Phsyiological basis of heath and disease BBMS2006 (BBMS2006)
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Ventilation and Perfusion
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Phsyiological basis of heath and disease BBMS2006 (BBMS2006)
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The University Of Hong Kong
Ventilation and perfusion rates and differences throughout the luung, relating to oxygen and carbon dioxide dissociation curves and the problems from ventilation-perfusion mismatching
Phsyiological basis of heath and disease BBMS2006 (BBMS2006)
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Ventilation and Perfusion
Monday, 20 September 2021 5:01 PM
Matching of alveolar ventilation to perfusion (Blood flow)
- Normal range of alveolar ventilation – 4 L/min
- Perfusion – 5 L/min
o Determined by cardiac output
§ Heartbeat x stroke volume
Ratio between ventilation / perfusion – 0.8 in normal conditions
- Effects of altering V/Q ratio
o Blood flow obstruction
§ Va/Q = infinity
§ E.g. blood clot / air bubble
§ Q becomes zero
§ Oxygen increases and CO2 decreases
o Airway obstruction
§ Va/Q ratio becomes zero
§ E.g. lung tumor / excessive mucous secretion
§ Higher RAW
§ Va becomes very low
§ Alveolar oxygen and CO2 levels near systemic venous levels
• Level of O2 in the blood entering the lungs – low O2 and high CO2
- An upright lung curve
o Blood flow
§ Decreases from bottom to top of lung
• Higher blood flow at base of lung
• Blood flow not evenly distributed in lung due to gravitational force
o ventilation
§ decreases slightly from bottom to top
§ more ventilation at base
§ degree of uneven distribution is less than blood flow
o Va/Q
§ Low at base
• Because of high blood flow compared to ventilation
§ Gradually increases
§ Very high at apex
• Because less blood flow
- Regional difference in V/Q ratio leads to regional differences in gas exchange
Oxygen dissociation curve
- At apex with high Va/Q ratio
o Oxygen level is high
o Oxygen content – 20% - fully saturated
- At normal ratio
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