During respiration, our lungs work to help the body take in oxygen while removing carbon dioxide. This cycle is referred to as gas exchange.

For it to function properly, the amount of ventilation must match the amount of perfusion in the alveoli of the lungs.

There should be a balance in the Ventilation/Perfusion Ratio.

This topic can be quite complex, however, we created this study guide to (hopefully) make the learning process easier for you. So, if you’re ready, let’s get started.

What is Ventilation?

Ventilation (V) refers to the amount of air that enters and leaves the alveoli.

For the body to obtain oxygen, there first must be sufficient amounts of air that reaches the alveolar region of the lungs.

What is Perfusion?

Perfusion (Q) refers to the amount of blood that flows to the alveolar capillaries.

For the body to obtain oxygen, there also must be sufficient amounts of blood passing through the lungs to pick up oxygen molecules so that it can be transported to other organs and tissues.

What is the Ventilation/Perfusion Ratio?

The V/Q ratio refers to the amount of air that reaches the alveoli per minute compared to the amount of blood that reaches the alveoli per minute.

Ideally, the amount of oxygen and blood reaching the alveoli would be a perfect match. This would result in a V/Q ratio of 1.0.

However, of course, this isn’t always the case — especially for people with cardiopulmonary conditions.

High V/Q Ratio

The patient would have a high V/Q ratio if there was more ventilation or less perfusion.

For example, this could occur when there is decreased blood flowing through the lungs and normal ventilation, as seen during a pulmonary embolism.

An area with ventilation but no perfusion is known as dead space.

Low V/Q Ratio

The patient would have a low V/Q ratio if there was less ventilation or more perfusion.

For example, this could occur during atelectasis because perfusion would be normal, but there would be decreased ventilation.

An area with perfusion but no ventilation is referred to as a shunt.

What is a Ventilation/Perfusion Imbalance?

A V/Q imbalance simply means that the amount of ventilation in the alveoli does not match the amount of perfusion. As you just learned, there could be either a high or low V/Q ratio.

You should know that a V/Q imbalance is typically the most common cause of hypoxemia in patients with respiratory diseases.

That is because any respiratory condition that causes decreased ventilation would result in a V/Q mismatch and, eventually, lead to low oxygen levels in the blood.

Ventilation-Perfusion Practice Questions:

1. What is a normal VQ ratio?
4:5 or 0.8

2. How is the VQ ratio calculated?
By dividing alveolar ventilation by pulmonary capillary blood flow.

3. What would be the ventilation and perfusion difference in the upper lobes when in an upright position?
Moderate ventilation and little blood flow.

4. What is the VQ ratio in the upper lung?

5. What would be the ventilation and perfusion difference in the lower lobes when in an upright position?
Alveolar ventilation is moderately increased and blood flow is greatly increased because blood flow is gravity dependent.

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6. What is the VQ ratio in the lower lung?

7. What would be the effect on VQ ratio which an obstructed airway?
VQ ratio decreases.

8. What would be the effect on PAO2 with an obstructed airway?
PAO2 levels fall.

9. What would be the effect on PACO2 with an obstructed airway?
PACO2 levels rise.

10. When the VQ ratio decreases, what happens to PAO2 and PACO2 levels?
PAO2 falls and PACO2 rises.

11. When the VQ ratio increases, what happens to PAO2 and PACO2 levels?
PAO2 rises and PACO2 falls.

12. What does VQ ratio stand for?
Ventilation-Perfusion ratio.

13. What is the meaning of VQ ratio?
It is the relationship of the overall alveolar ventilation (L/min) to the overall pulmonary blood flow (L/min). 4:5 or 0.8.

14. What is the meaning of respiratory quotient?
The ratio between the volume of oxygen consumed and the volume of carbon dioxide produced.

15. What is the average respiratory quotient?

16. What is the respiratory exchange ratio defined as?
The quantity of oxygen and carbon dioxide exchanged during a period of 1 minute.

17. Under normal conditions, what is the respiratory exchange ratio equal to?
The respiratory quotient is 0.8.

18. What is ETCO2?
It is a measurement of exhaled CO2.

19. What is the normal range of ETCO2?
NThe normal range is 35 – 40 mmHg or about 5%.

20. When will ETCO2 increase? Decrease?
It will increase with hypoventilation and decreases with hyperventilation.

21. When will the ETCO2 remain at zero?
It will remain at zero if esophageal intubation or in cardiac arrest.

22. What is a capnogram?
It is the waveform produced from capnography.

23. What is a capnography?
A rapid, continuous, noninvasive monitoring and graphic display of the patient’s inhaled and exhaled CO2 plotted against time.

