0826 - Alveolar Ventilation Flashcards
What are the components that make up ventilation?
Total Ventilation = Frequency x Tidal Volume
2 Components of air volume:
- Dead Space (physiological in healthy, pathological in ill) where no gas exchange takes place (around 30% of TV)
- Alveoli where gas exchange takes place.
What are the components of physiological dead space
Anatomical dead space -airways - nose to bronchioles.
Functional dead space - ventilated lung parts which are not perfused (~0 in a healthy person due to ventilation/perfusion relationship)
Briefly discuss anatomical dead space
First few cm prepare air for gas exchange (cleaning, warming, humidifying).
For a given TV, volume of dead space limits how much CO2 you can breath off (i.e. you have to re-breathe the air in your trachea), and also sets the FRC (a certain amount of air will always remain in the dead space).
Modulates airway resistance (by CO2 concentration - see perfusion lecture).
Briefly discuss functional dead space
Ventilated lung that is not perfused (should be ~0 in a healthy human).
Rises in pathology “atelectasis” - air free areas.
Can become an effective shunt if the pathology is in a diffusion barrier and arteriole supply is not cut off. Leads to hypoxaemia (low O2, high CO2 - essentially venous blood).
Explain how ventilation determines partial pressure of CO2
Alveolar ventilation (relevant part of ventilation) is around 0.7 of total ventilation.
Around 15% of alveolar gas volume is refreshed every tidal breath.
Gives you an exponential decay in CO2 concentration (half life around 17 seconds).
Alveolar ventilation is thus inversely proportional to CO2 partial pressure - doubling ventilation would half the half life).
Alveolar ventilation decreases significantly when dead space increases. Increases in exercise as more areas are being ventilated - leaving blood gases largely identical.
Why is the slow replacement of alveolar gas important?
Prevents rapid changes of blood gas concentrations, providing the body with a significant buffer if respiration is interrupted, and providing stability in respiratory control.
What factors determine the speed of gas exchange in alveoli?
Airway resistance and lung tissue compliance (time constant of filling is a product of the two). Altering either of these variables will impact on the time it takes to fill the alveolus.
How can uneven ventilation of lung tissue arise?
Filling determined by airway resistance and tissue compliance.
If either one of these factors is altered within a lung area, it will impact on ventilation.
Increased airway resistance (slow filling time) OR compliance (‘too full’) both impede gas exchange (make it take longer). Resistance prevents the flow getting to the alveolus, compliance prevents the tissue ‘bouncing back’ to expel the gas on expiration - both of these result in raised CO2 and lower O2 in alveoli. The reverse is also true.
Where is lung ventilation largest? Why is this the case?
Key Exam Concept
Ventilation is largest at the base and smallest at the apex due to hydrostatic (gravity) pressure. The apex is more inflated than the base, thus receives less actual ventilation. This difference largely disappears when laying down or in zero gravity.
