[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Anaesthesia and ICU](https://durgical.com/en/anaesthesia/) › [Anaesthesia basics](https://durgical.com/en/anaesthesia/areas/basics/) › The anaesthesia machine and breathing circuits, in principle

# The anaesthesia machine and breathing circuits, in principle

- Concept

The anaesthesia machine mixes oxygen, air and anaesthetic vapour in safe proportions and delivers them to the patient through a breathing circuit.

Updated Nov 13, 2026

## Key points

- Safety features: pin-index and gas-specific connectors, oxygen failure alarm, fail-safe valve, hypoxic guard, inspired oxygen analyser.
- The oxygen flush delivers pure oxygen straight to the circuit, bypassing the vaporisers; it can cause barotrauma and dilute the anaesthetic.
- The circle system recycles exhaled gas through a carbon dioxide absorbent with one-way valves, allowing low fresh gas flows.
- An exhausted absorbent changes colour and causes rebreathing: inspired carbon dioxide appears on the capnography trace.
- Always have a self-inflating bag and an independent oxygen source ready in case the machine fails.

On this page

- The basics Key points
- What it is
- Key points
- Components
- How it works
- Step by step
- Next step
- Elsewhere in DurgiPedia

- Safety Machine safety features and what they prevent

- Study Test yourself

- Connections Connections

## What it is

The anaesthesia machine takes gases from the hospital pipelines or from cylinders, reduces them to a safe working pressure, mixes them in chosen proportions, adds an exact concentration of volatile anaesthetic and delivers the fresh gas to a breathing circuit connected to the patient. It can be described in three parts. The high-pressure system holds the cylinders, with pin-index connectors so that a cylinder cannot be fitted to the wrong yoke. The intermediate-pressure system takes pipeline gas through gas-specific, non-interchangeable connectors, and includes the oxygen failure alarm, the fail-safe valve that cuts other gases when oxygen pressure falls, and the oxygen flush. The low-pressure system holds the flowmeters, the hypoxic guard that links oxygen and nitrous oxide flows so that a hypoxic mixture cannot be set, and the agent-specific, temperature-compensated vaporisers. The fresh gas reaches the patient through a breathing circuit: most adults are anaesthetised with a circle system, in which exhaled gas passes through a carbon dioxide absorbent and back to the patient through one-way valves, so low fresh gas flows can be used and agent and heat are saved; open circuits without absorbent (Mapleson types and the T-piece used in children) need higher flows to wash out carbon dioxide. The circuit includes a reservoir bag, an adjustable pressure-limiting valve, a ventilator and scavenging of waste gas. A full machine check before each list, and a shorter one before each case, is mandatory.

## Key points

- Gas pipelines and cylinders are colour-coded, but the codes differ between countries: always read the label.
- Vaporisers are agent-specific and filled with keyed fillers to prevent the wrong agent being added.
- Desflurane needs a heated, pressurised vaporiser because it boils near room temperature.
- Scavenging removes exhaled anaesthetic gases so the theatre staff are not exposed to them.
- Dry absorbent can react with volatile agents and produce carbon monoxide or other harmful compounds; change it as the protocol says.

## Components

- Gas supply: pipelines with wall outlets and backup cylinders on the machine.
- Pressure regulators, oxygen failure alarm and fail-safe valve.
- Flowmeters with the hypoxic guard, and the oxygen flush.
- Vaporisers and the common gas outlet.
- Breathing circuit: inspiratory and expiratory limbs, one-way valves, absorbent, reservoir bag, adjustable pressure-limiting valve, Y-piece, filter.
- Ventilator, airway pressure and volume monitors, and scavenging.

## How it works

- Gas flows from high pressure to low pressure: regulators protect the machine and the patient from pipeline and cylinder pressures.
- Part of the fresh gas passes through the vaporising chamber and picks up saturated vapour; the dial sets how much, so the output concentration is accurate.
- In the circle, one-way valves keep gas moving in one direction, so exhaled carbon dioxide passes the absorbent before it can be breathed again.
- The adjustable pressure-limiting valve lets excess gas out to scavenging during spontaneous or manual ventilation; the ventilator takes over this during mechanical ventilation.

## Step by step

- Check that a self-inflating bag and backup oxygen are present — they are the rescue if the machine fails.
- Confirm pipeline connections by tugging and check cylinder contents — a wrong or empty supply is a classic cause of hypoxia.
- Check flowmeters, the hypoxic guard and the oxygen analyser calibration — they stop a hypoxic mixture reaching the patient.
- Check vaporisers are filled, seated and locked, with no leak — a loose vaporiser leaks agent and gas.
- Leak-test the breathing circuit and test the one-way valves and absorbent — leaks and stuck valves cause awareness or rebreathing.
- Test the ventilator, alarms, suction and scavenging, and record the check — each is needed during the case.

