilation at the base of the lungs compared to the top.
How does ventilation affect gas composition?
Ventilation is not the same in all parts of the lung.
So there will be different gas composition in different parts of the lung.
This is due to different distending pressures and compliance.
This will also change where blood equilibrium is, which impacts gas exchange.
How does gravity affect blood pressure?
In systemic circulation:
Above heart level, blood pressure decreases.
Below heart level, blood pressure increases.
In the lungs, there is still a height difference, so gravity still affects blood pressure, but to a less extent.
How does gravity affect perfusion?
Blood flow is perfusion, so perfusion is dependent on pressure gradient.
In pulmonary circulation, the arteries are less muscular than in systemic circulation.
This is because the overall system is at a low pressure.
So the main properties of pulmonary arteries is compliance.
What is compliance?
How easily the vessel distends.
Pulmonary arteries are very compliant.
As the pressure increases, they will expand and distend.
How is blood flow dependent on pressure?
The blood vessels in the lungs have thin walls, so distend and can collapse.
For pulmonary vessels to stay open, the pressure on the inside needs to be bigger than the outside (in the alveolar space).
If the pressure on the outside is greater, the vessels will collapse and there will be no blood flow.
What is zone 3 of blood flow?
There is continuous flow.
The pressure inside the vessel is always greater than outside.
This is nearer the base of the lungs.
What is zone 4 of blood flow?
There is still continuous flow, but the blood vessels easily distend (compliant), so as the pressure increases, they will expand and distend.
The pressure is greatest at the base of the lung, so the vessels will be very large and distended.
What is zone 2 of blood flow?
Intermittent flow.
There is still an arterial-venous pressure gradient, but at some points the pressure outside the vessel is greater than inside, so the vessel collapses and flow stops.
Intermittent flow is dependent on the respiratory cycle.
How is intermittent flow dependent on the respiratory cycle?
When breathe out, generates positive alveolar pressure due to elastic recoil. This allows air to flow down the pressure gradient and be expired. The positive pressure may collapse the vessels.
When breathe in, alveolar pressure is sub-atmospheric. The pressure inside the vessel is greater than outside, so the blood vessel becomes open.
Where is zone 2 blood flow?
At the top of the lung.
This is because it is above the heart, so the hydrostatic pressure of the vessel is low.
Therefore arterial blood pressure is lower, and venous pressure even lower.
What is zone 1 blood flow?
There is no flow, because the vessel is always closed.
The pressure outside the vessel is always greater than inside, because the pressure inside the vessel is so low.
This is not present in healthy lungs, it may occur in a haemorrhage where there is lots of blood loss.
This means the heart pressure is low, so above the heart, pressure will be even less.
How does blood flow change through the zones?
Zone 2 is at the top, with zone 4 at the base of the lung.
So there will be more blood flow at the base compared to the top.
What is vessel distension?
Vessels are distended by internal pressure.
As the pressure increases, they distend, and resistance goes down, so there is more blood flow through the vessels.
How is pulmonary capillary flow controlled?
Recruitment and distension.
Mechanical effects.
Vasoactive factors.
What is recruitment of blood vessels?
During intermittent flow, if you increase cardiac output, this increases perfusion, so blood vessels that are intermittently or not perfused can be recruited and distended.
This increases blood for diffusion for gas exchange.
e.g. during exercise when metabolic rate is higher.
What mechanical effects can control pulmonary capillary flow?
For gas exchange to happen, there is close proximity of alveoli and blood vessels.
As increase size of alveoli and airspace, they pinch on the blood vessels, which reduces flow through the vessels by preventing them getting bigger.
The blood vessels are thin walled and easily deformed.
How do vasoactive factors control pulmonary capillary flow?
Vascular smooth muscle can be acted on by agonists and antagonists.
These will either dilate or constrict the muscle, controlling the diameter and therefore blood flow.
What is the difference in Ventilation and perfusion from the top to the base of the lung?
Perfusion is 6x better at the base than top.
Ventilation is 2x better at the base than top.
This produces 2 straight lines, where perfusion is steeper because of the greater difference.
The 2 lines divide to form V/Q which is a curved line.
See picture.
