Ventilation–perfusion matching and West zones !

Mechanism
Exam priority: high

To exchange gas, air and blood must reach the same alveolus. If either is missing, that alveolus is wasted.

Updated 2026-11-01

Also called: V/Q ratio, West zones

See it in 3D: Pulmonary arteries · Pulmonary trunk · Right lower lobe · Right upper lobe

What it is

The V/Q ratio compares alveolar ventilation with blood flow in each region; overall it is ≈ 0.8. A V/Q of 0 is shunt (blood without air); an infinite V/Q is dead space (air without blood). Gravity gives the base of the upright lung more ventilation and much more blood than the apex.

How it works, step by step

Upright, gravity raises hydrostatic pressure in the pulmonary arteries toward the base.
The base receives much more blood flow than the apex.
The base also ventilates more, because its alveoli start smaller and more compliant.
Perfusion changes more than ventilation from apex to base, so V/Q falls from top to bottom.
Where air is lacking, low PAO₂ constricts the local arterioles and cuts flow there.
Where blood is lacking, low local PCO₂ mildly constricts bronchioles; both reflexes limit waste.

Approximate values

Parameter	Value	Note
Overall V/Q	≈ 0.8
Apical V/Q	≈ 3
Basal V/Q	≈ 0.6
Physiological shunt	≈ 2–5% of cardiac output	bronchial and Thebesian veins

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

West zones (upright lung)

Zone	Pressures	Flow	When it appears
1 (apex)	PA > Pa > Pv	None	Only with hypotension or high positive-pressure ventilation
2 (middle)	Pa > PA > Pv	Intermittent, driven by Pa − PA	Upper half of the lung at rest
3 (base)	Pa > Pv > PA	Continuous, the highest	Bases, and most of the lung when supine

Clinical notes

Pneumonia: alveoli full of exudate with preserved flow (shunt); hypoxemia responds poorly to O₂.
Pulmonary embolism: dead space in the occluded area and over-perfusion with low V/Q elsewhere; hypoxemia comes mostly from those zones.
Reactivated tuberculosis settles in the apices, where PAO₂ is highest.

Test yourself

Think of the answer, then tap the question to see it.

Why does supplemental O₂ correct true-shunt hypoxemia poorly?

Shunted blood never passes ventilated alveoli, so it never meets the extra O₂; blood from healthy alveoli is already nearly saturated and cannot make up the difference.

Where in the upright lung is PAO₂ highest, and why?

At the apex: its V/Q is high (little blood for the air it gets), so alveolar gas is closer to inspired gas.

In a pulmonary embolism, what value does the V/Q of the occluded segment tend toward?

Infinity (ventilation without perfusion): alveolar dead space.

Which West zone can appear in a hypovolemic patient on positive-pressure ventilation?

Zone 1: alveolar pressure exceeds pulmonary arterial pressure and the capillary is squeezed shut, with no flow.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 22.4 Gas Exchange (OpenStax (Rice University), 2022); Acute Pulmonary Embolism (StatPearls (NCBI Bookshelf), 2024); Community-Acquired Pneumonia (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 40). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.