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Lungs > Pulmonary perfusion, ventilation and VQ mismatch > Flashcards

Pulmonary perfusion, ventilation and VQ mismatch Flashcards

1
Q

ventilation definition

A

the process of moving air into and out of the lungs to facilitate gas exchange with the internal environment

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2
Q

what does ventilation depend upon?

A

diffusion

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3
Q

diffusion definition

A

the net movement of molecules from a region of higher concentration to a region of lower concentration until equilibration

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4
Q

what occurs in the respiratory zone?

A

carbon dioxide diffuses out of the venous blood and oxygen diffuses into the venous blood across the blood-air barrier

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5
Q

blood-air barrier definition

A

capillary endothelium, alveolar epithelium and their fused basement membranes and associated structures that form a very thin diffusion pathway

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6
Q

respiratory zone definition

A

region where gaseous exchange takes place

10% in bronchioles and alveolar ducts

90% in alveoli

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7
Q

diffusion capacity definition

A

the measurement of the transfer of gas from the air in the lung to the red blood cells in lung blood vessels

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8
Q

what normally impacts the diffusion capacity?

A

a misnomer, as most commonly diffusion is not the limiting factor but instead it is perfusion

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9
Q

Fick’s Law of diffusion

A

Ficks Law= DxAx∆P/d

D= diffusion constant

A= surface area

∆P= partial pressure difference

d= diffusion distance

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10
Q

What does Fick’s law measure?

A

the diffusion flux, of which the dimension is amount of substance per unit area per unit time

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11
Q

what is nick’s law proportional to?

A

the gas’s solubility in the liquid

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12
Q

how is the diffusion speed increased in the lungs?

A

increase in surface area of alveoli

thin blood-air barrier

good blood supply, maintaining concentration gradient

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13
Q

pathologies that affect diffusion + why

A

pulmonary fibrosis- thicker alveoli, longer diffusion distance

pulmonary oedema- fluid in lungs increases effective thickness of alveoli

emphysema- alveoli and lung tissue destroyed, smaller surface area available for exchange

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14
Q

explain perfusion limitation

A

the blood comes into equilibrium with the alveolar gas

doubling the diffusing capacity won’t alter the gas exchange

doubling the blood flow will double the gas exchanged

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15
Q

explain diffusion limitation

A

Partial pressure gradient essentially constant along the capillary

doubling blood flow will not change gas exchange

doubling diffusing capacity will double gas exchange

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16
Q

whether there is diffusion or perfusion capacity depends upon?

A
  1. diffusing capacity, the bigger this is the more likely it is to get equilibration
  2. speed of chemical reaction with blood
  3. the solubility in blood, including the gas reacting with the blood
  4. blood flow, faster flow less time to equilibrate
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17
Q

where does pulmonary circulation begin?

A

at the main pulmonary arteries which receive mixed venous blood from the right ventricle

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18
Q

where do the pulmonary arteries accompany?

A

accompany the airways as far as the terminal bronchioles

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19
Q

what happens to the blood vessels at the terminal bronchioles?

A

arterioles form a capillary network to supply the alveoli

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20
Q

adaptations of the pulmonary capillaries

A

very large surface area

short diffusion distance formed by the close apposition of the alveoli and the vessels

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21
Q

what is the mean pressure in the main pulmonary artery?

A

15mmHg

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22
Q

Systolic and diastolic pressure in the main pulmonary artery

A

25mmHg and 8mmHg

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23
Q

what type of circulation is pulmonary?

A

low pressure circulation

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24
Q

why is the pressure likely to be lower in pulmonary compared to systemic?

A

reduced height through which the blood is required to ascend, so not pumped as much

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25
Q

what is a consequence of the low pressure?