What is the relationship of V/Q?
Ventilation and perfusion are both less at the top, but ventilation is greater than perfusion, so V/Q ratio is greater than 1 at the top.
At the base, perfusion is greater than ventilation, so the V/Q ratio is less than 1.
The V/Q ratio is 1 when ventilation and perfusion are the same - where the lines cross on the graph.
How is V/Q affected in extremes?
If there is no ventilation, V/Q is 0.
If there is no perfusion, Q = 0, and V/Q is infinity.
What is a V/Q of 0 in individual alveoli?
When perfusion is normal, but ventilation is reduced.
This happens in a physiological shunt - Thebesian veins in bronchial circulation (supply to lung tissue).
The venous blood drains into the left ventricle and bypasses the lungs.
How does a shunt affect PO2?
Arterial vessels from the left ventricle will have a lower PO2 than predicted because deoxygenated blood is pumped into the left ventricle.
Alveolar PO2 will be normal.
What does an infinity V/Q mean?
When ventilation is normal, but perfusion is 0.
No gas exchange takes place.
V/Q is infinity because of the dead space - there is ventilation but no perfusion so no gas exchange.
How does V/Q affect gas exchange?
V/Q less than 1 is under ventilated.
V/Q of 1 is ideal, ventilation and perfusion are matched.
V/Q more than 1 is under perfused.
The change in ratio will change alveolar partial pressures.
What are alveolar gas levels?
The balance of what is added and what is removed.
In breathing, the first part of tidal volume comes from the dead space, and the rest comes from the air.
The air from the dead space has higher PO2 and lower PCO2.
Perfusion then removes O2 and delivers CO2 to the alveoli, and this determines the alveolar gas composition.
How does a shunt affect alveolar gas composition?
The R-L shunt means blood bypasses gas exchange, so there is no ventilation.
The blood is taken and CO2 is added until it reached equilibrium.
V/Q = 0.
The alveolar gases will equilibrate to the venous gases coming back to the heart.
How does dead space affect alveolar gas composition?
V/Q = infinity.
No CO2 is added as there is no perfusion, and no O2 is removed.
So the alveolar gases will match the air composition.
What is the O2-CO2 diagram?
Each V/Q value from 0 to infinity will have a different gas composition.
See picture.
What happens when the blood gases from each V/Q region are mixed together?
The blood equilibrates with alveolar gas, so the blood leaving will have a different gas composition.
This will average out the contents.
How is V/Q mismatch fixed?
A slightly disturbed V/Q can be minimised by physiological measures.
Both ventilation and perfusion can be altered to change the ratio.
This is not in big disturbance, such as a blood clot.
How does the body counteract a high V/Q ratio?
PACO2 is low due to hyperventilation.
Low CO2 causes bronchoconstriction, which leads to a decrease in ventilation and therefore regional V/Q.
PaO2 is increased, which causes vasodilation on the vessels in the lung. This increases perfusion, and returns V/Q towards lower.
How does the body counteract a low V/Q ratio?
Hypoventilation, so PACO2 is increased, which causes bronchodilation.
PaO2 is decreased, so causes vasoconstriction on the vessels in the lung.
These return regional V/Q to be higher and normal.
How does O2 have a different effect on systems?
In systemic circulation, low O2 would cause vasodilation.
But in lungs, low PaO2 causes vasoconstriction in the blood vessels in the lungs - hypoxic pulmonary vasoconstriction.
This restores V/Q ratio back to normal, so gas exchange is normal.
What are the symptoms in pulmonary embolism?
A deep vein thrombosis clot is dislodged, and travelled to the alveoli so reduced gas exchange.
Not enough oxygen is delivered to brain, so feel light headed and dizzy - hypoxia.
Shortness of breath due to lack of oxygen so try to increase through breathing rate.
Blood clot also causes a backflow of blood which causes inflammation.
What will happen to the V/Q in the region of the lung affected by the clot?
The clot obstructs perfusion to the lung.
Ventilation is about normal.
So V/Q ratio increases – dead space ventilation.
Which part of the lung is at most risk from a pulmonary embolism?
The lower lobes because the blood flow is greatest here due to gravity, so are more likely to trap blood clots.
