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# Alveolar air: composition, slow renewal and how ventilation sets PAO₂ and PACO₂

- Concept
- Exam priority: medium

The air deep in the lungs is not fresh air. Each breath replaces only a small part of it, which keeps blood gases steady.

Updated Nov 13, 2026

Also called: Alveolar gas, Inspired, alveolar and expired air

## Key points

- Each breath renews only about one seventh of alveolar gas; the FRC acts as a buffer that smooths breath-to-breath swings.
- PACO₂ ∝ CO₂ production / alveolar ventilation: double ventilation and PACO₂ halves.
- PAO₂ rises with ventilation but can never exceed the humidified inspired PO₂ (≈ 150 mmHg breathing air at sea level).
- Expired air is a mixture of dead-space gas (like inspired air) and alveolar gas, so its values sit between the two.
- End-tidal CO₂ approximates PACO₂ (and PaCO₂) in healthy lungs; the gap widens when dead space rises.

## What it is

Alveolar air differs from room air: it is saturated with water vapour, poorer in O₂ and richer in CO₂, because blood constantly takes O₂ out and adds CO₂. Each normal breath brings only about 350 mL of fresh air into the roughly 2.3 L of gas left in the lungs at the end of expiration (the FRC), so the mix changes slowly. Alveolar PO₂ and PCO₂ are set by the balance between alveolar ventilation and the rates of O₂ uptake and CO₂ output.

## How it works, step by step

- Of a 500 mL tidal breath, about 150 mL stays in the conducting airways; about 350 mL reaches the alveoli.
- This fresh gas mixes with about 2300 mL already in the lungs, so each breath renews only about 15% of alveolar gas.
- Blood takes up O₂ (≈ 250 mL/min) and adds CO₂ (≈ 200 mL/min) continuously, between and during breaths.
- If ventilation rises for the same uptake, more O₂ is brought in than removed: PAO₂ rises and PACO₂ falls.
- If O₂ uptake rises (exercise) without more ventilation, PAO₂ falls; in practice ventilation rises in step.
- The large, slowly renewed alveolar volume prevents sudden swings in arterial PO₂, PCO₂ and pH from breath to breath.

## Equations

**Alveolar CO₂**PACO2 = (VCO2 × 0.863) / VA

**VCO 2**
CO₂ output, mL/min (STPD)

**V A**
alveolar ventilation, L/min (BTPS)

**0.863**
unit conversion constant

At rest: 200 × 0.863 / 4.3 ≈ 40 mmHg.

**Alveolar gas equation (simplified)**PAO2 = PiO2 − PACO2 / R

**PiO 2**
humidified inspired PO₂

**R**
respiratory exchange ratio ≈ 0.8

Sea level, air: 150 − 40/0.8 ≈ 100 mmHg.

## Reference values (healthy adult at rest)

| Parameter | Value |
| --- | --- |
| Functional residual capacity | ≈ 2.3 L |
| Fresh alveolar gas per breath | ≈ 350 mL |
| Alveolar ventilation | ≈ 4–5 L/min |
| PAO₂ / PACO₂ (sea level, air) | ≈ 100–104 / 40 mmHg |
| End-tidal CO₂ | ≈ 35–45 mmHg |

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

## From room air to expired air (sea level, mmHg, approximate)

| Air | PO₂ | PCO₂ | What changed it |
| --- | --- | --- | --- |
| Room air (dry) | ≈ 160 | ≈ 0 | Nothing: 21% of 760 mmHg |
| Humidified in the trachea | ≈ 150 | ≈ 0 | Water vapour (47 mmHg) dilutes the other gases |
| Alveolar | ≈ 100 | ≈ 40 | Blood removes O₂ and adds CO₂; slow renewal by the FRC |
| Expired (mixed) | ≈ 115–120 | ≈ 27–32 | Dead-space gas mixes with alveolar gas |

## Clinical notes

- Hyperventilation from anxiety lowers PACO₂ and raises PAO₂ slightly; the low PaCO₂ causes tingling and light-headedness.
- Because the FRC holds an O₂ reserve, breathing 100% O₂ before intubation (preoxygenation) replaces its nitrogen with O₂ and lengthens safe apnoea time.
- A widening gap between PaCO₂ and end-tidal CO₂ suggests more dead space, as in pulmonary embolism or low cardiac output.

## Memory aid

The alveolus is a big pot of soup: each breath adds a small ladle, so the taste changes slowly.

## Test yourself

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

## Why is expired PO₂ higher than alveolar PO₂?

Expired air includes the dead-space gas from the end of the previous inspiration, which never exchanged gas and still resembles inspired air.

## Alveolar ventilation doubles with constant CO₂ production. What happens to PACO₂?

It halves, for example from 40 to about 20 mmHg, because PACO₂ is proportional to CO₂ output divided by alveolar ventilation.

## Why can breathing room air never raise PAO₂ above about 150 mmHg at sea level?

Alveolar gas cannot contain more O₂ than the humidified inspired gas; even infinite ventilation only brings PAO₂ toward PiO₂ (≈ 150 mmHg).

## What is the advantage of the slow renewal of alveolar air?

The large FRC buffers each breath, so arterial PO₂, PCO₂ and pH stay steady instead of swinging with every inspiration and expiration.

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## Connections

## Related topics 4

- [The alveolar gas equation and the A–a gradient](https://durgical.com/en/physiology/the-alveolar-gas-equation-and-the-a-a-gradient/)
- [Dead space and alveolar ventilation](https://durgical.com/en/physiology/dead-space-and-alveolar-ventilation/)
- [Lung volumes, capacities and spirometry](https://durgical.com/en/physiology/lung-volumes-capacities-and-spirometry/)
- [Diffusion across the respiratory membrane and DLCO](https://durgical.com/en/physiology/diffusion-across-the-respiratory-membrane-and-dlco/)

[← 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: [Arterial Blood Gas (ABG) Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/arterial-blood-gas-abg-test/) (MedlinePlus, US National Library of Medicine, 2025); [Anatomy, Thorax, Lungs](https://www.ncbi.nlm.nih.gov/books/NBK470197/) (StatPearls (NCBI Bookshelf), 2025).

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Representative model, for educational purposes only.

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