24. What are three disease states that would cause an increased VQ ratio?
Pulmonary emboli, complete or partial obstruction of the pulmonary artery, and decreased cardiac output.

25. What are the three diseases that would cause a decreased VQ ratio?
Asthma, pneumonia, and hypoventilation.

26. How will a capnogram change during bronchospasm?
It will produce a “shark-fin” type pattern due to uneven exhalation. As bronchodilator treatment is effective, the exhalation line will become more normal.

27. How will a capnogram change during esophageal intubation?
The capnogram will read zero if the ETT is placed in the esophagus.

28. How will a capnogram change during cardiac arrest?
If ETCO2 drops after CPR stops, CPR needs to be resumed as there is no or inadequate blood flow through the lungs. Effective CPR will produce a capnogram reading

29. How will a capnogram change with a patient who has a fever?
ETCO2 rises with hyperthermia as metabolism increases and produces an increased metabolism.

30. How will a capnogram change with a patient having a seizure?
It will increase ETCO2 which is secondary to a great increase in muscle use/muscle spasm.

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31. How will a capnogram change with a patient experiencing pulmonary emboli?
A blockage of the pulmonary artery from an embolus will cause a rapid decrease in ETCO2 due to decreased blood flow.

32. What is the V/Q if your patient’s RR is 15 with a tidal volume of 450. (they weigh 160 pounds) and their cardiac output is 5.6 liters?

33. What are the normal values for ventilation and capillary blood flow?
Normal Ventilation: 4L/min and normal capillary blood flow: 5L/min.

34. What is the normal overall V/Q ratio for the lung and how is it determined?
Normal Ventilation-Perfusion (V/Q) is .8 which is normal ventilation 4 divided by normal capillary blood flow which is 5. 4/5=.8

35. How does the upper lung region V/Q compare to the normal V/Q ratio?
Upper lung region is higher than .8 and ventilation best at top of the lung.

36. How does the lower lung region V/Q compare with the normal V/Q ratio?
Lower lung region is lower than .8. Progressively decreases from top to bottom in the upright lung. And perfusion is best at the bottom of the lung.

37. What is the cause of an increased ratio?
An increase in ventilation and decrease in perfusion.

38. What is the cause of a decreased ratio?
A decrease in ventilation and an increase in perfusion.

39. What is the classic dead space condition?
Pulmonary embolus and a blood clot in the lung, blood can’t go through.

40. How to determine PAO2 balance?
Amount of O2 entering the alveoli and removal of O2 by capillary blood flow.

41. How to determine PACO2 balance?
Amount of CO2 diffusing into the alveoli from the capillary and removal CO2 from the alveoli by ventilation.

42. What can be said about V/Q ratios as you move from the apex of the lung to the base in an upright individual?
In the apex, V/Q ratio is high. PAO2 increasing & PACO2 decreasing. In the base, V/Q ratio is low, PAO2 is decreasing and PACO2 increasing.

43. What changes in pH would occur in capillary blood from the apex of the lung to the base of the lung?
It would be at 7.45 at the apex of the lung and 7.35 at the base of the lung.

44. What causes the change in pH that occur in capillary blood from the apex of the lung to the base of the lung?
The PaCO2 decreases from the bottom to the top of the lungs, the progressive reduction of the CO2 level in the end capillary blood causes the pH to become more alkaline. At bottom pH is 7.35 and PACO2 is 46 at top pH7.45 and PaCO2 is 30.

45. What is the respiratory zone of the lungs?
It refers to the respiratory bronchioles, alveoli, alveolar ducts, and alveolar sacs.

46. What is the respiratory exchange ratio?
Quantity of O2 and CO2 exchanged in one minute.

47. What is the relationship between RQ and RR under normal conditions?
They are equal.

48. What type of V/Q ratio is associated with dead space ventilation?
An increase in V/Q ratio leads to wasted ventilation or dead space.

