Respiratory 5: Perfusion of lungs Flashcards
Why is the pulmonary circulation a low pressure system
The heart is working as 2 pumps, pumping the same volume per minute. However the left side of the heart supplying the systemic circuit has to pump blood across larger distance so generates higher pressures compared to right side of heart.
What is sheet flow
Capillaries are so dense that their walls touch each other.
The side walls of two adjoining capillaries (arteriole and venule) have erroded away to form a flat sheet held up by pillars of interstitial tissue to allow greater SA contact of Blood with Alveoli membrane.
What is vessel distension
decrease in pulmonary resistance
How do you measure mean pressure, and why is it measured that way.
MAP= Diastolic pressure + (1/3 (Diastolic-Systolic pressure).
Bc as a pulsatile pump heart spends more time in diastole than systole
What is Pulmonary Hypoxic vasoconstriction
This is caused by a restriction in airway to alveoli which means that air takes the path of least resistance. This causes that alveoli to become hypoxic and this causes a local vasocontriction around the blood vessels going to the hypoxic alveoli, directing blood flow to not go there where there isn’t much O2
What are the differences in hypoxia in the Arterial circulation vs Pulmonary circulation
Hypoxia causes vasodilation in systemic whereas in pulmonary its vasoconstriction
How can you measure the distribution of blood flow up and down the lung
Inject an aqueous solution of 133Xe intravenously. The radiation counters can be placed along the chest wall. the amount of radiation will reflect how much blood is flowing through the lung per time.
What is the difference in blood flow in the top part, middle part and bottom part of lung (upright, resting condition)
In the top, there is not much blood flow because of there is not enough pressure from the heart to push against gravity,
In the middle, there is starting to have some blood flow because there is enough pressure to driving the blood.
At bottom: there is lots of blood flow. This better perfused and better ventilated
What is the difference between the PA (alveolar pressure), Pa (artery pressure) and Pv (venous pressure) in the top, middle and bottom of the lung
In the top: PA>Pa>Pv
Therefore the PA squashes down the artery and vein so no blood flows through
In Middle: Pa>PA>Pv
The pressure in arteries is now greater than PA so the vessel can open
At Bottom: Pa>Pv>PA
The alveoli pressure is lowest compared to Pa and Pv
Which part of the lung has the most O2 before taking another breath
The top because the blood flow is poor so much of the O2 it had before hasn’t been taken away yet
What is perfusion (Q)
The amount of blood going to lungs -> cardiac output: SV * HR
What is the ideal ventilation- perfusion ratio (V:Q) and what is it in real life
If we had uniform perfusion and ventilation across the lung, V:Q would be 1 but actually its 0.85 due to uneven blood flow across the lung
How do you find ventilation for ventilation perfusion ratio
Minute ventilation: Vt*f
Then Alveoli minute ventilation: (Vt - Dedspce)*f
What is pulmonary hypertension and how does it affect ventilation
This is failure of the right heart which leads to hypoxia in the lungs which leads to vasoconstriction
What are the Factors regulating the movement of Gas across the respiratory surface (alveolar membrane) (from alveoli to blood)
- Area of the lungs
- Thickness of alveolar membrane
- Partial pressure differential across tissue of the gases involved (O2 and CO2)
- Solubility of gas in blood
- Molecular weight in gas
How does Area help regulate movement of gas across the respiratory surface
Lots of small alveoli are packed into the same area to increase SA to huge area which increases gas exchange.
How does Thickness of tissue help regulate movement of gas across the respiratory surface
Small distance for diffusion increases rate of gas exchange. There is a thickness of only 0.5 um tissue thickness between air and blood.
How does Partial pressures of gas help regulate movement of gas across the respiratory surface
The pressure difference for O2 to go to the blood is 60 so big driving force and the pressure difference for CO2 to go from the blood to air is 6 so small driving force
What is the limitation of rate of O2 and N2O you can uptake across the alveoli membrane
The amount of O2 you can take up is dependent on blood flow (perfusion) which is proportional to rate of uptake because the rate of diffusion is very quick so the Haemoglobin is already saturated pretty quick
How does solubility of gas in blood and molecular weight of gas affect movement of gas across respiratory surface
Solubility more important than MWt of gas.
CO2 is 25x more soluble than O2 in blood (so quicker get into the blood) but CO2 is released from haemoglobin slower than O2 so this balances the movement of gases across the alveolar membrane
What are the two pulmonary circulations
1st is to the alveoli to collect oxygen,
2nd is to the tracheobronchial tree.
Describe the path of the Tracheobronchial tree circulation: receives, feeds, drains
The tracheobronchial tree receives blood from the systemic circuit as its artery comes from the aorta and this feeds the trachea bronchi and bronchioles. It drains by two veins, one going back to the right side of the heart for reoxygenation and the other an anatomical shunt into the left side of the heart, contaminating the freshly oxygenated blood.
Compare the pressure in the pul. artery, pul. capillaries and left atrium
~22 from the Pul. Artery, still pulsatile but dampening as vessels get smaller
~6 at Pul. Capillaries and now smooth laminar flow
~4 at Left Atrium so overall nice pressure gradient is maintained
Compare the relationship between pressure and resistance in the Pulmonary circuit and the Systemic circuit. What accounts for this difference.
In the Systemic circuit, if the resistance decreases, then the pressure will decrease
However in the pulmonary circuit, if the resistance if the resistance decreases the pressure will increase.
This is because an increase in pressure will cause distension and recruitment of capillaries which ends up reducing resistance
What is distension and recruitment
Distension is the widening of the capillary vessels with increase in pressure due to higher compliance. Whereas Recruitment is the opening of previously closed vessels with increase in pressure.
Why is distension and recruitment important
This means that the lung can absorb any large increases in pressure and thus prevent pul. Oedema where fluid from the capillaries gets pushed out into the alveoli due to high pressure, increasing distance for diffusion.
What is another name for hypoxic alveoli
dead space
What is pulmonary Oedema caused by specifically
left heart failure causing congestion in the pulmonary veins as it can’t bail out the blood, therefore increasing the pressure and leading to fluid leaking out and breathlessness (dyspnoea).
How is the diffusion of a volume of gas affected by lung area, gas density, (pressure differential) and thickness of the alveolar membrane
Diffusion is proportional to lung area, gas density, and (pressure differential)
and Inversely proportional to thickness of the alveolar membrane
What is the limitation of rate of CO you can uptake across the alveoli membrane
The rate that it is uptaken in the blood is diffusion limited by its really long time to diffuse and bind the haemoglobin but that means that its hard to get off.
