[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) › Dead space and alveolar ventilation

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

# Dead space and alveolar ventilation

- Concept
- Exam priority: high

Part of every breath stays in the airway tubes and never reaches the alveoli. What matters for CO₂ is the air that does arrive.

Updated 2026-11-01

Also called: Physiological dead space, Bohr equation

**See it in 3D:** [Trachea](https://durgical.com/en/anatomy/trachea/) · [Main bronchi](https://durgical.com/en/anatomy/right-main-bronchus/) · [Lobar bronchi](https://durgical.com/en/anatomy/lobar-bronchus/) · [Segmental bronchi](https://durgical.com/en/anatomy/segmental-bronchus/)

## Key points

- Anatomical dead space ≈ 150 mL (scales with body size); normal VD/VT ≈ 0.2–0.35.
- Alveolar ventilation = (VT − VD) × RR; PaCO₂ is inversely proportional to it.
- Rapid shallow breathing: same minute ventilation, less alveolar ventilation, more CO₂.
- Pulmonary embolism increases alveolar dead space (ventilation without perfusion).
- Bohr equation: VD/VT = (PaCO₂ − PECO₂) / PaCO₂.

## What it is

Anatomical dead space is the volume of the conducting airways (≈ 150 mL), where no gas exchange happens. Alveolar dead space is alveoli that are ventilated but not perfused; together they make the physiological dead space. Alveolar ventilation, not total ventilation, sets the PaCO₂.

## How it works, step by step

- CO₂ reaches the alveoli from the blood at a rate set by metabolism.
- Only fresh air that reaches the alveoli dilutes and carries away that CO₂.
- If tidal volume approaches dead space, most air just moves in the tubes and CO₂ builds up.
- Doubling alveolar ventilation halves PaCO₂, and vice versa.

### Lung volumes and capacities

**TLC** 6.0 L **VC** 4.8 L **FRC** 2.4 L **RV** 1.2 L **FEV1/FVC** 0.80

Normal adult pattern (approximate values: they vary with height, age and sex).

Tap a volume, a capacity or FEV1.

- IRV · Inspiratory reserve volume
- TV · Tidal volume
- ERV · Expiratory reserve volume
- RV · Residual volume
- Capacity (sum of volumes)
- Current forced expiration
- Normal

Tidal volume: about 150 of its 500 mL stays in the conducting airways.

## Equations

**Minute ventilation**VE = VT × RR

**V T**
tidal volume

**RR**
respiratory rate

**Alveolar ventilation**VA = (VT − VD) × RR

**V D**
dead space

The ventilation that actually exchanges gas.

**Bohr equation**VD/VT = (PaCO2 − PECO2) / PaCO2

**P E CO 2**
PCO₂ of mixed expired gas

Measures physiological dead space.

## Reference values (resting adult)

| Parameter | Value | Note |
| --- | --- | --- |
| Anatomical dead space | ≈ 150 mL | |
| Minute ventilation | ≈ 6 L/min | 500 mL × 12 |
| Alveolar ventilation | ≈ 4.2 L/min | (500 − 150) × 12 |
| VD/VT | ≈ 0.2–0.35 | |
| PaCO₂ at sea level | 35–45 mmHg | usually lower in Bogotá, ≈ 30–35 mmHg |

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

## Clinical notes

- Pulmonary embolism: ventilated alveoli with no blood flow; dead space rises and end-tidal CO₂ falls (capnography).
- Sedation or opioids: slow, shallow breathing; alveolar ventilation falls and PaCO₂ rises.
- On a ventilator, long tubing between the Y-piece and the patient adds dead space.

## Memory aid

Fast, shallow breathing is like stirring the air in a hallway: lots of movement, but none of it gets into the rooms.

## Test yourself

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

## Two patients both ventilate 6 L/min: one at 500 mL × 12, the other at 250 mL × 24. Which retains CO₂, and why?

The second: alveolar ventilation is (250 − 150) × 24 = 2.4 L/min against 4.2 L/min for the first, so PaCO₂ rises.

## What happens to end-tidal CO₂ in a massive pulmonary embolism?

It falls: unperfused alveoli receive no CO₂ and their CO₂-free gas dilutes the exhaled air; dead space increases.

## If alveolar ventilation halves while CO₂ production stays constant, what happens to PaCO₂?

It doubles (e.g. from 40 to 80 mmHg), because PaCO₂ is inversely proportional to alveolar ventilation.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/physiology/dead-space-and-alveolar-ventilation/)

## See it in 3D

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

- [Trachea](https://durgical.com/en/anatomy/trachea/)
- [Main bronchi](https://durgical.com/en/anatomy/right-main-bronchus/)
- [Lobar bronchi](https://durgical.com/en/anatomy/lobar-bronchus/)
- [Segmental bronchi](https://durgical.com/en/anatomy/segmental-bronchus/)

## Related topics

- [Ventilation–perfusion matching and West zones](https://durgical.com/en/physiology/ventilation-perfusion-matching-and-west-zones/)
- [The alveolar gas equation and the A–a gradient](https://durgical.com/en/physiology/the-alveolar-gas-equation-and-the-a-a-gradient/)
- [CO₂ transport and the Haldane effect](https://durgical.com/en/physiology/co-transport-and-the-haldane-effect/)
- [Lung volumes, capacities and spirometry](https://durgical.com/en/physiology/lung-volumes-capacities-and-spirometry/)
- [Respiratory acidosis and alkalosis](https://durgical.com/en/physiology/respiratory-acidosis-and-alkalosis/)

## 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/)
- [Respiratory acidosis](https://durgical.com/en/conditions/respiratory-acidosis/)

## In Pharmacology

- [Opioids and antagonists](https://durgical.com/en/pharmacology/classes/opioids-and-antagonists/)

[← 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: [22.3 The Process of Breathing](https://openstax.org/books/anatomy-and-physiology-2e/pages/22-3-the-process-of-breathing) (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), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 38). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

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/dead-space-and-alveolar-ventilation/