[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/) › What anaesthesia does to the heart, lungs, brain and kidneys

# What anaesthesia does to the heart, lungs, brain and kidneys

- Concept

Anaesthetic drugs switch off awareness but also relax blood vessels, slow breathing and change blood flow to the brain and kidneys.

Updated Nov 13, 2026

## Key points

- Induction hypotension comes from vasodilation, myocardial depression and loss of sympathetic tone; it is worst in hypovolaemic and older patients.
- Anaesthesia lowers functional residual capacity: atelectasis forms in dependent lung within minutes and widens the alveolar–arterial gradient.
- Volatile agents reduce cerebral metabolism but dilate cerebral vessels; propofol reduces both metabolism and blood flow, which suits raised intracranial pressure.
- Reduced urine output during surgery is usually the stress response and lower renal perfusion, not established kidney injury.
- Volatile agents are bronchodilators and blunt hypoxic pulmonary vasoconstriction; even low concentrations depress the ventilatory response to hypoxia.

On this page

- The basics Key points
- What it is
- Key points
- Components
- How it works
- Main effects of general anaesthesia by organ (in principle; they vary by drug and depth)
- In practice
- Next step
- Elsewhere in DurgiPedia

- Study Test yourself

- Connections Connections

## What it is

General anaesthesia is a reversible, drug-induced loss of consciousness with amnesia, analgesia and immobility, but the drugs that produce it act on almost every organ. Volatile agents and propofol relax vascular smooth muscle and depress the myocardium, and all hypnotics reduce sympathetic tone and blunt the baroreceptor reflex, so blood pressure falls, most at induction and most in patients who are hypovolaemic, old or already on vasodilators. Positive-pressure ventilation adds to this by reducing venous return. In the lungs, anaesthesia depresses the ventilatory response to carbon dioxide and to hypoxia, relaxes the upper airway and abolishes protective reflexes, and lowers functional residual capacity, so dependent lung regions collapse (atelectasis) within minutes and ventilation–perfusion matching worsens. In the brain, intravenous hypnotics such as propofol lower metabolic rate and cerebral blood flow together, whereas volatile agents lower metabolism but dilate cerebral vessels, which can raise intracranial pressure at higher concentrations; ketamine tends to raise sympathetic tone. The kidneys receive less blood because of lower arterial pressure and the surgical stress response (antidiuretic hormone, renin–angiotensin–aldosterone, catecholamines), so urine output falls and salt and water are retained. Anaesthesia also impairs temperature regulation, causing early heat redistribution from core to periphery. Knowing these effects lets the anaesthetist anticipate hypotension, hypoxaemia and low urine output instead of being surprised by them.

## Key points

- All general anaesthetics depress the brain and, to differing degrees, the heart, vessels and breathing.
- The effects are dose-related and add up when drugs are combined (hypnotic plus opioid plus volatile agent).
- Neuraxial blocks cause sympathetic block: vasodilation below the level of the block and, if high, bradycardia.
- The surgical stress response raises cortisol, catecholamines and glucose and retains salt and water.
- Hypothermia begins in the first hour from redistribution of heat to the periphery.

## Components

- Heart and vessels: lower systemic vascular resistance, reduced contractility, blunted baroreflex; ketamine and etomidate keep blood pressure better.
- Lungs and airway: reduced respiratory drive, loss of upper-airway tone and reflexes, fall in functional residual capacity, atelectasis, impaired mucociliary clearance.
- Brain: loss of consciousness and memory; changes in cerebral blood flow and intracranial pressure that depend on the agent.
- Kidneys: lower renal blood flow and glomerular filtration rate, hormonal water and salt retention.
- Liver and temperature: lower hepatic blood flow; loss of shivering and vasoconstriction thresholds, so the patient cools.

## How it works

- Hypnotics enhance inhibitory and reduce excitatory transmission in the brain, which removes consciousness and also central sympathetic outflow.
- Volatile agents relax vascular and airway smooth muscle; some, given rapidly, can briefly stimulate the sympathetic system and raise heart rate.
- Nitrous oxide mildly stimulates the sympathetic system and can raise pulmonary vascular resistance.
- Supine position, loss of muscle tone and a cephalad diaphragm shrink lung volume; high inspired oxygen speeds absorption atelectasis.
- Positive-pressure ventilation raises intrathoracic pressure and lowers venous return, which matters most in the hypovolaemic patient.

## Main effects of general anaesthesia by organ (in principle; they vary by drug and depth)

| Organ | Usual effect | What to watch |
| --- | --- | --- |
| Heart and vessels | Vasodilation, myocardial depression, blunted baroreflex | Blood pressure, heart rate, signs of poor perfusion |
| Lungs | Less drive, lower lung volume, atelectasis, loss of airway reflexes | Oxygen saturation, end-tidal carbon dioxide, airway pressures |
| Brain | Unconsciousness; agent-dependent change in blood flow and pressure | Depth of anaesthesia, perfusion pressure in brain injury |
| Kidneys | Lower blood flow and filtration, salt and water retention | Urine output, blood pressure, volume status |
| Temperature | Heat redistribution and loss of thermoregulation | Core temperature, active warming |

## In practice

- Anticipate hypotension at induction: titrate the hypnotic, choose a more cardiostable agent when the reserve is poor, and have a vasopressor ready.
- Limit atelectasis with good preoxygenation, lung recruitment and positive end-expiratory pressure set by the anaesthetist, not by a fixed rule.
- In raised intracranial pressure, intravenous anaesthesia and attention to carbon dioxide and blood pressure protect cerebral perfusion.
- Low urine output under anaesthesia: check the catheter, blood pressure and volume status before assuming kidney injury.
- Older adults and patients with heart failure, sepsis or bleeding have less reserve and show these effects sooner and more strongly.