## Machine safety features and what they prevent

| Feature | What it prevents |
| --- | --- |
| Pin-index and gas-specific pipeline connectors | Connecting the wrong gas |
| Oxygen failure alarm and fail-safe valve | Delivering other gases unnoticed when oxygen pressure falls |
| Hypoxic guard (linked oxygen and nitrous oxide flows) | Setting a hypoxic fresh gas mixture |
| Inspired oxygen analyser with low alarm | A hypoxic mixture from any cause reaching the patient |
| Agent-specific vaporisers with keyed fillers and interlock | Filling with the wrong agent or using two vaporisers at once |
| Adjustable pressure-limiting valve and pressure alarms | Excessive airway pressure and barotrauma |

## Next step

- Never start an anaesthetic without a completed machine check and a self-inflating bag within reach.
- If you suspect machine failure, ventilate the patient with the self-inflating bag and independent oxygen, keep the patient anaesthetised intravenously and call for help.
- If inspired carbon dioxide rises, check the absorbent and the one-way valves, increase fresh gas flow and change the absorbent.
- Report any fault to the biomedical engineering team and label the machine out of service until repaired.

## Test yourself

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

## What does the hypoxic guard on an anaesthesia machine do?

It links oxygen and nitrous oxide flows so that a fresh gas mixture with too little oxygen cannot be set.

## How does a circle breathing system remove carbon dioxide?

One-way valves direct exhaled gas through a carbon dioxide absorbent before it returns to the patient, so gas can be rebreathed and low fresh gas flows used.

## What should you do if you suspect the anaesthesia machine has failed during a case?

Ventilate with a self-inflating bag on an independent oxygen source, keep the patient anaesthetised intravenously and call for help.

## Why is the oxygen flush used with caution during a case?

It delivers pure oxygen at a high flow straight into the circuit, bypassing the vaporisers: it can cause barotrauma and lighten anaesthesia, risking awareness.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/anaesthesia/the-anaesthesia-machine-and-breathing-circuits-in-principle/)

## Elsewhere in DurgiPedia

- [Bag-mask ventilation](https://durgical.com/en/clinical-skills/bag-mask-ventilation/)
- [Dead space and alveolar ventilation](https://durgical.com/en/physiology/dead-space-and-alveolar-ventilation/)

## Connections

## Related 5

- [Standard intraoperative monitoring](https://durgical.com/en/anaesthesia/standard-intraoperative-monitoring/)
- [Capnography: the waveform and what it tells you](https://durgical.com/en/anaesthesia/capnography-the-waveform-and-what-it-tells-you/)
- [Awareness and depth of anaesthesia](https://durgical.com/en/anaesthesia/awareness-and-depth-of-anaesthesia/)
- [General anaesthesia: induction, maintenance and emergence](https://durgical.com/en/anaesthesia/general-anaesthesia-induction-maintenance-and-emergence/)
- [Intraoperative hypoxaemia: a stepwise approach](https://durgical.com/en/anaesthesia/intraoperative-hypoxaemia-a-stepwise-approach/)

## Related conditions 1

- [Carbon monoxide poisoning](https://durgical.com/en/conditions/carbon-monoxide-poisoning/)

## In Pharmacology 6

- [Inhaled anaesthetics](https://durgical.com/en/pharmacology/classes/inhaled-anaesthetics/)
- [Sevoflurane](https://durgical.com/en/pharmacology/sevoflurane/)
- [Desflurane](https://durgical.com/en/pharmacology/desflurane/)
- [Isoflurane](https://durgical.com/en/pharmacology/isoflurane/)
- [Nitrous oxide](https://durgical.com/en/pharmacology/nitrous-oxide/)
- [Oxygen](https://durgical.com/en/pharmacology/oxygen/)

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

Educational content; it does not replace supervised training, your institution's protocols or the anaesthetist's or intensivist's decision. It contains no doses and no ventilator or infusion settings.

Sources: [Anesthesia Machine](https://www.ncbi.nlm.nih.gov/books/NBK572060/) (StatPearls (NCBI Bookshelf), 2022).

## **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.

---
Source: https://durgical.com/en/anaesthesia/the-anaesthesia-machine-and-breathing-circuits-in-principle/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anaesthesia/the-anaesthesia-machine-and-breathing-circuits-in-principle/