Ventilation and Perfusion Matching

Question 1

What must be closely macthed for effective gas exchange?

Answer 1

ventilation and perfusion

Question 2

What is ventilation?

Answer 2

the change in volume through the respiratory cycle

Question 3

What type of alveolus will have less ventilation?

Answer 3

an alveolus that is already open and extended at the start of the cycle

Question 4

Where does more gas go in the lung?

Answer 4

More gas goes to the base of the lung than the apex.

Question 5

Why does the base of the lung change volume more than the apex?

Answer 5

• Lower ribs are more curved and mobile than upper, the action of the diaphragm expands the lower lobes more than the upper, partly because upper lobes are attached to main bronchi and upper airways, so less easily stretched than tissue of lower lobes, and compliance alters across lung – recall larger compliance means a larger change in volume – lower lobes have greater compliance
• Small role due to gravity - apex has more negatuve ontrapleural pressure so the alveoli are extended already

Question 6

What two circuits nourish the lung tissue?

Answer 6

pulmonary and bronchial

Question 7

Where do the bronchial arteries branch from?

Answer 7

the descending aorta

Question 8

What is the function of the bronchial circulation

Answer 8

to supply oxygen to lung parenchyma, airway smooth muscle, pulmonary arteries and veins and pleura. Additional function is warming of inspired air

Question 9

What is flow through the pulmonary artery considered to be equal to?

Answer 9

cardiac output

Question 10

What is the volume of the pulmonary capillary network at rest

Answer 10

~100ml

Question 11

What is the volume of stroke volume?

Answer 11

~70ml

Question 12

What type of system is the pulmonary circulation?

Answer 12

low pressure, low resistance system

Question 13

What is the mean arterial pressure in the pulmonary circulation and what does it need to be sufficient to do?

Answer 13

~15mmHg

pressure only needs to be sufficient to lift blood to the apex of the lung

Question 14

How thick are the walls of the pulmonary arteries?

Answer 14

thin (mistaken for systemic veins)

Question 15

What are pulmonary capillaries in effect surrounded by?

Answer 15

surrounded by air - can collapse or be extended according to balance of blood pressure and alveolar pressure

Question 16

What is transmural pressure?

Answer 16

the balance between alveolar and blood pressure which determines the blood flow through the pulmonary capillaries

Question 17

Why is blood floe low at the apex and high at the base of the lung?

Answer 17

• due to the hydrostatic pressure differnce between the base and apex of the lung (~23mmHg)
• pressure in capillaries is lower at the apex than the base of the lung

Question 18

When will blood only flow in the lungs?

Answer 18

if blood pressure is greater than alveolar pressure

Question 19

Draw a diagram and graph showing the blood flow in each zone of the lung and the relative size of the capillary in that section

Answer 19

Question 20

Describe the nature of the capillaries in each zone of the lungs and relate how this would affect blood flow

Answer 20

• zone 1 = closed (low)
• zone 2 = open/closed (medium)
• zone 3 = open (higher)

Question 21

What is the consequence of changes in hydrostatic pressure at the apex of the lung? And when would this occur?

Answer 21

alveolar pressure > blood hydrostatic pressure - the capillary will be closed

occurs with low BP or raised alveolar pressure

Question 22

What occurs in zone one if capillaries are closed? And when does this not tend to exist?

Answer 22

if capillaries are closed then they are ventilated but not perfused and this is considered alveolar dead space

Doesnt usually exist in normal people

Question 23

under what conditions is there not normally a zone 1 and why?

Answer 23

Normal conditions

there is sufficient pressure to perfuse the apices

Question 24

Describe the pressure and flow in zone 2

Answer 24

• alveolar pressure is lower than systolic arterial pressure, but may be higher than diastolic arterial pressure and venous pressure
• flow is determined by the arterial-alveolar pressure difference
• blood pressure must be greater than alveolar for flow to occur

Question 25

Describe the pressure and flow in zone 3

Answer 25

• alveolar pressure is lower than arterial and venous pressure
• the capillaries are distended as a consequence of transmural pressure and there is continuous flow

Question 26

What is the V/Q ratio?

Answer 26

Ventilation and perfusion ratio

Question 27

What is the average V/Q ratio over the whole lung?

Answer 27

0.85

Question 28

What is the V/Q ratio in the base of the lung and what does this mean?

Answer 28

0.6

good perfusion and good ventilation (but more perfusion occurs)

Question 29

What is the V/Q ratio in the apex of the lung and what does this mean?

Answer 29

3

More ventilation than perfusion

Question 30

Draw a graph indicating the changes in V/Q ratio, blood flow and ventilation between the top and bottom of the lung

Answer 30

Question 31

Describe perfect matching

Answer 31

well ventilated alveoli with good perfusion of blood. Blood will equilibriate with alveolar air and be rich in oxygen and low in CO₂

Question 32

What occurs in poorly ventilated alveoli with a rich blood supply?

Answer 32

• alveolar air will equilibiriate with the blood and the blood will tend towards the same composition as venous, as there is a decrease in fresh air being brought in
  
  • Low PO₂
  • High PCO₂

Question 33

What occurs in well ventilated alveoli that are poorly perfused with blood?

Answer 33

Blood leaving the alveoli will be low in CO₂ (due to high concentration gradient and it is blown off) but as the Hb is fully saturated there will not be a singificant increase in O₂ levels

Question 34

What occurs when there is lowered PO₂ of the blood?

Answer 34

leaving poorly ventilated parts of the lung is not compensated for by blood leaving well ventilated areas, because of the shape of the oxygen dissociation curve

Question 35

What is the effect on CO₂ concentration at a low ventilation to perfusion ratio? (base of the lung)

Answer 35

CO₂ diffuses from blood to alveoli, but because ventilation is low, CO₂ os not taken away as rapidly.

CO₂ accumulates in the alveoli and results in a higher state PCO₂

Question 36

What is the effect on O₂ concentration at a low ventilation to perfusion ratio? (base of the lung)

Answer 36

O₂ diffuses from alveoli to blood, but because ventilation is low O₂ is taken up by the blood is not fully replenished by new air entering the lungs

O₂ is depleted in the lungs leaving a lower new steady state PO₂

Question 37

What is the effect on CO₂ concentration at a high ventilation to perfusion ratio? (apex of the lung)

Answer 37

CO₂ diffusing from the blood is nearly all blown away, thus CO₂ in alveoli is depleted until a new lower steady state level occurs

Question 38

What is the effect on O₂ concentration at a high ventilation to perfusion ratio? (apex of the lung)

Answer 38

O₂ diffusing from alveoli is not taken away by blood as much as normal because relative blood flow is lower. O₂ is replenished with each breath causing it to accumulate. Diffusion carries on and a new PO₂ level occurs

Question 39

What are the PO₂ and PCO₂ in the apex of the lung and how does this differ from normal values?

Answer 39

• PO₂ = 130mmHg - normally ~100mmHg
• PCO₂ = 28mmHg - normally ~40mmHg

Question 40

What is an active control mechanism of blood flow?

Answer 40

Hypoxic vasocontriction

• intrinsic effect
• diverts blood away from poorly ventilated areas

Question 41

When is hypoxic vasocontriction important?

Answer 41

At birth

• before 1st breath, lung is vasocontricted and resistance is high
• With 1st breath, vasconstriction and resistance decreases