The lung is divided into three zones based on blood flow. The apex has little to no blood flow and is a zone of wasted ventilation. The base has the highest blood flow and is a zone of wasted perfusion. Ventilation is highest at the apex while perfusion is lowest. Perfusion is highest at the base while ventilation is lowest. Normally the V/Q ratio is 0.8, allowing for optimal gas exchange. A V/Q of 0 means no ventilation while a V/Q of infinity means no perfusion. Various lung diseases can cause changes in V/Q ratios through effects on ventilation and/or perfusion.

1. Diffusion across
respiratory membrane

2. ZONES OF THE LUNG
• THERE ARE THREE ZONES INTO WHICH LUNG IS DIVIDED:
1-ZONE WITH NO BLOOD FLOW:
gravity diminishes blood flow to the apex of lung hence it makes zone of least or no
blood flow
2-ZONE OF INTERMITTENT BLOOD FLOW:
this zone receives almost intermittent blood flow that may vary during systole and
diastole.its usually the portion somewhat between apex and base of lung
3-ZONE OF CONTINUOUS BLOOD FLOW:
With positive support from gravity,blood flow is most effiecient and continuous
towards base of the lung

3. THE VENTILATION AT THE APEX OF THE LUNG IS HIGHEST WHILE PERFUSION IS THE
LOWEST.THE VENTILATION AT BASE OF THE LUNG IS LOWEST WHILE PERFUSION IS
THE HIGHEST WHICH CONCLUDES THAT APEX IS ZONE OF WASTED VENTILATION
WHIE BASE IS ZONE OF WASTED PERFUSION

4. NOTE THAT BOTH VENTILATION AND PERFUSION ARE GREATER AT BASE OF
THE LUNG THAN TOP OF THE LUNG BUT VENTILATION IS GREATER THAN
PERFUSION AT TOP OF THE LUNG AND PERFUSION IS GREATER THAN
VENTILATION AT BASE HENCE VENTILATION PERFUSION(V/Q) RATIO IS
GREATER AT TOP OF LUNG THAN AT BASE THUS V/Q DECREASES FROM TOP
TO BOTTOM

5. THEREFORE AT THE TOP OF LUNG,ALVEOLAR PRESSURE IS GREATER THAN
PULMONARY ARTERY PRESSURE AND AT THE BASE OF LUNG,PULMONARY
ARTERY PRESSURE IS GREATER THAN THE ALVELAR PRESSURE WHILE IN
ZONE OF INTERMITTENT BLOOD FLOW,PULMONARY ARTERY PRESSURE IS
GREATER THAN ALVEOLAR PRESSURE DURING SYSTOLE WHILE ALVEOLAR
PRESSURE IS GREATER THAN PULMONARY ARTERY PRESSURE DURING
DIASTOLE

6. NORMAL V/Q
• AVERAGE V/Q IS 0.8 WHICH MEANS ON AVERAGE IN A PERSON
DENOMINATOR=PERFUSION IS GREATER THAN VENTILATION
• GASEOUS EXCHANGE AT MEMBRANE IS OPTIMAL
• ALVEOLAR PO2 IS 104mmHg
• ALVEOLAR PCO2 IS 40mmHg
• THESE ALVEOLAR PRESSURES FOR BOTH GASES ARE BETWEEN THE
EXTREMES OF THEIR PRESSURE IN VENOUS BLOOD AND THEIR
PRESSURE IN INSPIRED AIR

7. WHEN V/Q IS ZERO
• THIS HAPPENS WHEN NUMERATOR=V BECOMES ZERO
• ZERO VENTILATION MEANS AIR IN THE ALVEOLUS COMES TO
EQUILIBRIUM WITH BLOOD O2 AND CO2 BECAUSE NO EXCHANGE OR
EVEN NO AIR IS BEING VENTILATED OR REACHED TO THE ALVEOLI
• VENOUS BLOOD PARTIAL PRESSURES OF ALL GASES ARE CONSIDERED
IN THIS CASE SO;
• PO2 =40mmHg
• PCO2=45mmHg

8. WHEN V/Q IS INFINITY
• THIS OCCURS WHEN DENOMINATOR =Q BECOMES ZERO
• THIS MEANS NO BLOOD IS RECHING THE ALVEOLI,NO CAPILLARY
BLOOD FLOW REACHES ALVEOLI TO CARRY O2 AWAY OR BRING CO2
• THIS TIME NO EXCHANGE MEANS THAT ALVEOLAR AIR EQUILIBRIATES
WITH SIMPLE INSPIRED HUMIDIFIED AIR BECAUSE IT NEVER
UNDERWENT ANY CHANGE AFTER INSPIRATION
• HUMIDIFIED PARTIAL PRESSURES OF GASES WILL BE CONSIDERED;
• PO2=149mmHg
• PCO2=0mmHg

10. PHYSIOLOGICAL SHUNT
• WHEN V/Q IS BELOW NORMAL,THERE IS INADEQUATE VENTILATION
TO FULLY OXYGENATE BLOOD THROUGH PULMONARY CAPILLARIES
• SHUNTED BLOOD=A CERTAIN FRACTION OF VENOUS BLOOD PASSING
THROUGH PULMONARY CAPILLARIES THAT DOES NOT BECOME
OXYGENATED
• 2% OF THE CARDIAC OUTPUT FLOWS THROUGH THE BRONCHIAL
VESSELS RATHER THAN PULMONARY CAPILLARIES ALSO REEMAINS
DEOXYGENATED AND IS COUNTED UNDER SHUNTED BLOOD
• PHYSIOLOGICAL SHUNT=TOTAL QUANTITATIVE AMOUNT OF
SHUNTED BLOOD PER MINUTE

11. IN PATHOLOGIES LIKE PNEUMONIA(CONSOLIDATION),EARLY COPD(MUCUS
PLUGS),PULMONARY OEDEMA(EXCESS FLUID IN LUNGS),VENTILATION FALLS VERY
LOW AND V/Q ALMOST REACHES ZERO;WHEREAS IN LATE COPD (DESTRUCTION OF
CAPILLARY WALLS) AND PULMONARY EMBOLISM(EMBOLUS),PERFUSION FALLS VERY
LOW HENCE V/Q ALMOST GOES NEAR INFINITY
IN LATE COPD (V WAS ALREADY LOW EARLIER),Q ALSO FALLS LOW SO RATIO
BECOMES NORMAL OR NEAR 1 WHICH DOESN’T MEAN IT IS NOT PATHOLOGICAL
BECAUSE RATIO IS NORMAL BUT BOTH V AND Q ARE EXTREMELY LOW