pk5 Ventilation and Perfusion Flashcards
What is the partial pressure of oxygen in the air?
What is the alveolar oxygen partial pressure?
What is the partial pressure of oxygen in venous blood?
What is the partial pressure of oxygen in arterial blood?
- The partial pressure of oxygen in the air is 20 kPa.
- Alveolar oxygen partial pressure is 13 kPa.
- The partial pressure of oxygen in venous blood is 5 kPa.
- The partial pressure of oxygen in arterial blood is 12 kPa.
What is the alveolar-arterial PO2 difference in a healthy young person?
How does the alveolar-arterial PO2 difference change with age?
- The alveolar-arterial PO2 difference is 13-12 = 1 kPa in a healthy young person.
- Alveolar-arterial PO2 difference increases with age.
Describe the causes of the alveolar-arterial PO2 difference.
What contribution does each cause make to the overall alveolar-arterial PO2 difference?
Causes of the alveolar-arterial PO2 difference are:
1 - Shunting.
- Not all blood entering the left aorta is oxygenated at the lungs.
- Shunted blood is normally 1-2% of the cardiac output.
2 - V/Q mismatch.
- At rest, the lungs receive:
- 4L/min alveolar ventilation (V).
- 5L/min pulmonary blood flow (Q).
- Total V/Q is therefore 0.8.
- However, V/Q varies throughout the lung. 0.8 is the total V/Q.
- Both causes contribute equally to the alveolar-arterial PO2 difference.
- V and Q are not evenly distributed throughout the lung.
Describe the natural causes of shunting.
1 - Thebesian veins (venae cordis minimae).
- These are small valveless veins that open directly into the left chamber of the heart from the capillary bed supplying the cardiac tissue.
2 - Bronchial circulation.
- This arises from the thoracic aorta, and empties into the pulmonary vein to drain directly into the left atrium.
How can pathological R-L shunts occur?
1 - Any airway block or collapse of a bronchus / alveolus will inhibit downstream flow.
- This means that blood supplying these small airways will not be oxygenated.
2 - Cardiovascular anatomical abnormalities.
- E.g. a patent ductus arteriosus can cause a L-R shunt.
- The flow of oxygenated blood from the aorta back into the right chamber of the heart can put an excessive load on the pulmonary circulation, causing congestion, which can ultimately impact back on the left heart. *This is explained in more detail in RED ‘Foetal and Neonatal Cardiorespiratory Physiology’.
- In these circumstances, the blood is shunted (moved to the arterial circulation without being oxygenated) and V/Q decreases.
List the causes of variation in V/Q throughout the lung.
Causes of V/Q variation throughout the lung include:
1 - Gravity.
2 - Variation in compliance.
- These factors lead to a variation in pleural pressure throughout the lung, impacting V/Q.
How does gravity cause variation in ventilation (V) throughout the lung?
Ventilation is greater at the base than at the apex:
- Gravity causes the base of the lung to sit closer to the visceral pleura than the apex of the lung (because the weight of the lung imposes on the pleural cavity).
- This means that the intrapleural pressure is more negative at the apex of the lung (-1 kPa) and less negative at the base of the lung (-0.25 kPa).
- This means that, at FRC, the apex of the lung is more inflated than the base.
- In order to inflate the lung, the intrapleural pressure needs to fall from resting (FRC) intrapleural pressure.
- A lung compliance curve is an S shape - lung compliance is not constant. The greatest change in lung volume occurs between 0 and -1 kPa. Before and after this range, compliance decreases.
- The base of the lung sits within this range, whereas the apex of the lung is just outside of this range, meaning the lung is more compliant at the base than at the apex.
- This means that ventilation is 2.5x greater in the base than at the apex.
How does gravity cause variation in perfusion (Q) throughout the lung?
- Gravity causes blood to pool to the arteries lower down the lung. This means that blood pressure increases towards the base of the lung.
- The lung is divided into 3 zones, with consecutive zones representing lower areas of the lung.
- In zone 3, the arterial pressure > venous pressure > intra-alveolar pressure. This means that the arterial-venous pressure difference is the driving force for blood flow through zone 3.
- In zone 2, the arterial pressure > intra-alveolar pressure > venous pressure. This means that the arterial-alveolar pressure difference is the driving force for blood flow through zone 2.
