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

# The pulmonary circulation and hypoxic vasoconstriction

- Regulation
- Exam priority: medium

The right heart pumps all the blood through the lungs at low pressure. Where oxygen is missing, the vessels narrow to send blood to well-aired areas.

Updated 2026-11-01

Also called: Hypoxic pulmonary vasoconstriction, Pulmonary vascular resistance

**See it in 3D:** [Pulmonary trunk](https://durgical.com/en/anatomy/pulmonary-trunk/) · [Pulmonary arteries](https://durgical.com/en/anatomy/right-pulmonary-artery/) · [Pulmonary veins](https://durgical.com/en/anatomy/pulmonary-veins/) · [Segmental lung vessels](https://durgical.com/en/anatomy/pulmonary-trunk-chest/) · [Right ventricle](https://durgical.com/en/anatomy/right-ventricle/)

## Key points

- Pulmonary artery pressure ≈25/8 mmHg, mean ≈15 mmHg; resistance is ≈1/10 of systemic.
- Alveolar hypoxia = pulmonary vasoconstriction (the opposite of systemic vessels, where hypoxia dilates).
- With exercise pressure rises little because closed capillaries are recruited and open ones distend: resistance falls.
- Resistance is lowest near functional residual capacity; it rises at very low and very high lung volumes.
- Chronic hypoxia (COPD, altitude) causes pulmonary hypertension and right-ventricular enlargement (cor pulmonale).
- Pulmonary hypertension: resting mean pressure >20 mmHg (current threshold; formerly ≥25).

## What it is

The pulmonary circulation takes the entire right-ventricular output, but at pressures about five times lower than the systemic circulation, thanks to short, thin-walled, highly distensible vessels. Unlike systemic vessels, pulmonary arterioles constrict when alveolar oxygen is low: blood is diverted away from poorly ventilated areas and ventilation–perfusion matching improves. When hypoxia is widespread, the same reflex raises pulmonary pressure.

## How it works, step by step

- Alveolar PO₂ falls in one region (atelectasis, pneumonia, bronchospasm).
- Smooth muscle of nearby arterioles senses the hypoxia: potassium channels close, the cell depolarizes and calcium enters.
- The arterioles constrict and blood is diverted toward better-ventilated alveoli.
- This limits shunt and protects PaO₂ when the problem is local.
- If hypoxia is global (altitude, hypoventilation), all arterioles constrict: pulmonary pressure and right-ventricular load rise.
- With sustained hypoxia the arteriolar wall remodels and pulmonary hypertension becomes fixed.

## Reference values (resting adult)

| Parameter | Value | Note |
| --- | --- | --- |
| Pulmonary artery pressure | ≈25/8 mmHg | mean ≈15 mmHg |
| Pulmonary capillary pressure | ≈7–10 mmHg | |
| Wedge pressure (left atrium) | ≈6–12 mmHg | |
| Pulmonary blood volume | ≈450 mL | ≈9 % of total |

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

## Regulation

- *−* **Alveolar hypoxia**: Constricts pulmonary arterioles and raises resistance.
- *−* **Acidosis and hypercapnia**: Strengthen hypoxic vasoconstriction.
- *+* **Endothelial nitric oxide and prostacyclin**: Dilate; their loss contributes to pulmonary hypertension.
- *+* **Capillary recruitment and distension**: Lower resistance as cardiac output rises.
- *−* **Embolism or thrombosis**: Obstructs the bed and raises pulmonary artery pressure.

## Clinical notes

- At Bogotá's altitude (≈2,600 m) pulmonary pressure is somewhat higher than at sea level; at greater heights, uneven vasoconstriction can cause high-altitude pulmonary edema.
- Massive pulmonary embolism: the right ventricle cannot handle the sudden afterload rise and dilates.
- Systemic vasodilators can worsen oxygenation by abolishing hypoxic vasoconstriction in poorly ventilated areas.

## Memory aid

In the lung, "no air, no blood": the vessel closes where the alveolus gets no oxygen.

## Test yourself

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

## Why does hypoxic vasoconstriction improve PaO₂ in lobar pneumonia?

It diverts blood away from the consolidated lobe, which does not ventilate, toward healthy lung, reducing shunt and venous admixture.

## Why does pulmonary pressure barely rise when cardiac output triples during exercise?

Previously closed capillaries open and open ones distend; resistance falls almost in proportion to the rise in flow.

## How does COPD with chronic hypoxemia lead to right-ventricular failure?

Diffuse alveolar hypoxia constricts and then remodels pulmonary arterioles; pulmonary pressure rises and the right ventricle hypertrophies and eventually fails.

## Why does pulmonary vascular resistance rise at both very high and very low lung volumes?

At high volume, stretched alveoli compress alveolar capillaries; at low volume, extra-alveolar vessels lose radial traction and narrow.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/the-pulmonary-circulation-and-hypoxic-vasoconstriction/)

## See it in 3D

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

- [Pulmonary trunk](https://durgical.com/en/anatomy/pulmonary-trunk/)
- [Pulmonary arteries](https://durgical.com/en/anatomy/right-pulmonary-artery/)
- [Pulmonary veins](https://durgical.com/en/anatomy/pulmonary-veins/)
- [Segmental lung vessels](https://durgical.com/en/anatomy/pulmonary-trunk-chest/)
- [Right ventricle](https://durgical.com/en/anatomy/right-ventricle/)

## Related topics

- [Ventilation–perfusion matching and West zones](https://durgical.com/en/physiology/ventilation-perfusion-matching-and-west-zones/)
- [High altitude and acclimatization](https://durgical.com/en/physiology/high-altitude-and-acclimatization/)
- [Physiology of pulmonary edema](https://durgical.com/en/physiology/physiology-of-pulmonary-edema/)
- [The bronchial circulation](https://durgical.com/en/physiology/the-bronchial-circulation/)
- [Fetal circulation and the transition at birth](https://durgical.com/en/physiology/fetal-circulation-and-the-transition-at-birth/)
- [The causes of hypoxemia](https://durgical.com/en/physiology/the-causes-of-hypoxemia/)

## Related conditions

- [Pulmonary embolism](https://durgical.com/en/conditions/pulmonary-embolism/)
- [Chronic obstructive pulmonary disease (COPD)](https://durgical.com/en/conditions/chronic-obstructive-pulmonary-disease/)
- [Community-acquired pneumonia](https://durgical.com/en/conditions/community-acquired-pneumonia/)

[← 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: [20.5 Circulatory Pathways](https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways) (OpenStax (Rice University), 2022); [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), 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 39). 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/the-pulmonary-circulation-and-hypoxic-vasoconstriction/