[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Respiratory](https://durgical.com/en/physiology/#s-resp) › Ventilation–perfusion matching and West zones

[Physiology](https://durgical.com/en/physiology/) · [Respiratory](https://durgical.com/en/physiology/#s-resp)

# 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](https://durgical.com/en/anatomy/right-pulmonary-artery/) · [Pulmonary trunk](https://durgical.com/en/anatomy/pulmonary-trunk/) · [Right lower lobe](https://durgical.com/en/anatomy/inferior-lobe-of-right-lung/) · [Right upper lobe](https://durgical.com/en/anatomy/superior-lobe-of-right-lung/)

## Key points

- Overall V/Q ≈ 0.8 (≈ 4 L/min alveolar ventilation / ≈ 5 L/min blood flow).
- Apex: high V/Q (≈ 3), high PAO₂, low PaCO₂. Base: low V/Q (≈ 0.6).
- V/Q = 0: shunt (pneumonia, atelectasis, ARDS). V/Q = ∞: dead space (pulmonary embolism).
- V/Q mismatch is the commonest cause of hypoxemia and does respond to supplemental O₂.
- Hypoxic pulmonary vasoconstriction diverts blood away from poorly ventilated alveoli.

## 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.

## Memory aid

Shunt is 'blood without air'; dead space is 'air without blood'. Oxygen only fixes what still gets some air.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/ventilation-perfusion-matching-and-west-zones/)

## See it in 3D

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

- [Pulmonary arteries](https://durgical.com/en/anatomy/right-pulmonary-artery/)
- [Pulmonary trunk](https://durgical.com/en/anatomy/pulmonary-trunk/)
- [Right lower lobe](https://durgical.com/en/anatomy/inferior-lobe-of-right-lung/)
- [Right upper lobe](https://durgical.com/en/anatomy/superior-lobe-of-right-lung/)

## Related topics

- [The causes of hypoxemia](https://durgical.com/en/physiology/the-causes-of-hypoxemia/)
- [The pulmonary circulation and hypoxic vasoconstriction](https://durgical.com/en/physiology/the-pulmonary-circulation-and-hypoxic-vasoconstriction/)
- [Dead space and alveolar ventilation](https://durgical.com/en/physiology/dead-space-and-alveolar-ventilation/)
- [The alveolar gas equation and the A–a gradient](https://durgical.com/en/physiology/the-alveolar-gas-equation-and-the-a-a-gradient/)
- [Diffusion across the respiratory membrane and DLCO](https://durgical.com/en/physiology/diffusion-across-the-respiratory-membrane-and-dlco/)

## Related conditions

- [Pulmonary embolism](https://durgical.com/en/conditions/pulmonary-embolism/)
- [Community-acquired pneumonia](https://durgical.com/en/conditions/community-acquired-pneumonia/)
- [Chronic obstructive pulmonary disease (COPD)](https://durgical.com/en/conditions/chronic-obstructive-pulmonary-disease/)
- [Acute respiratory distress syndrome (ARDS)](https://durgical.com/en/conditions/acute-respiratory-distress-syndrome/)
- [Pulmonary tuberculosis](https://durgical.com/en/conditions/pulmonary-tuberculosis/)

[← All of physiology](https://durgical.com/en/physiology/)

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](https://openstax.org/books/anatomy-and-physiology-2e/pages/22-4-gas-exchange) (OpenStax (Rice University), 2022); [Acute Pulmonary Embolism](https://www.ncbi.nlm.nih.gov/books/NBK560551/) (StatPearls (NCBI Bookshelf), 2024); [Community-Acquired Pneumonia](https://www.ncbi.nlm.nih.gov/books/NBK430749/) (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.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/ventilation-perfusion-matching-and-west-zones/