49. What type of V/Q ratio is associated with shunt conditions?
A decrease in V/Q ratio leads to a shunt.

50. What is the number one cause of hypoxemia?
V/Q mismatching.

51. What disorder always affects V/Q ratios?

52. Can it be able to indicate whether it is more likely dead space disorder or shunt disorder in a given V/Q ratio?
If you have 10/5, 4/5, 2/5 they would equal 2,.8,.4 respectively. Top number is the fraction in ventilation which would follow that since dead space is ventilation without perfusion. It would have the higher ventilation number and higher number so 10/5 and the 2. Shunt is less ventilation to perfusion–consolidation, atelectasis, alveoli collapse, higher number in bottom so the 2/5 and decreased number.

53. What are the disorders associated with increased V/Q ratio?
Pulmonary emboli, partial or complete obstruction of the pulmonary artery or arterioles, atherosclerosis, collagen disease, extrinsic pressure on pulmonary vessels,
tumor compressing vessel, destruction of pulmonary vessels, and decrease cardiac output.

54. What disorders associated with decreased V/Q ratios?
Obstructive lung disease, emphysema, bronchitis, asthma, restrictive lung disease,
atelectasis, pneumonia, silicosis, pulmonary fibrosis, and hypoventilation from any cause.

55. What are the signs of diminished pulmonary perfusion?
Little or no blood flow in relation to ventilation, ventilation without perfusion, increase V/Q ratio, increase dead space ventilation, PAO2 increases and PACO2 decreases.

56. What are the signs of diminished ventilation?
Little or no ventilation in relation to blood flow, perfusion without ventilation, decrease V/Q ratio, increase in shunted blood, decrease PAO2 and increase PACO2.

57. What is internal respiration?
It is the gas exchange between systemic capillaries and cells.

58. What is external respiration?
It is the gas exchange between pulmonary capillaries and alveoli.

59. What is the quick description of a shunt?
It is less ventilation to perfusion.

60. What is the quick description of dead space?
It Is ventilation without perfusion.

61. Where does pulmonary circulation begin?
At the main pulmonary artery.

62. What artery follows the path of branching airways to the level of terminal bronchioles?
Main pulmonary artery.

63. When does the pulmonary artery break up to supply the pulmonary capillary bed (alveoli)?
Beyond the terminal bronchioles.

64. What percentage of the alveolar surface area do the pulmonary capillaries cover?

65. What is bronchial circulation?
It is when bronchial arteries branch from the descending aorta and supply oxygenated blood to intrapulmonary structures and return to the lungs.

66. What is anastomosis?
It is when vessels are joined together in a short circuit.

67. What is right to left shunt?
It is the addition of deoxygenated venous blood to the oxygenated blood.

68. Which is higher, pulmonary circulation or systemic circulation?
Systemic circulation is much higher.

69. What is the difference between the walls of the aorta and the walls of the pulmonary artery?
Pulmonary artery walls are much thinner and have less smooth muscle and elastin than the walls of the aorta.

70. What is the significance of pulmonary and systemic circulation running in series?
Both receive entire cardiac output (Q).

71. What does it mean for the resistance of pulmonary vasculature if the cardiac output is high and the pulmonary circulation pressure is low?
High cardiac output + low pulmonary pressure = low pulmonary vasculature resistance.

72. How does the pulmonary circulation have such low resistance?
It is because of the low smooth muscle tone.

73. What is the equation for vascular resistance?
(Input pressure – Output pressure)/ (Blood flow)

74. How does pulmonary vascular resistance decrease when pulmonary arterial or venous pressure increases?
Recruitment of new pulmonary beds to open and distension of already opened pulmonary vessels.

75. How to measure the distribution of pulmonary blood flow?
Using radioactive xenon dissolved in saline and injected into the peripheral vein and xenon is counted by radiation detectors place over the chest.

76. Which part of the lung receives more blood flow?
It is at the base of the lung which receives more blood flow per unit volume than does the apex of the lung.

77. Why does the base of the lung receive more blood flow than the apex?
Gravity produces a gradient of pulmonary blood flow because of hydrostatic pressures within pulmonary vessels and effect of Starling forces.

78. How do Starling resistors affect blood flow to alveoli?
If surrounding alveolar pressure is greater than arterial pressure the capillary will be squeezed shut and blood flow will cease.

79. What is another name for pulmonary capillaries?
Alveolar vessels.

80. What is another name for large pulmonary vessels?
Extra-alveolar vessels.

81. Where are extra-alveolar vessels located?
They run through the lung parenchyma.

82. Does the sympathetic or parasympathetic nervous system supply the pulmonary blood vessels?
Both! However, the autonomic nervous system probably does not have a major function in the normal control of pulmonary blood flow.

