Pulmonary Circulation Flashcards
What is the pulmonary circulation?
Is it a high pressure or low pressure circuit?
What does the Bronchial Circulation do?
Bronchial circulation comes out of the ………………. ………………….. and drains into the veins within the ………………….circulation
What is the pulmonary circulation?
The pulmonary circulation is everything coming out of the right ventricle and returning to the left atrium
It is a low pressure circuit
It is NOT the same as the bronchial circulation
The bronchial circulation keeps the airways tissue alive and humid whereas the pulmonary circulation is mainly involved with gas exchange
Bronchial circulation comes out of the THORACIC AORTA and drains into the veins within the pulmonary circulation
Pulmonary arteries carry mixed venous blood
Compare the pulmonary circulation to the systemic circulation?
Pulmonary vs Systemic
Pulmonary arteries have a greater lumen: wall thickness ratio and so they are more distensible and have GREATER COMPLIANCE
The pulmonary arteries still need to be elastic to convert the pulsatile flow into continuous flow
There is LOWER PRESSURE in the pulmonary circulation
The LEFT ventricle is almost a full circle
The left ventricle needs to be able to pump to much greater distances
Resistance is proportional to the length of the circuit so there is much MORE RESISTANCE against the systemic circulation
Circuit Pressures
SYSTEMIC = ……………PRESSURE
Pulmonary = ……… Pressure
There is then a pressure gradient between the right ventricle and the left atrium. What do this mean?
Circuit Pressures
SYSTEMIC = HIGH PRESSURE
Pulmonary = Low Pressure
Blood will flow down the pressure gradient and will return to the right atrium at very low pressure
There is then a pressure gradient between the right ventricle and the left atrium which allows the blood flow to take place
Pulmonary vs Systemic continued…
Cardiac output on both sides is around 5 L/min
The volume enclosed in the systemic circulation is much greater because there is more pipework
………… ………………. ……………… …………….. in the pulmonary circuit is 15% that of the systemic circuit - this is because we aren’t pumping as far, there is less pipework, so less pressure is needed
The ………………….. …………………. is much greater in the systemic (92) compared to pulmonary (9) - the gradient in the pulmonary is around 10% that of the systemic
The ………………… against the pulmonary circulation is 10% that of the systemic
The …………….. of the flow is also going to be greater in the systemic compared to the pulmonary (due to the much greater pressure gradient)
…………………. is higher in the pulmonary circulation (the arteries are more distensible) so they can easily handle an increase in cardiac output
NOTE: compliance = willingness to distend under pressure
Pulmonary vs Systemic continued…
Cardiac output on both sides is around 5 L/min
The volume enclosed in the systemic circulation is much greater because there is more pipework
Mean arterial blood pressure in the pulmonary circuit is 15% that of the systemic circuit - this is because we aren’t pumping as far, there is less pipework, so less pressure is needed
The pressure gradient is much greater in the systemic (92) compared to pulmonary (9) - the gradient in the pulmonary is around 10% that of the systemic
The resistance against the pulmonary circulation is 10% that of the systemic
The velocity of the flow is also going to be greater in the systemic compared to the pulmonary (due to the much greater pressure gradient)
Compliance is higher in the pulmonary circulation (the arteries are more distensible) so they can easily handle an increase in cardiac output
NOTE: compliance = willingness to distend under pressure
List 3 functions of the pulmonary circulation?
Gas Exchange
Metabolism of Vasoactive Substances
Filtration of Blood
Functions of the Pulmonary Circulation
Gas Exchange
Metabolism of Vasoactive Substances
………. is exhibited within the walls of the pulmonary endothelium
……… participates in a cascade where ………………….. (from the ………….) is converted by ………….. (from the …………….. cells in the kidneys) to ………………..
…………………. is then converted by ……….. to …………………….
