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Alveolar ventilation
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Alveolar ventilation and relationship with oxygen and carbon dioxide partial pressures and variations throughout the lung
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Alveolar VentilationMonday, 20 September 2021 5:00 PM
- Tidal volume (TV)
o the amount of air you inhale and exhale
§ Dead space ventilation (Vd) + Alveolar ventilation (Va)
- Tidal volume moves into conducting airway
- Do not obtain full volume of fresh air (TV) because of dead space
o Some air remains in airway – doesn’t take part in gas exchange
- Anatomical dead space
o For conducting air but not for gas exchange
- Physiological dead space
o Anatomical dead space plus space receives no blood supply and space with excess
ventilation
o Conducting airway plus alveolar dead space
o No gas exchange
o This amount can be elevated in diseased conditions
CO2 and alveolar ventilation relationship
- Partial pressure of CO2 in alveolus
- Alveolar co2 level over alveolar ventilation
o Co2 level kept constant at 40 mmhg at resting level
§ Isocapnic line
§ 4-5L/min alveolar ventilation
o Hypoventilation
§ Inadequate ventilation to meet metabolic demand
§ Breathing less than what you need – e.g., holding your breath
§ CO2 level increases due to CO2 retention
• Since you’re not breathing CO2 out
• Not getting cO2 out of your lungs fast enough
o Hyperventilation
§ Increase alveolar ventilation
§ Curve move downwards
§ CO2 level decreases below 40mmhg
§ Exhale more CO2 out
o Hyperpnea
§ Ventilation increased to match increased metabolic activity
• Moderate exercise
§ Normal CO2 concentration
§ Curve shift to right
§ Increased alveolar ventilation and increase CO2 level
Alveolar O2 level and Alveolar ventilation relationship
- Vo2 – amount of oxygen uptake by the body in a minute
o Oxygen consumption
, § Curve shift to right
§ Increased alveolar ventilation and increase CO2 level
Alveolar O2 level and Alveolar ventilation relationship
- Vo2 – amount of oxygen uptake by the body in a minute
o Oxygen consumption
- Amount of oxygen left in the lung is Po2
- Alveolar oxygen level varies directly with alveolar ventilation
o VO2 depends on metabolic activity
- PiO2 – oxygen inspired - depends on atmospheric pressure
o Higher altitude means PiO2 level is lower
- Curve
o Hyperventilation
§ Higher alveolar ventilation, higher alveolar oxygen level
§ Breath in more fresh air so increase amount of inspired oxygen in the lungs
o Iso-oxic line
§ Ideally 100mmhg PaO2 and 4-5 liters per min of alveolar ventilation
o Hypoventilation
§ Not enough alveolar ventilation, lower alveolar oxygen level
• E.g. holding your breath
§ More oxygen is consumed and not adequate inspired oxygen coming into the
lungs to maintain at the iso-oxic level
o Hypernea – increased ventilation
§ To match the increased metabolic activity
• During exercise
§ Curve shifts to the right
§ Leading to increase in oxygen consumption
• Increased metabolic demand
• Alveolar oxygen level remains at 100mmhg but alveolar ventilation
increases
- Alveolar gas equation
o To calculate the alveolar oxygen level
o PaO2 = PiO2 ( Inspired oxygen) - PaCO2 / R
§ If we know inspired oxygen level
• Constant at sea level
• 150mmhg
• (Barometric pressure - water pressure) x FiO2
• (760-47) x 0.21 mm Hg
§ And alveolar CO2 level
• Measured by collecting inspired gas
o Indicates CO2 level in the lungs
• Generally 40mmhg
§ R – respiratory quotient
• Co2 produced / O2 consumed
o VCO2 / VO2
o 200/250
• About 0.8 in a health individual at sea level – also depends on diet
o If diet is pure carbohydrate – R becomes 1
o If pure fat diet – R lowers to 0.7
o Average = mixed diet, r = 0.8
§ Ideal – 100mhg
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