Ventilation and Perfusion Matching Flashcards
What must be closely macthed for effective gas exchange?
ventilation and perfusion
What is ventilation?
the change in volume through the respiratory cycle
What type of alveolus will have less ventilation?
an alveolus that is already open and extended at the start of the cycle
Where does more gas go in the lung?
More gas goes to the base of the lung than the apex.
Why does the base of the lung change volume more than the apex?
- 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
What two circuits nourish the lung tissue?
pulmonary and bronchial
Where do the bronchial arteries branch from?
the descending aorta
What is the function of the bronchial circulation
to supply oxygen to lung parenchyma, airway smooth muscle, pulmonary arteries and veins and pleura. Additional function is warming of inspired air
What is flow through the pulmonary artery considered to be equal to?
cardiac output
What is the volume of the pulmonary capillary network at rest
~100ml
What is the volume of stroke volume?
~70ml
What type of system is the pulmonary circulation?
low pressure, low resistance system
What is the mean arterial pressure in the pulmonary circulation and what does it need to be sufficient to do?
~15mmHg
pressure only needs to be sufficient to lift blood to the apex of the lung
How thick are the walls of the pulmonary arteries?
thin (mistaken for systemic veins)
What are pulmonary capillaries in effect surrounded by?
surrounded by air - can collapse or be extended according to balance of blood pressure and alveolar pressure
What is transmural pressure?
the balance between alveolar and blood pressure which determines the blood flow through the pulmonary capillaries
Why is blood floe low at the apex and high at the base of the lung?
- 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
When will blood only flow in the lungs?
if blood pressure is greater than alveolar pressure
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
Describe the nature of the capillaries in each zone of the lungs and relate how this would affect blood flow
- zone 1 = closed (low)
- zone 2 = open/closed (medium)
- zone 3 = open (higher)
What is the consequence of changes in hydrostatic pressure at the apex of the lung? And when would this occur?
alveolar pressure > blood hydrostatic pressure - the capillary will be closed
occurs with low BP or raised alveolar pressure
What occurs in zone one if capillaries are closed? And when does this not tend to exist?
if capillaries are closed then they are ventilated but not perfused and this is considered alveolar dead space
Doesnt usually exist in normal people
under what conditions is there not normally a zone 1 and why?
Normal conditions
there is sufficient pressure to perfuse the apices
Describe the pressure and flow in zone 2
- 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
Describe the pressure and flow in zone 3
- alveolar pressure is lower than arterial and venous pressure
- the capillaries are distended as a consequence of transmural pressure and there is continuous flow
What is the V/Q ratio?
Ventilation and perfusion ratio
What is the average V/Q ratio over the whole lung?
0.85
What is the V/Q ratio in the base of the lung and what does this mean?
0.6
good perfusion and good ventilation (but more perfusion occurs)
What is the V/Q ratio in the apex of the lung and what does this mean?
3
More ventilation than perfusion
Draw a graph indicating the changes in V/Q ratio, blood flow and ventilation between the top and bottom of the lung
Describe perfect matching
well ventilated alveoli with good perfusion of blood. Blood will equilibriate with alveolar air and be rich in oxygen and low in CO2
What occurs in poorly ventilated alveoli with a rich blood supply?
- 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
What occurs in well ventilated alveoli that are poorly perfused with blood?
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
What occurs when there is lowered PO2 of the blood?
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
What is the effect on CO2 concentration at a low ventilation to perfusion ratio? (base of the lung)
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
What is the effect on O2 concentration at a low ventilation to perfusion ratio? (base of the lung)
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
What is the effect on CO2 concentration at a high ventilation to perfusion ratio? (apex of the lung)
CO2 diffusing from the blood is nearly all blown away, thus CO2 in alveoli is depleted until a new lower steady state level occurs
What is the effect on O2 concentration at a high ventilation to perfusion ratio? (apex of the lung)
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
What are the PO2 and PCO2 in the apex of the lung and how does this differ from normal values?
- PO2 = 130mmHg - normally ~100mmHg
- PCO2 = 28mmHg - normally ~40mmHg
What is an active control mechanism of blood flow?
Hypoxic vasocontriction
- intrinsic effect
- diverts blood away from poorly ventilated areas
When is hypoxic vasocontriction important?
At birth
- before 1st breath, lung is vasocontricted and resistance is high
- With 1st breath, vasconstriction and resistance decreases