A

pulsatile

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26
Q

structure of pulmonary artery wall

A

very thin, containing relatively little smooth muscle, often mistaken for veins

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27
Q

two types of pulmonary vessels

A

alveolar vessels- capillaries and slightly larger vessels in the corners of the alveolar walls, which are exposed to alveolar pressure

extra-alveolar vessels- larger pulmonary vessels that run through the lung parenchyma

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28
Q

adaptations and structures of alveolar vessels

A

virtually surrounded by gas

thing, single cel endothelial layer with little support

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29
Q

caliber definition

A

the diameter of the vessels

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30
Q

what controls the caliber in alveolar vessels?

A

caliber is determined by the pressure difference between the alveoli and capillaries

if the alveolar pressure rises above the pressure in the capillaries they become compressed and collapsed, negative transmural pressure

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31
Q

what controls the caliber of extra alveolar vessels? + explained

A

lung volume

as the lung expands the vessels are pulled open by radial traction of the elastic parenchyma

results in low pressure surrounding the extra alveolar vessels, causing them to increase in diamater

32
Q

experiment to show the distribution of blood flow

A

radioactive xenon can be used to demonstrate regional blood flow

xenon dissolved in saline is injected into the peripheral vein, when it reaches the pulmonary capillaries its evolved into alveolar gas due to its low solubility

the distribution of radioactvity, and thus perfusion, can be measured by counters over the chest during breath holding

33
Q

how lung perfusion changes in an upright position

A

decreases linearly from the base to the apex

34
Q

how lung perfusion changes in a supine position

A

apical zone blood flow increases but the basal zone remains unchanged, meaning the distribution becomes almost uniform

blood flow in posterior exceeds anterior regions

35
Q

how lung perfusion changes during exercise

A

both upper and lower zone blood flow increases upon mild exercise and the regional diffrences become less

36
Q

what is the major determinant in perfusion of the lung?

A

gravity

37
Q

how is the lung divided?

A

into 4 zones

38
Q

explain perfusion and ventilation in zone 1

A

region at the top of lung where pulmonary atrerial pressure falls below alveolar pressure, capillaries are squashed and there is no flow

perfect ventilation

39
Q

what is zone 1 often referred to as? + WHY

A

alveolar dead space, the area is ventilated but not perfused, no exchange despite the alveoli working fine

40
Q

when does alveolar dead space occur?

A

arterial pressure is decreased during a haemorrhage

alveolar pressure is increased due to positive pressure ventilation and deep inspiration

41
Q

explain perfusion and ventilation in zone 2

A

pulmonary arterial pressure increases because of the hydrostatic effect and exceeds alveolar pressure

venous pressure is still much lower than alveolar, blood flow determined by arterial-alveolar pressure differences, not the venous pressure

42
Q

what effect takes place in zone 2 explained

A

starling resistor/waterfall effect

capillaries collapse downstream when the alveolar pressure exceeds the pressure in the capillaries

43
Q

explain perfusion and ventilation in zone 3

A

hydrostatic pressure increases both the arterial and venous pressure, so flow is determined by the arterial venous pressure difference

perfusion increases due to the distension of capillaries

44
Q

explain perfusion and ventilation in zone 4

A

region of reduced blood flow at the base of the lungs due to extra alveolar vessels narrow from reduced radial traction

45
Q

ventilation perfusion relationships definition

A

concerns the matching of ventilation and blood flow within various regions in the lung which is critical for adequate gaseous exchange

46
Q

what does mismatching of V and Q cause?

A

responsible for most defective gas exchange in pulmonary diseases

47
Q

what is alveolar PO2 determined by?

