Ventilation and Perfusion Matching Flashcards
Explain why ventilation and perfusion matching is important.
For effective gas exchange to occur, ventilation and perfusion need to be closely matched- clinically very important in a number of conditions. This is not straightforward, since there are variations in both lung ventilation, and perfusion in different parts of the lung.
Define ventilation.
Ventilation is the change in volume through the respiratory cycle. An alveolus that is already open and extended at the start of the cycle will have less ventilation (i.e. less capacity to be ventilated) than a small and non extended one. In other words, ventilation is the change in volume compared to the resting volume.
Identify the main pattern of variations in ventilation in different areas of the lung, and explain why these variations occur.
♦ Inspired air is not evenly distributed throughout the lung in the upright human (i.e. more gas goes to the base of the lung than the apex)
Why:
♦ 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. Hence, base of the lung changes volume more than the apex.
♦ Small role due to gravity – weight of the lung pulls down on pleura, so the apex has a more negative intrapleural pressure, with so alveoli are more extended already (so less ability to undergo ventilation) (also observed in low gravity environments in earth orbit, so gravity not major factor).
Identify the circuits which nourish lung tissue.
1) Pulmonary circulation
2) Bronchial circulation
Describe the main features of the pulmonary circulation as a circuit for the nourishment of lung tissue.
♣ Approximately 98% of all blood to lungs – respiratory portion. Almost entire output of right ventricle, deoxygenated blood, into pulmonary artery, then branching to supply lobes, alongside bronchial tree until reach bronchioles where a dense capillary network is formed.
♣ Possible to consider blood flow in this region as a sheet of flowing blood rather than individual capillaries.
♣ Oxygenated blood returns through pulmonary venules and veins to the left atrium.
♣ Flow through the pulmonary artery is considered to be equal to cardiac output (i.e. 5 L entering the pulmonary circulation per minute), but actually slightly less – see bronchial circulation (i.e. volume of pulmonary capillary network at rest is ~ 100ml, and stroke volume is ~70ml so blood is almost completely exchanged each beat)
State the PCO2 and PO2 values in the tissues and lungs.
TISSUES (tissues, blood leaving tissue capillaries, blood entering alveolar capillaries)
- PCO2 45 mmHg (greater than 45 mmHg in tissues)
- PCO2 40 mmHg (less than 40 mmHg in tissues)
LUNGS (alveoli of the lungs, blood leaving alveolar capillaries, blood entering tissue capillaries)
- PCO2: 40 mmHg
- PO2: 104 mmHg
Describe the main features of bronchial circulation as a circuit for the nourishment of lung tissue.
☻ Approximately 2% of blood to lungs – conducting portion (re-joins circulation in pulmonary vein) thus ‘diluting’ slightly the oxygenated blood with deoxygenated (drops the oxygen saturation by a couple percentages)
☻ Part of systemic circulation – bronchial arteries are branches of descending aorta.
☻ Function to supply oxygen to lung parenchyma, airway smooth muscle pulmonary arteries and veins, and pleura + additional function is conditioning (warming) of inspired air
Describe the pressures in the pulmonary circulation.
♦ The pulmonary circulation is a low pressure, low resistance, high flow system (average pressures are 25/8, mean pressure is ~15 mmHg) since pressure only needs to be sufficient to lift blood to apex of lung
♦ Able to recruit more vessels with only a small increase in arterial pulmonary pressure.
♦ Walls of pulmonary artery are thin (can be mistaken for a systemic vein)
♦ Pulmonary capillaries are in effect surrounded by air – can collapse or be extended according to balance of blood pressure and alveolar pressure (i.e. blood flow through them depends on the balance between alveolar pressure and blood pressure called - transmural pressure))
♦ Arteries and veins also affected by surrounding tissue
Identify the main pattern of variations in perfusion in different areas of the lung, and explain why these variations occur.
○ Blood flow at apex is low and blood flow at the base is large
○ Due to hydrostatic pressure difference between base and apex of the lung ~23 mmHg (i.e. pressure in capillaries is lower at apex than at base of lung)
○ Blood will only flow through capillary network if blood pressure is greater than alveolar pressure (i.e. relying on P difference between what’s in alveoli and capillaries to determine if capillaries in different parts of the lungs are open ).
○ In zone 1 (top of the lung), capillary networks are closed because P is higher in alveoli than capillaries (i.e. alveolar pressure> blood hydrostatic pressure)
In zone 2, capillaries can open and close open and close depending on variance in individual heartbeat (at some points P in capillaries will be higher than that inside alveoli, whilst at others it will be lower)
In zone 3 (base of the lung), capillary networks open because P is higher in capillaries than in alveoli.
Explain the implications of different ventilation-perfusion matching on blood flow in different areas of the lung.
♪ In zone 1, apex of lungs, if capillaries are closed, then they are ventilated but not perfused and so are considered alveolar dead space (under normal conditions there is no zone 1, because normally there is sufficient pressure to perfuse the apices, so zone 1 only becomes significant in pathologies)
♪ 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 arterial- alveolar pressure difference.
♪ In zone 3, alveolar pressure is lower than both arterial pressure and venous pressure – the capillaries are distended as a consequence of the transmural pressure and there is continuous flow
What is the perfusion to ventilation ratio of the lung ?
♥ (V/Q)
♥ The ratio over the whole lung is about 0.85
♥ At the base, there is good perfusion and good ventilation, and the ratio is about 0.6 (more perfusion than ventilation)
♥ About 2/3 up from the base the ratio is 1
♥ At the apex, it is about 3
In graphical form, show the change in ventilation, and blood flow, as you go down the lung and for different V/Q ratios.
Refer to slide 17 in lecture on “Ventilation and Perfusion Matching”
Describe the consequences of perfect matching between perfusion and ventilation.
Blood will equilibrate with alveolar air and be rich in oxygen and low in CO2.
Give an example of somewhere in the lung with low ventilation to perfusion ratio. Describe the consequences of this on CO2 and O2 concentrations.
BASE OF LUNG
• Low PO2: O2 diffuses from alveoli into blood, but because ventilation is low the O2 taken up by the blood is not fully replenished by new air entering the lungs. O2 is therefore depleted in alveoli, and new steady state low PO2 occurs.
• High PCO2: CO2 diffuses from blood to alveoli, but because ventilation is low, CO2 is not taken away as rapidly. Thus CO2 accumulates in alveoli and higher steady state PCO2 occurs.
Give an example of somewhere in the lung with high ventilation to perfusion ratio. Describe the consequences this on O2 and CO2 concentrations.
APEX OF LUNG
• Lower CO2: CO2 diffusing from blood is nearly all blown away, thus CO2 in
alveoli is depleted until a new lower steady state level occurs
• Higher O2: O2 diffusing from alveoli is not taken away by blood as much as normal because relative blood flow is lower. But as O2 is replenished with each breath O2 accumulates, diffusion carries on and a new higher PO2 level occurs.
• PO2 is no higher than normal, so haemoglobin saturation is normal and dissolved O2 is normal
