L2 - Respiration - Ventilation and Perfusion Flashcards
What is ventilation?
Ventilation is the volume of air moved out of the lungs per unit time
How do you calculate ventilation?
Tidal volume - 0.5 litres
Breathing frequency - 12 breaths per minute
Total ventilation = 0.5 x 12 = 6 litres/min
Why does not all the fresh air reach the alveoli in the respiratory zone?
First portion is stale air from conducting zone (anatomical dead space) - 0.15 litres
How do you calculate alveolar ventilation?
Alveolar ventilation = total ventilation – dead space ventilation
= 6 litres/min - (0.15 x12) litres/min
= 4.2 litres/min
How does ventilation rate have an impact on alveolar and arterial gas composition?
As alveolar ventilation (l/min) increases alveolar partial pressure of
- Oxygen increases
- Carbon dioxide decreases
How does lung ventilation differ in the different areas of the lung?
Ventilation is greater at the base compared to the apex – linked to starting volume of the alveoli
- At apex - larger starting volume - low compliance
- At base - smaller starting volume - high compliance (small change in pressure gives big change in volume)
When is pulmonary resistance at its lowest?
When lungs are at functional residual capacity
Total resistance is sum of alveolar and extra-alveolar resistances
Pulmonary resistance in alveolar vessels?
Capillaries and slightly larger vessels surrounded by alveoli
Resistance linked to transmural pressure and lung volume
Pulmonary resistance in extra-alveolar vessels?
Vessels not surrounded by alveoli
Sensitive to changes in intra-pleural pressure
What is the importance of capillary recruitment?
Pulmonary circulation can accommodate increases in pulmonary pressure and flow by recruitment
- This reduces the system resistance
What are the 3 different states capillary vessels can be found in during capillary recruitment?
Collapsed
Open but do not conduct blood
Open and conduct blood
Capillary recruitment method
In first stage of recruitment
- Previously collapsed vessels now become patent but still do not conduct blood
- Previously open but non-conducting vessels now conduct blood
- Distension widens vessels that previously had been open and conducting
Later during recruitment the previously collapsed vessels now conduct blood
All vessels now dilate causing resistance to fall
What impacts lung perfusion?
Perfusion varies according to posture and gravity
Perfusion is greater at the base compared to the apex
What is the reference point for pulmonary circulation pressures?
Outside the heart at the left atrium level
What is Ppa?
Pressure in pulmonary arterioles
Mean of about 15mmHg (20 cm H2O)
What is Ppv?
Pressure in pulmonary venules
Mean of about 8mmHg (10 cmH2O)
How do Ppa and Ppv change when moved above or below the left atrium?
Fall by about 1 cm H2O for every 1cm above the left atrium
Increase by 1 cm H2O for every 1cm below the left atrium
What is Pa?
Alveolar pressure
Is relative to atmospheric pressure
Model considers the point when there is no movement of air
- At this time, no difference between atmospheric pressure and PA so PA = 0 cmH2O.
What is Ptm?
Transmural pressure gradient across the wall of the vessel
How many zones does the lung have?
4
What factors alter perfusion?
Dilators - Increase in alveolar oxygen - Decrease in alveolar carbon dioxide - Increased pH Constrictors - Decrease in alveolar oxygen - Increase in alveolar carbon dioxide - Decreased pH
Ratio of the rate of alveola ventilation and pulmonary blood flow influences?
Gas composition
If alveoli are not ventilated the ventilation/perfusion ration becomes?
0
Gas composition will become the same as that of mixed venous blood
40mmHg O2 and 46 mmHg CO2
If alveoli are not perfused the ventilation/perfusion ration becomes?
Infinity
Gas composition will become the same as that of inspired humified air
149mmHg O2 and 0mmHg CO2
How does the ventilation/perfusion ratio vary down the lung?
Apex – 3.3
Base – 0.6
Overall – 0.84
Ventilation/perfusion mismatch - Alveolar dead space ventilation
Local reduction of perfusion e.g. pulmonary embolism
No exchange means gas composition in affected alveoli becomes like moist inspired air
Ventilation/perfusion ratio becomes ∞
Compensation
- Blood redirected to other areas
- Bronchiolar constriction
- Reduction in surfactant production
Ventilation/perfusion mismatch - shunt
Local reduction of ventilation e.g. tumor or foreign body
Gas can’t be removed so gas composition becomes like mixed venous blood
Compensation
- Air redirected to other areas of the lung
- Hypoxic pulmonary vasoconstriction