## Next step

- Before induction, judge each patient's reserve (heart, lungs, volume status, age) and choose drugs and monitoring to match.
- Treat a sustained fall in blood pressure early: reduce anaesthetic depth if possible, give fluid if hypovolaemic and a vasopressor as needed.
- Call a senior anaesthetist for hypotension or hypoxaemia that does not respond to first measures.
- Monitor core temperature in longer cases and warm actively from the start.
- After surgery, follow breathing, oxygen saturation, blood pressure and urine output in recovery until they return towards baseline.

## Test yourself

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

## Why does blood pressure often fall at induction of general anaesthesia?

Hypnotics and volatile agents cause vasodilation and myocardial depression and reduce sympathetic tone and the baroreflex; the fall is larger in hypovolaemic and older patients.

## Why does atelectasis develop during general anaesthesia?

Loss of muscle tone, the supine position and a higher diaphragm lower functional residual capacity, and high inspired oxygen is absorbed from poorly ventilated alveoli, so dependent lung collapses.

## How do volatile agents and propofol differ in their effect on cerebral blood flow?

Both lower cerebral metabolism, but volatile agents dilate cerebral vessels and can raise intracranial pressure at higher concentrations, whereas propofol lowers blood flow along with metabolism.

## What usually explains low urine output during surgery?

Lower renal perfusion from reduced arterial pressure plus the stress response (antidiuretic hormone, renin–angiotensin–aldosterone, catecholamines), which retains salt and water.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/anaesthesia/what-anaesthesia-does-to-the-heart-lungs-brain-and-kidneys/)

## Elsewhere in DurgiPedia

- [Cardiac output and what sets it](https://durgical.com/en/physiology/cardiac-output-and-what-sets-it/)
- [Lung and chest wall compliance](https://durgical.com/en/physiology/lung-and-chest-wall-compliance/)
- [Ventilation–perfusion matching and West zones](https://durgical.com/en/physiology/ventilation-perfusion-matching-and-west-zones/)
- [Cerebral blood flow, brain metabolism and intracranial pressure](https://durgical.com/en/physiology/cerebral-blood-flow-brain-metabolism-and-intracranial-pressure/)
- [Renal blood flow, autoregulation and tubuloglomerular feedback](https://durgical.com/en/physiology/renal-blood-flow-autoregulation-and-tubuloglomerular-feedback/)
- [Body temperature regulation](https://durgical.com/en/physiology/body-temperature-regulation/)

## Connections

## Related 7

- [General anaesthesia: induction, maintenance and emergence](https://durgical.com/en/anaesthesia/general-anaesthesia-induction-maintenance-and-emergence/)
- [Intraoperative hypotension](https://durgical.com/en/anaesthesia/intraoperative-hypotension/)
- [Intraoperative hypoxaemia: a stepwise approach](https://durgical.com/en/anaesthesia/intraoperative-hypoxaemia-a-stepwise-approach/)
- [Perioperative hypothermia and shivering](https://durgical.com/en/anaesthesia/perioperative-hypothermia-and-shivering/)
- [Postoperative oliguria and fluids](https://durgical.com/en/anaesthesia/postoperative-oliguria-and-fluids/)
- [High or total spinal block](https://durgical.com/en/anaesthesia/high-or-total-spinal-block/)
- [The older patient](https://durgical.com/en/anaesthesia/the-older-patient/)

## Related conditions 5

- [Atelectasis](https://durgical.com/en/conditions/atelectasis/)
- [Acute kidney injury](https://durgical.com/en/conditions/acute-kidney-injury/)
- [Raised intracranial pressure and brain herniation](https://durgical.com/en/conditions/raised-intracranial-pressure-and-brain-herniation/)
- [Accidental hypothermia](https://durgical.com/en/conditions/accidental-hypothermia/)
- [Malignant hyperthermia](https://durgical.com/en/conditions/malignant-hyperthermia/)

## In Pharmacology 6

- [Inhaled anaesthetics](https://durgical.com/en/pharmacology/classes/inhaled-anaesthetics/)
- [Intravenous anaesthetics](https://durgical.com/en/pharmacology/classes/intravenous-anaesthetics/)
- [Propofol](https://durgical.com/en/pharmacology/propofol/)
- [Ketamine](https://durgical.com/en/pharmacology/ketamine/)
- [Etomidate](https://durgical.com/en/pharmacology/etomidate/)
- [Nitrous oxide](https://durgical.com/en/pharmacology/nitrous-oxide/)

[← 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 Inhalation Agents and Their Cardiovascular Effects](https://www.ncbi.nlm.nih.gov/books/NBK541090/) (StatPearls (NCBI Bookshelf), 2025); [General Anesthesia for Surgeons](https://www.ncbi.nlm.nih.gov/books/NBK493199/) (StatPearls (NCBI Bookshelf), 2023).

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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/anaesthesia/what-anaesthesia-does-to-the-heart-lungs-brain-and-kidneys/