1. Ventilation-perfusion ratio
Normally and more in disease, some areas of the lungs are well ventilated but have almost no blood flow, whereas other areas may have excellent blood flow but little or no ventilation, this will seriously impair gas exchange.
Normal ventilation and perfusion might be deceiving if ventilation and blood flow going to different parts of the lungs.
if V= 0 & Q=N V/Q= 0  no gas exchange
if V= N & Q= 0 V/Q= ∞  also no gas exchange

2. Ventilation-perfusion ratio
When V/Q= 0 :
 alveolar air will reach equilibrium with blood CO2 & O2 (venous blood)
PO2= 40mmHg PCO2= 45mmHg
When V/Q= ∞ :
there’s no blood so no gas exchange
the alveolar air will equal the inspired air
PO2=149mmHg PCO2= 0

5. Ventilation-perfusion ratio
Physiologic shunt:
when V/Q is below normal due to inadequate ventilation
part of the venous blood passing the capillary does not become oxygenated
total amount of shunted blood per minute:
_Qps_= _CiO2 -_CaO2
Qt CiO2 – CvO2

6. Qps : Physiological shunt per minute
QT : Cardiac output
Cio2 : O2 concentration in arterial blood with ideal ventilation perfusion ratio
Cao2 : O2 concentration measure in arterial blood
Cvo2 : O2 concentration measured in mixed venous Bl

7. Ventilation-perfusion ratio
Physiological dead space:
when V/Q is above normal
considered as wasted ventilation
measured by the following equation:
_Vds_ = _PaCO2_-_PECO2_
Vt PaCO2

8. Ventilation-perfusion ratio
Abnormalities :
1- V/Q in upright chest :
ventilation & blood flow are less at top
blood flow is decreased more than vent.
Top = 2.5x ideal V/Q (phys. dead space)
Bottom = 0.6x ideal V/Q (phys. shunt)

9. Ventilation-perfusion ratio
2-V/Q in chronic obstructive lung disease: in chronic smoker, bronchial damage will occur leading alveolar air trapping and emphysema, this will lead to: a-unventilated alveoli because of obstruction b-wasted ventilation because of damaged blood vessels