Ventilation & perfusion in the lung

 

It is the ratio of ventilation to pulmonary capillary blood flow [VA/Q] that determines what the alveolar gas composition will be.

 

 

1) VA/Q = 0

·      zero alveolar ventilation but still blood flow through alveolar capillaries

·      air in alveolus comes into equilibrium with blood gases

·      alveolus partial pressures PO2 = 40; PCO2 = 45 [ie venous blood gases]

 

2) VA/Q = ¥

·      there is alveolar ventilation but no blood flow through alveolar capillaries

·      air in alveolus comes in equilibrium with inspired air

·      alveolus partial pressures PO2 =149; PCO2 = 0 [ie inspired air]

 

3) VA/Q =normal

·      there is both normal alveolar ventilation and blood flow in pulmonary capillaries

·      oxygen is absorbed from inspired air in alveolus

·      carbon dioxide is transfered to alveolar air from venous blood

·      alveolus partial pressures PO2 =104; PCO2 = 40 [ie between inspired air & alveolus air]

 

A) Physiological Shunt VA/Q = < normal

·      not enough ventilation for the pulmonary blood flow

·      some of the venous blood is not oxygenated: shunted blood

·      note physiological shunt seen in:

  1. bottom of lung
  2. bronchial blood flow (2% total CO)

 

B) Physiological Dead Space VA/Q = > normal

·      excess ventilation for pulmonary blood flow

·      some of the ventilation is wasted

·      note physiological dead space:

  1. seen in top of lung
  2. includes lung dead space

 

K. C. Potger
Copyright © 2001