……. also degrades BRADYKININ (which works antagonistically with angiotensin II)
Metabolism of Vasoactive Substances
ACE is exhibited within the walls of the pulmonary endothelium
ACE participates in a cascade where angiotensinogen (from the LIVER) is converted by RENIN (from the juxtaglomerular cells in the kidneys) to ANGIOTENSIN I
Angiotensin I is then converted by ACE to ANGIOTENSIN II
ACE also degrades BRADYKININ (which works antagonistically with angiotensin II)
List 3 ways the pulmonary circulation acts as a filter for the blood?
Filtration of Blood
Embolus = mass within the circulation that is capable of causing obstruction
Embolism = event characterised by obstruction of a major artery
Although the entire circulation is a closed circuit, things can get caught in the blood (usually on the venous side)
The pulmonary circulation filters before the systemic arteries
Small emboli could get trapped and then eliminated in the pulmonary circulation
Air bubbles can be compacted and moved back out into the airspaces
Fatty plaques and thrombi can be enzymatically degraded
A large embolus could get stuck in a bigger artery which would decrease the amount of local perfusion and could potentially lead to sudden death
Cancer cells that are spreading can also get caught in the pulmonary circulation
The pulmonary circulation is a good defence system to prevent things from getting stuck in the brain or heart and causing stroke or death
On the venous side, there is more stasis so you are more likely to form clots
Define shunt?
List 3 examples of shunts?
It is any thing that allows gas via blood to bypass the pulmonary circulation
Pulmonary Shunts
Pulmonary Shunts = circumstances associated with bypassing the respiratory exchange surface
Why is the Bronchial circulatuon an example of a pulmonary shunt?
The BRONCHIAL CIRCULATION in itself is a pulmonary shunt because it comes out of the thoracic aorta and goes and perfuses the airways and helps humidify the air and then it returns to the pulmonary veins and goes back to the left side of the heart
This means that it goes through the left side of the heart twice before returning to the right side - it bypasses the respiratory exchange surface
About 1% of cardiac output goes to the bronchial circulation
The FOETAL CIRCULATION has TWO shunts: name them?
The FOETAL CIRCULATION has TWO shunts:
Foramen Ovale
Ductus Arteriosus
The foramen ovale is a hole between the right and left atria - it creates a low pressure alternative for blood flow
Foetal blood gets oxygen from the mother via the placenta so the blood will come to the right side of the heart and it will follow the path of least resistance
The blood would much more favourably go through the foramen ovale or ductus arteriosus to get back to the systemic circulation - there is no need to go to the lungs because the oxygen is coming from the placenta
NOTE: the pulmonary circulation CONSTRICTS in response to HYPOXIA
How can a congenital heart defect act as a pulmonary shunt?
Some people have a CONGENITAL HEART DEFECT:
Atrial Septal Defect or Patent Foramen Ovale
Ventricular Septal Defect
In ASD, mixed venous blood moves from the right atrium to the left atrium
VSD is more a congenital defect rather than mal-correction after birth
Pulmonary Vascular Resistance
Effect of Increasing Cardiac Output
The pulmonary circulation is a low resistance high capacity circuit at a resting perfusion of 5 L/min
The resistance of the pulmonary circulation is 10% of the systemic circulation
Imagine the pulmonary circulation is made up of rigid tubes:
An increase in cardiac output would lead to an increase in MAP in the pulmonary circulation
This will increase the hydrostatic pressure and push more fluid into the interstitial space
This leads to pulmonary oedema
This reduces the ability for your respiratory exchange surface to work - you decrease pulmonary function
Why does an oedema not happen in the lungs of a healthy person?