- In zone 1, the intra-alveolar pressure > arterial pressure > venous pressure. At this zone, there is no driving force for blood flow through the lung (the intra-alveolar pressure overcomes blood pressure).
- Blood flow therefore becomes increasingly small towards the apex - because the intra-alveolar pressure inhibits the driving force for blood flow, which decreases from zone 3 - zone 1.
- This is why the perfusion is 6x greater in the base than at the apex.
- Arterial-venous pressure difference is the same at all levels, it’s just that their individual values decrease with decreasing zones.
- Zone 1 is not seen in normal health - explained in a later card
What is recruitment and distension?
Where do these processes occur?
What is the impact of these processes on lung perfusion?
- Recruitment is the increase in formation of blood vessels in the lung with rising pressure.
- Recruitment occurs at zone 2 of the lung.
- Recruitment accounts for a large increase in blood flow in the lung.
- Distension is the expansion of recruited vessels in the lung.
- Distension occurs as zone 3 of the lung, where blood pressure is highest.
- Distension accounts for a moderate increase in blood flow in the lung, but not as significant as recruitment.
What is recruitment and distension?
Where do these processes occur?
What is the impact of these processes on lung perfusion?
- Recruitment is the increase in formation of blood vessels in the lung with rising pressure.
- Recruitment occurs at zone 2 of the lung.
- Recruitment accounts for a large increase in blood flow in the lung.
- Distension is the expansion of recruited vessels in the lung.
- Distension occurs as zone 3 of the lung, where blood pressure is highest.
- Distension accounts for a moderate increase in blood flow in the lung.
How does the V/Q ratio change from the apex to the base of the lung?
- At the apex, V is greater than Q - V/Q here is 3.0.
- Both V and Q increase from the apex to the base.
- However, remember that Q increases 6 fold from apex to base whereas V only increases 2.5x from apex to base.
- This means that flow of blood (Q) overtakes flow of air (V) with increasing depth in the lung, and at the base, Q is greater than V - V/Q here is 0.6.
- The average V/Q throughout the lung is 0.8.
What is the V/Q ratio in a shunt?
What will the PO2 and PCO2 be in a shunt?
- The V/Q ratio in a shunt is 0 because V is 0.
- Here, and anywhere with a V/Q lower than 0.8, the PO2 will be low and the PCO2 will be high.
What is the V/Q ratio in alveolar deadspace?
What will the PO2 and PCO2 be in alveolar deadspace?
- The V/Q ratio in alveolar deadspace is infinity because Q is 0.
- Here, and anywhere with a V/Q higher than 0.8, the PO2 will be high and the PCO2 will be low.
List 2 mechanisms by which the V/Q ratio can be adjusted to match physiological demands.
What is the physiological effect of an excessively high V/Q and an excessively low V/Q?
Which mechanism is appropriate in response to a high V/Q and to a low V/Q?
The V/Q ratio can be adjusted by:
1 - Altering bronchial tone to alter V.
2 - Altering arteriolar tone to alter Q.
- An excessively high V/Q ratio causes hyperoxia and hypocapnia.
- The appropriate mechanism to a high V/Q is bronchoconstriction + vasodilation.
- The vasodilation acts to divert blood to this area of better ventilation, and the bronchoconstriction acts to divert the high ventilation to other under-ventilated areas of the lung.
- An excessively low V/Q ratio causes hypoxia and hypercapnia.
- The appropriate mechanism to a low V/Q is bronchodilation + vasoconstriction.
- The vasoconstriction acts to divert blood away to other areas with better ventilation, and the bronchodilation acts to increase ventilation to this under-ventilated area.
Describe the cellular mechanism that controls vasoconstriction at the pulmonary vessels.
- Remember, when hypoxia occurs in the lung, the appropriate response is vasoconstriction:
1 - Hypoxia results in the blockage of potassium channels on the pulmonary artery,
2 - This reduces efflux of potassium from the endothelium cell.
3 - This causes membrane depolarisation.
4 - Membrane depolarisation leads to the opening of voltage-gated Ca2+ channels.
5 - This increases influx of Ca2+ into the endothelium cell.
6 - Ca2+ causes vasoconstriction.
