COMPOSITION OF ALVEOLAR AIR
COMPOSITION OF ALVEOLAR AIR
- Concentrations of gases in alveolar air differ from atmospheric air (inspired air)
- Atmospheric air contains approximately 21% O2, 79% N2 & 0.04% of CO2.
- Immediately after inspiration, there is no change in composition.
Factors causing difference:
1. Constant gaseous exchange process
- Continous removal of oxygen from alveolar air into blood vessels.
- CO2 addition to alveolar air from blood vessels.
2. Humidification:
- Dry atmospheric air humidified by upper respiratory passages before alveolar entry.
3. Partial/slow replacement with atmospheric air:
- Only small amount of fresh air (350 ml) added to alveolar air (2.2 L) during each inspiration.
- Slow replacement of atmospheric air into alveolar air happens.
Advantages:
- Prevents sudden changes in gas concentrations in blood.
- Stabilizes respiratory control mechanism.
- Prevents excessive fluctuation in tissue oxygenation, CO2 concentration, & tissue pH,
Gaseous concentration & its partial pressure in alveoli:
1. Oxygen:
Concentration & partial pressure controlled by,
- Rate of absorption of O2 into blood
- Rate of entry of new O2 into lungs by ventilatory process.
Values:
- Partial pressure of O2 in inspired air (Pi O2)
– 158 mm Hg
- Partial pressure of O2 in alveolar air (PA O2)
– 100 mm Hg
– Calculated by “Alveolar gas equation”.
- Partial pressure of O2 in expired air (PE O2)
– 116 mm Hg
2. Carbon-di-oxide:
- Partial pressure of CO2 in inspired air (Pi CO2)
– 0.3 mm Hg
- Partial pressure of CO2 in alveolar blood (PA CO2)
– 40 mm Hg
- Partial pressure of CO2 in expired air (PE CO2)
– 32 mm Hg
Exam Important
COMPOSITION OF ALVEOLAR AIR
Factors causing difference:
- Constant gaseous exchange process
- Humidification:
- Partial/slow replacement with atmospheric air:
Gaseous concentration & its partial pressure in alveoli:
1. Oxygen:
- Partial pressure of O2 in inspired air (Pi O2)
– 158 mm Hg
- Partial pressure of O2 in alveolar air (PA O2)
– 100 mm Hg
– Calculated by “Alveolar gas equation”.
- Partial pressure of O2 in expired air (PE O2).
– 116 mm Hg
2. Carbon-di-oxide:
- Partial pressure of CO2 in inspired air (Pi CO2)
– 0.3 mm Hg
- Partial pressure of CO2 in alveolar blood (PA CO2)
– 40 mm Hg
- Partial pressure of CO2 in expired air (PE CO2)
– 32 mm Hg.
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