Pulmonary Vascular Resistance
Effect of Increasing Cardiac Output
The pulmonary circulation is a low resistance high capacity circuit at a resting perfusion of 5 L/min
The resistance of the pulmonary circulation is 10% of the systemic circulation
Imagine the pulmonary circulation is made up of rigid tubes:
An increase in cardiac output would lead to an increase in MAP in the pulmonary circulation
This will increase the hydrostatic pressure and push more fluid into the interstitial space
This leads to pulmonary oedema
This reduces the ability for your respiratory exchange surface to work - you decrease pulmonary function
However, the pulmonary circulation is actually a low resistance high capacity circuit - we can increase cardiac output with a small increase in MAP:
An increase in cardiac output is not a problem because the pulmonary arteries are distensible so will increase the diameter to accommodate this
There is increased perfusion to the hypoperfused capillary beds
So there is capacity to increase perfusion if cardiac output increases
Minimal changes in MAP means that there is normal fluid leakage and no pulmonary oedema hence no detriment to pulmonary function
Why is pulmonary perfusion not equal at rest?
PULMONARY PERFUSION IS NOT EQUAL AT REST:
Basal capillary beds are much more perfused at rest because blood leaving the heart wants to go with gravity and follow the path of least resistance
The difference between the apex and the base is still present in increased vascular recruitment but to a lesser extent
Vascular Recruitment = increased use of the vascular beds which were not being used because there wasn’t enough pressure to access them
What effect does inspiration and expiration have on extra-alveolar vessels?
Effect of Increasing Ventilation
Inspiration compresses alveolar vessels and expiration compresses extra-alveolar vessels
Alveolar Vessels = RED
Extra-Alveolar Vessels = PURPLE
Influencing these vessels will lead to changes in the resistance of the pulmonary circulation
Resistance will increase when you’re at the EXTREMITIES of your vital capacity
Resistance will increase when you’re at the EXTREMITIES of your vital capacity
LEFT - the chest is getting smaller during expiration meaning that there is more pressure on the extra-alveolar vessels making them constrict
RIGHT - when you inspire, the alveoli expand and compress the alveolar vessels
What is the systemic and pulmonary vascular response to hypoxia?
Why is this useful (both the systemic and pulmonary )?
When is this reaction useful and detrimental?
Effect of Hypoxaemia
SYSTEMIC vascular response to hypoxia is vasodilation
PULMONARY vascular response to hypoxia is VASOCONSTRICTION
This is different to the systemic circulation which vasodilates in response to hypoxia - this is because the low oxygen environment will mean that we are underperfusing the tissues distal to the vessel so we need to increase blood flow to meet demands
The vasoconstriction in response to hypoxia is useful in the pulmonary circulation because perfusing a non-ventilated alveolus is wasted perfusion - the blood will not get oxygenated
Pulmonary Fluid Balance
There are TWO pressures and TWO regions involved in pulmonary fluid balance
name them
Pulmonary Fluid Balance
There are TWO pressures and TWO regions involved in pulmonary fluid balance
Regions: plasma + interstitium
Pressures: hydrostatic + oncotic
Plasma hydrostatic pressure is greater at the arterial end and lower at the venous end
The interstitial hydrostatic pressure is the TINY RED ARROW - this is pretty much zero in healthy individuals
Plasma oncotic pressure is a pulling force
The lymph system is like a plug hole - it drains excess fluid
As soon as deposition of fluid exceeds the capacity of the lymphatics to drain fluid - fluid accumulates and you get oedema
Overall, in health, there is 1 mm Hg of fluid moving out of the vessel which is mopped up by the lymphatics
How can mitral valve stenosis cause pulmonary oedema?
If we increase the hydrostatic pressure (e.g. due to pulmonary hypertension) then there will be a much bigger force pushing fluid out than in
More fluid enters the interstitium and you get oedema
How can infection cause pulmonary oedema?
If you have an infection where proteins and white blood cells accumulate in the interstitium then there is going to be a bigger pulling force pulling water into the interstitium
More fluid leaves the capillaries and you get oedema
How can hypoproteineamia cause pulmonary oedema?
If we are hypoproteinaemic then the pulling force back into the vessels is going to be less
More fluid remains in the interstitium and you get oedema
How can cancer cause pulmonary oedema?
If the ability of the lymphatics to clear the excess fluid is obliterated (e.g. due to cancer) and everything else stays the same, then the normal 1 mm Hg net movement of fluid into the interstitium is going to build up and lead to oedema