83. What is hypoxic pulmonary vasoconstriction?
It is the distribution of pulmonary blood flow is decreased whenever the PO2 of small airways or alveoli is reduced.

84. What is the benefit of hypoxic pulmonary vasoconstriction?
It diverts mixed venous blood away from poorly ventilated areas of the lung that have a low PAO2 by locally increasing vascular resistance and mixed venous blood is sent to better ventilated areas of the lung.

85. How is the hypoxic reaction in the pulmonary circulation different from the hypoxic reaction in the system circulation?
In pulmonary circulation the mixed venous blood is sent away from hypoxic area while systemic circulation is localized vasodilation to the hypoxic area, increasing the blood flow to that area.

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86. Which part of the lungs receives greater ventilation per unit volume?
The base of the lungs receives greater ventilation.

87. Why do the bases of the lungs receive more ventilation?
Due to gravity, there is distortion of the lung tissue at the bottom of the lung while the apex of the lung is expanded which means the base alveoli need more ventilation to fill to max capacity.

88. How does the intrapleural pressure compare from the base to the apex of the lungs?
The base of the lungs has a greater (less negative) intrapleural pressure than the apex.

89. Describe the resting volume of alveolus at the base of the lungs:
It is small.

90. When is gas exchange maximally efficient?
When both ventilation and perfusion are appropriately matched in all regions of the lungs.

91. What will happen if perfusion in alveolus increases, but ventilation is unchanged?
More CO2 in the blood will be delivered to alveolus and more O2 will be moved from alveolus to the blood. PACO2 will increase and PAO2 will decrease.

92. What will happen if ventilation stays the same in an alveolus, but perfusion decreases?
Less CO2 will be brought to alveolus in the blood, and less O2 will move from alveolus into the blood. Also, PACO2 will decrease and PAO2 will increase.

93. How does the ventilation to perfusion ratio (V/Q) change compared from apex to base of lungs?
V/Q is high at apex (over ventilated) and V/Q is low at base (over perfused).

94. What is a normal V/Q ratio?

95. What is the expected PO2 and PCO2 of blood entering the pulmonary capillary In a normal alveolus (V/Q = 1)?
PO2 = 40 mmHg and for PCO2 = 46 mmHg.

96. Normally, what pressures will the blood leaving the pulmonary capillaries have?
PO2 = 100 mmHg and for PCO2 = 40 mmHg.

97. What two things need to happen for gas exchange to occur?
Ventilation and perfusion

98. Would a pulmonary embolism result in a high or low V/Q ratio?
It would result in a high V/Q ratio.

99. Would atelectasis result in a high or low V/Q ratio?
It would result in a low V/Q ratio.

100. What is the most common cause of hypoxemia in patients with a respiratory condition?
A V/Q imbalance.

Final Thoughts

Understanding the relationship between ventilation and perfusion is an important fundamental topic that must be learned by each and every Respiratory Therapist and student.

Again, hopefully, this study guide can help make the learning process easier for you. We also have a guide on the topic of Gas Exchange and Transport that I think you will enjoy. Thank you so much for reading and as always, breathe easy my friend.


The following are the sources that were used while doing research for this article:

  • Egan’s Fundamentals of Respiratory Care. Mosby, 2020. [Link]
  • Jardins, Des Terry. Cardiopulmonary Anatomy & Physiology: Essentials of Respiratory Care. 7th ed., Cengage Learning, 2019. [Link]
  • Rrt, Des Terry Jardins MEd, and Burton George Md Facp Fccp Faarc. Clinical Manifestations and Assessment of Respiratory Disease. 8th ed., Mosby, 2019. [Link]
  • “Physiology, Pulmonary, Ventilation and Perfusion.” National Center for Biotechnology Information, 6 Aug. 2020, www.ncbi.nlm.nih.gov/books/NBK539907.
  • “Blood Flow Redistribution and Ventilation-Perfusion Mismatch during Embolic Pulmonary Arterial Occlusion.” PubMed Central (PMC), 2011, www.ncbi.nlm.nih.gov/pmc/articles/PMC3224428.
  • Petersson, Johan. “Gas Exchange and Ventilation-Perfusion Relationships in the Lung.” PubMed, 25 July 2014, pubmed.ncbi.nlm.nih.gov/25063240.

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