Nausea and vomiting: causes by mechanism and fluid-electrolyte effects
- Topic
Feeling sick and throwing up can come from the gut, the brain, the inner ear, pregnancy or the blood chemistry, and repeated vomiting drains water and salts.
What it is
Nausea is the unpleasant sense of needing to vomit; vomiting is the forceful expulsion of stomach contents, unlike regurgitation, which is effortless and without nausea. Signals reach the vomiting center in the brainstem from the gut, the chemoreceptor trigger zone (drugs, toxins, uremia, ketosis, pregnancy hormones), the vestibular system and the higher brain, so causes are best sorted by these pathways. Repeated vomiting causes dehydration, hypokalemia and metabolic alkalosis.
Key facts
- Timing: soon after a medicine, toxin or motion points to that cause; early morning suggests pregnancy, alcohol or raised intracranial pressure; hours after meals suggests delayed gastric emptying.
- Content: undigested food, bile, blood, coffee-ground material or feculent material; also the volume and smell.
- Associated symptoms: abdominal pain, diarrhea, last stool and flatus, fever, headache, vertigo, chest pain, polyuria and thirst, missed period.
- Background: previous abdominal surgery (adhesions), diabetes, kidney or liver disease, chemotherapy, cannabis use, alcohol, eating disorders.
- Fluid effects: loss of hydrochloric acid raises bicarbonate; volume depletion activates aldosterone, so the kidney wastes potassium and holds sodium.
- Other complications: esophageal tear (Mallory-Weiss), rupture (Boerhaave), aspiration pneumonia, malnutrition with chronic vomiting.
Step by step
- Assess hydration and vital signs first; look for confusion, a rigid abdomen or a distended, tympanic abdomen.
- Separate vomiting from regurgitation: 'Do you feel sick first? Do you have to push it up, or does it just come back?'
- Ask about timing, content (bile, blood, food from earlier) and frequency.
- Ask about pain, bowel movements and flatus, fever, headache, vertigo, and the last menstrual period.
- Review medicines, alcohol, cannabis, toxins and possible poisoning, especially in children.
- Examine the abdomen (distension, scars, hernias, bowel sounds, peritoneal signs) and do a neurologic exam with fundoscopy when the history suggests it.
- Measure intake and output and order electrolytes when vomiting is severe or prolonged.
Causes of nausea and vomiting by pathway
| Pathway | Clue | Example |
|---|---|---|
| Gut irritation | Acute onset with diarrhea, contacts ill | Viral or bacterial gastroenteritis |
| Mechanical obstruction | Colicky pain, distension, no flatus, bilious or feculent vomiting | Small bowel obstruction |
| Poor gastric emptying | Early fullness, vomiting food eaten hours before | Gastroparesis in diabetes |
| Chemoreceptor trigger zone, metabolic | Thirst, polyuria, abdominal pain, deep breathing | Diabetic ketoacidosis |
| Chemoreceptor trigger zone, pregnancy | First trimester, weight loss, ketonuria | Hyperemesis gravidarum |
| Central nervous system | Morning headache, vomiting without nausea, papilledema | Raised intracranial pressure |
| Vestibular | Vertigo, worse with head movement or motion | Labyrinthitis, motion sickness |
Clinical notes
- Blood or coffee-ground vomit: upper gastrointestinal bleeding; assess for shock.
- Vomiting with headache, stiff neck or altered consciousness: think of meningitis, hemorrhage or raised intracranial pressure.
- Vomiting with peritoneal signs or a distended tympanic abdomen: acute abdomen or obstruction; surgical review.
- Vomiting followed by severe chest pain and subcutaneous emphysema suggests esophageal rupture.
- Vomiting in a diabetic patient may be the first sign of ketoacidosis, even with modest glucose levels in some cases.
- In infants and older adults, dehydration develops faster and is harder to see.
In the lab
- Pregnancy test in women of childbearing age.
- Electrolytes (sodium, potassium, chloride, bicarbonate), urea and creatinine, glucose and urine ketones when vomiting is severe, prolonged or with dehydration.
- Liver tests and lipase when pain is upper abdominal; blood gas when acid-base disorder is suspected.
- Abdominal radiographs or CT when obstruction is suspected; brain imaging when neurologic signs are present.
Test yourself
Think of the answer, then tap the question to see it.
How is vomiting distinguished from regurgitation?
Vomiting is forceful and preceded by nausea; regurgitation brings food back effortlessly, without nausea or abdominal effort.
Which acid-base and potassium disorder follows repeated vomiting?
Hypochloremic metabolic alkalosis with hypokalemia, from acid loss and aldosterone-driven renal potassium loss.
What does vomiting food eaten many hours earlier suggest?
Gastric outlet obstruction or gastroparesis.
Which test is ordered in every woman of childbearing age with vomiting?
A pregnancy test.
Connections
Educational content on the clinical exam; it is learned and practised under supervision. It does not replace assessment by a health professional and is not for self-diagnosis.
Sources: Nausea and vomiting - adults (MedlinePlus Medical Encyclopedia, U.S. National Library of Medicine, 2025); Nausea and Vomiting (MSD Manual Professional Edition, 2025).
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.