Respiratory 5: Perfusion of lungs

Question 1

Why is the pulmonary circulation a low pressure system

Answer 1

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.

Question 2

What is sheet flow

Answer 2

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.

Question 3

What is vessel distension

Answer 3

decrease in pulmonary resistance

Question 4

How do you measure mean pressure, and why is it measured that way.

Answer 4

MAP= Diastolic pressure + (1/3 (Diastolic-Systolic pressure).

Bc as a pulsatile pump heart spends more time in diastole than systole

Question 5

What is Pulmonary Hypoxic vasoconstriction

Answer 5

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

Question 6

What are the differences in hypoxia in the Arterial circulation vs Pulmonary circulation

Answer 6

Hypoxia causes vasodilation in systemic whereas in pulmonary its vasoconstriction

Question 7

How can you measure the distribution of blood flow up and down the lung

Answer 7

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.

Question 8

What is the difference in blood flow in the top part, middle part and bottom part of lung (upright, resting condition)

Answer 8

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

Question 9

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

Answer 9

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

Question 10

Which part of the lung has the most O2 before taking another breath

Answer 10

The top because the blood flow is poor so much of the O2 it had before hasn’t been taken away yet

Question 11

What is perfusion (Q)

Answer 11

The amount of blood going to lungs -> cardiac output: SV * HR

Question 12

What is the ideal ventilation- perfusion ratio (V:Q) and what is it in real life

Answer 12

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

Question 13

How do you find ventilation for ventilation perfusion ratio

Answer 13

Minute ventilation: Vt*f

Then Alveoli minute ventilation: (Vt - Dedspce)*f

Question 14

What is pulmonary hypertension and how does it affect ventilation

Answer 14

This is failure of the right heart which leads to hypoxia in the lungs which leads to vasoconstriction

Question 15

What are the Factors regulating the movement of Gas across the respiratory surface (alveolar membrane) (from alveoli to blood)

Answer 15

1. Area of the lungs
2. Thickness of alveolar membrane
3. Partial pressure differential across tissue of the gases involved (O2 and CO2)
4. Solubility of gas in blood
5. Molecular weight in gas

Question 16

How does Area help regulate movement of gas across the respiratory surface

Answer 16

Lots of small alveoli are packed into the same area to increase SA to huge area which increases gas exchange.

Question 17

How does Thickness of tissue help regulate movement of gas across the respiratory surface

Answer 17

Small distance for diffusion increases rate of gas exchange. There is a thickness of only 0.5 um tissue thickness between air and blood.

Question 18

How does Partial pressures of gas help regulate movement of gas across the respiratory surface

Answer 18

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

Question 19

What is the limitation of rate of O2 and N2O you can uptake across the alveoli membrane

Answer 19

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

Question 20

How does solubility of gas in blood and molecular weight of gas affect movement of gas across respiratory surface

Answer 20

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

Question 21

What are the two pulmonary circulations

Answer 21

1st is to the alveoli to collect oxygen,

2nd is to the tracheobronchial tree.

Question 22

Describe the path of the Tracheobronchial tree circulation: receives, feeds, drains

Answer 22

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.

Question 23

Compare the pressure in the pul. artery, pul. capillaries and left atrium

Answer 23

~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

Question 24

Compare the relationship between pressure and resistance in the Pulmonary circuit and the Systemic circuit. What accounts for this difference.

Answer 24

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

Question 25

What is distension and recruitment

Answer 25

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.

Question 26

Why is distension and recruitment important

Answer 26

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.

Question 27

What is another name for hypoxic alveoli

Answer 27

dead space

Question 28

What is pulmonary Oedema caused by specifically

Answer 28

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).

Question 29

How is the diffusion of a volume of gas affected by lung area, gas density, (pressure differential) and thickness of the alveolar membrane

Answer 29

Diffusion is proportional to lung area, gas density, and (pressure differential)
and Inversely proportional to thickness of the alveolar membrane

Question 30

What is the limitation of rate of CO you can uptake across the alveoli membrane

Answer 30

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.