A

the continual replenishment by alveolar ventilation

the removal of O2 by pulmonary capillaries

48
Q

explain replenishment

A

alveolar ventilation is not continuous, and fluctuates between each breath

tidal volume is small compared to volume of gas in the lung so the process is regarded as continuous

49
Q

explain removal

A

rate of removal of o2 is governed by the O2 consumption of the tissues

in resting conditions consumption is rarely steady so alveolar PO2 is largely determined by alveolar ventilation

50
Q

hypoventilation explained + causes

A

alveolar ventilation is abnormally low, alveolar PO2 levels fall, alveolar CO2 rises

fall in arterial PO2 and rise in PCO2

damage to the chest wall, paralysis of respiratory muscles, high resistance to breathing

51
Q

diffusion limitation explained

A

alveolar PO2 and pulmonary artery PO2 is not the same, however ideally should be

very rarely happens, however can be due to a low O2 mixture being inhaled reducing the PO2 gradient or due to exercise with faster blood flow and less time to equilibrate

52
Q

Shunt definition

A

refers to blood which enters the arterial system without passing through ventilated areas of the lung

53
Q

two examples of lung shunt + explained

A

bronchial artery blood- supplies lung parenchyma and drains into the pulmonary veins after it has perfused the bronchi

coronary venous blood- drains directly into the left ventricle ia the cardiac veins

54
Q

explain the shunt

A

there is perfectly fine ventilation, yet little perfusion due to a pulmonary arteriovenous fistula

55
Q

how can a shunt be tested?

A

inspire 100 O2, which would raise arterial PO2, so ventilation is fine

if arterial PO2 does not rise then there is a problem with perfusion

56
Q

what is a common cause of hypoxaemia?

A

VQ mismatch

57
Q

what is used to measure VQ mismatch?

A

VQ ratio

58
Q

what happens if V/Q is reduced by obstructing ventilation?

A

PO2 in the lung will decrease

PCO2 will rise

59
Q

what is V/Q=0?

A

No ventilation, arterial blood will have the same pO2 as venous blood

60
Q

V/Q is increased by obstructing blood flow explained

A

PO2 will increase

alveolar PO2 and CO2 becomes that of inspired gas

61
Q

where are both ventilation and perfusion greater in the lung?

A

at the bottom than the apex

62
Q

rate of increase

A

ventilation increases slowly

blood flow increases rapidly

63
Q

why does perfusion vary to a greater extent?

A

density of blood id greater than the density of air so gravity has a greater effect

64
Q

explain V/Q ratios near apex

A

abnormally high, as blood flow is minimal

65
Q

explain V/Q at base

A

perfusion is greatest so V/Q is smaller

66
Q

ventilation top and borrom

A

bottom of lung is under ventilated relative to increased blood flow

top of lung is over ventilated relative to the blood flow

67
Q

explain V/Q=1

A

well ventilated and well perfused alveoli, blood equilibrates with alveolar air

68
Q

explain V/Q <1

A

Poorly ventilated, well perfused

alveolar PO2 and PCO2 tend towards the levels found in mixed venous blood

found in the lower lung regions

69
Q

explain V/Q >1

A

well ventilated, poorly perfused

alveolar PO2 and PCO2 tend towards levels found in inspired gas

found in higher lung regions

70
Q

what V/Q does deadspace have?

A

infinity

71
Q

importance of the shape of the oxyhaemoglobin dissociation curve

A

the sigmoidal shape means that increasing alveolar PO2 cannot further increase Hb saturation, so increasing oxygen when V/Q is very small is unlikely to help

72
Q

explain hypoxic pulmonary vasoconstriction

A

physiological phenomenon where small pulmonary arteries constrict in the presence of alveolar hypoxia

73
Q

what does hypoxic pulmonary vasoconstriction allow + explained

A

better V/Q matching

blood floe becomes redirected from badly ventilated regions to well ventilated regions so more oxygen can be moved into the vlood

74
Q

molecular mechanism of hypoxic pulmonary vasocontriction

A

hypoxia sensitive voltage gated potassium channels in pulmonary artery smooth muscle cells leads to depolarisation

activates voltage gated calcium channels, increases intracellular calcium and activates smooth muscle contractile machinery

75
Q

explain acute pulmonary hypertension

A

condition where there is an increased blood pressure within the arteries of the lungs

occurs where perfusion is low to increase V/Q to 1

due to thromobosis, emboli clots

Lungs (10 decks)

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