In one breath
The ventilation-perfusion ratio (V/Q) is alveolar ventilation divided by pulmonary blood flow: 4 L/min ÷ 5 L/min, so the normal whole-lung value is 0.8. Gravity makes it regional, because from apex to base blood flow varies about five-fold while ventilation varies only about two-fold — so the ratio is about 3 at the apex and 0.6 at the base. A high ratio means a high alveolar PO₂ and a low PCO₂; a low ratio means the reverse, and blood leaving that region is not fully oxygenated. Blood that escapes oxygenation, whether through a shunt or through a low V/Q region, mixes with oxygenated blood as venous admixture.
Builds on: Alveolar ventilation · Pulmonary circulation · Leads to: Oxygen transport · Hypoxia
The ratio and its normal value
There are always regions of the lung where airflow and blood flow are not properly matched, so a small fraction of the cardiac output normally leaves the lung not fully oxygenated. The V/Q ratio is how that matching is measured.
| Quantity | Normal value at rest |
|---|---|
| Alveolar ventilation (VA) | 4 L/min |
| Pulmonary blood flow (Q) | 5 L/min |
| V/Q ratio | 0.8 |
Perfusion here means the same thing as cardiac output — strictly, right ventricular output.
Regional differences from apex to base
In the upright posture the base is both better ventilated and better perfused than the apex. What makes the ratio differ is that the two do not vary to the same degree.
- Blood flow shows about a five-fold difference between apex and base.
- Ventilation shows only about a two-fold difference.
- So at the base perfusion outruns ventilation, and at the apex ventilation outruns perfusion.
| Region | V/Q ratio | Consequence |
|---|---|---|
| Apex | about 3 | High PAO₂, low PACO₂ |
| Whole lung (average) | 0.8 | — |
| Base | 0.6 | Blood leaves not fully oxygenated |
- Worth knowing, though not in your pages: the chapter’s own figure caption puts the apical ratio at 3.6 while its text says about 3. Either number will be accepted; write the one your college’s paper uses, and always give 0.6 at the base and 0.8 as the average, which the book is consistent about.
Draw it: ventilation and perfusion from apex to base
Draw an upright lung outline with the vertical axis marked apex at the top and base at the bottom, and flow in L/min along the horizontal axis. Draw two lines rising from apex to base: a steep one for perfusion (Q) and a shallower one for ventilation (V). Make them cross in the middle third of the lung. Label the region above the crossing “V greater than Q, ratio about 3” and the region below it “Q greater than V, ratio 0.6”. Write “average V/Q = 0.8” beside the crossing point, and note against the axis “perfusion varies 5-fold, ventilation 2-fold”.
The apex has the high ratio, not the low one
The base receives more of both air and blood — so it is tempting to give it the higher ratio. It has the lower one (0.6), because its blood flow is boosted far more than its ventilation. And 0.8 is the average for the whole lung, not a value found in any particular region.
Importance of the V/Q ratio
Physiological importance
The ratio decides how well gas exchange actually works in each region.
- At the apex, ventilation is high relative to blood flow, so alveolar oxygen tension (PAO₂) is high and carbon dioxide tension (PACO₂) is low.
- At the base, perfusion is high relative to ventilation, so some blood passes through without being fully oxygenated.
Clinical importance
Regional differences in the ratio make some diseases settle in particular parts of the lung.
Applied: why tuberculosis prefers the apex
The apex has a high V/Q ratio and therefore a high alveolar oxygen tension, and Mycobacterium tuberculosis grows better where oxygen is plentiful. That is why pulmonary tuberculosis is commonest in the apical regions — a favourite one-mark question.
Effects of a change in the ratio
| Change | What happens | Alveolar PO₂ | Alveolar PCO₂ |
|---|---|---|---|
| Ratio falls | Ventilation reduced relative to perfusion | Falls (less O₂ delivered) | Rises (less CO₂ removed) |
| Ratio rises | Perfusion reduced relative to ventilation | Rises (less O₂ taken away) | Falls (less CO₂ delivered) |
Read both rows from the alveolus outwards: a low ratio starves the alveolus of fresh air, so its gas drifts towards venous values; a high ratio starves it of blood, so its gas drifts towards inspired air.
Venous admixture and physiological shunt
- Just as physiological dead space is wasted ventilation, there is wasted perfusion: venous blood that passes the lung without being fully oxygenated.
- The mixing of that unoxygenated blood with oxygenated blood is venous admixture.
- Venous admixture arises in two ways: through a shunt, or through a low V/Q ratio.
A shunt is a route that carries blood past the lungs, and it comes in two kinds.
Anatomical shunt
Blood bypasses the lung through an anatomical defect.
- A right-to-left shunt through an atrial or ventricular septal defect.
- A branch of the pulmonary artery joining a pulmonary vein directly.
Applied: how big a shunt gets
In health the venous admixture reaches at most 2% of cardiac output — that is the physiological shunt. In some bronchial diseases it rises to about 20%, and in congenital right-to-left shunts to about 50%. At those levels gas exchange is grossly impaired and arterial oxygen tension falls, producing hypoxemia.
Physiological shunt
Bronchial venous blood is deoxygenated, and it empties straight into the pulmonary veins (see Pulmonary circulation), mixing with blood that has just been oxygenated. This is the physiological shunt, and like physiological dead space it is present in every normal lung.
Low ventilation-perfusion ratio
A low ratio means a fraction of pulmonary capillary blood is not oxygenated because alveolar ventilation is too low for it.
- Normally this happens at the base of the lung.
- It also happens when an airway is partly obstructed, so the region behind it is hypoventilated.
- Blood passing through that hypoventilated region leaves unoxygenated, and the result is again venous admixture.
Exam-answer skeleton: "Ventilation-perfusion ratio: normal value, regional variation and its importance" (short note)
- Definition: alveolar ventilation ÷ pulmonary blood flow; 4 ÷ 5 = 0.8.
- Why it varies regionally: gravity; perfusion varies five-fold, ventilation only two-fold from apex to base.
- Values: about 3 at the apex, 0.6 at the base, 0.8 overall — draw the two crossing lines.
- Physiological importance: high PAO₂ and low PACO₂ at the apex; incompletely oxygenated blood leaving the base.
- Clinical importance: tuberculosis favours the apex because oxygen tension is high there.
- Effects of change: a fall lowers PO₂ and raises PCO₂; a rise does the opposite.
- Venous admixture: wasted perfusion, from shunt or from low V/Q.
- Shunts: anatomical (septal defect, right-to-left) and physiological (bronchial venous drainage), about 2% of cardiac output normally.
Asked in exams
- Ventilation–perfusion ratio, normal and from apex to base: Jan/Feb 2022, MCQ · Dec 2023, MCQ · Oct 2024, MCQ