GI motility, water flux and vomiting
How the gut moves food along and handles water, what goes wrong in motility disorders like gastroparesis, and how nausea and vomiting are controlled.
The gut has its own nervous system that coordinates muscle movement and fluid secretion, modulated by the brain and gut hormones. This page covers the physiology of motility and of water and electrolyte flux, motility disorders such as gastroparesis, and the vomiting pathways with the drug choices for each common setting.
Regulation of motility and secretion
The gut wall has two muscle layers coordinated by the enteric nervous system: the myenteric plexus mainly controls movement and the submucosal plexus mainly controls secretion and blood flow. Interstitial cells of Cajal act as pacemakers, setting the electrical rhythm of contractions. When the wall is stretched or the mucosa stimulated, enterochromaffin cells release serotonin, which activates sensory neurons; the reflex contracts the gut behind the bolus and relaxes it ahead, producing peristalsis. Acetylcholine is the main excitatory transmitter; nitric oxide and VIP relax. Vagal and sympathetic nerves and gut hormones adjust the system, and opioids and dopamine slow it.
- Prokinetic drug targets: D2 antagonism (metoclopramide, domperidone), 5-HT4 agonism (prucalopride), motilin receptor agonism (erythromycin).
Water and electrolyte flux
Large volumes of fluid enter the gut each day from food and secretions, and almost all is reabsorbed by the small and large intestine. Absorption depends on the sodium gradient made by the Na+/K+-ATPase: sodium enters enterocytes through ENaC, through the sodium-glucose co-transporter SGLT1 (the basis of oral rehydration solution) or by exchange with hydrogen (NHE), and water follows. Secretion depends on chloride leaving through CFTR and other channels, with water following through tight junctions. Many antidiarrhoeal and laxative drugs act on these transporters.
Motility disorders
Common disorders include achalasia, gastroparesis, intestinal pseudo-obstruction and functional bowel disorders. Gastroparesis is delayed gastric emptying without obstruction, often diabetic or idiopathic, with nausea, vomiting, early fullness and bloating. Management treats the cause, improves glucose control, adapts the diet (small, low-fat, low-fibre meals) and uses drugs: metoclopramide is first-line but limited to short courses because of tardive dyskinesia; erythromycin helps in the short term; domperidone is widely available outside the US; antiemetics relieve symptoms without speeding emptying. Feeding tubes, parenteral nutrition or gastric electrical stimulation are reserved for refractory cases.
Pathways of nausea and vomiting
Vomiting is coordinated by a centre in the brainstem reticular formation, next to the nucleus of the solitary tract and the chemoreceptor trigger zone in the area postrema. The trigger zone lies outside the blood-brain barrier, so it samples blood for toxins and drugs. Four main inputs reach the centre: the trigger zone, the gut through vagal afferents (serotonin released by chemotherapy acts on 5-HT3 receptors there), the vestibular system in motion sickness, and the cortex in anticipatory vomiting.
- Receptors and blockers: 5-HT3 (ondansetron), D2 (metoclopramide, prochlorperazine, droperidol), NK1 (aprepitant), muscarinic (hyoscine), H1 (promethazine, dimenhydrinate), CB1 (cannabinoids).
Drug choice by setting
- Chemotherapy: prevention is matched to how emetogenic the regimen is; highly emetogenic regimens combine an NK1 antagonist, a 5-HT3 antagonist and dexamethasone, with olanzapine as an option; delayed vomiting needs cover for several days.
- Postoperative: 5-HT3 antagonists and dexamethasone, with other classes such as D2 antagonists added in patients at higher risk.
- Motion sickness: hyoscine and H1 antihistamines; 5-HT3 antagonists do not work here.
- Pregnancy: diet changes first; doxylamine with pyridoxine is approved for nausea and vomiting not controlled by conservative measures; it causes drowsiness and is not combined with alcohol or other sedatives. Hyperemesis gravidarum needs specialist care.
Test yourself
Think of the answer, then tap the question to see it.
Why can the chemoreceptor trigger zone detect drugs and toxins in the blood?
It lies in the area postrema, outside the blood-brain barrier, so it samples blood and cerebrospinal fluid directly.
Why are 5-HT3 antagonists ineffective for motion sickness?
Motion sickness is driven by vestibular input through muscarinic and H1 pathways, not by serotonin released in the gut.
Why does oral rehydration solution contain glucose as well as sodium?
SGLT1 co-transports sodium with glucose into enterocytes, and water follows; this route keeps working in secretory diarrhoea.
Why is metoclopramide limited to short courses in gastroparesis?
Long exposure raises the risk of tardive dyskinesia, an often irreversible movement disorder from D2 blockade.
Example drugs
- Metoclopramide
- Domperidone
- Erythromycin
- Prucalopride
- Ondansetron
- Aprepitant
- Dexamethasone
- Olanzapine
- Droperidol
- Prochlorperazine
- Promethazine
- Dimenhydrinate
- Hyoscine hydrobromide (scopolamine)
- Pyridoxine
- Loperamide
Connections
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Your Digestive System & How it Works (National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), 2026); Treatment for Gastroparesis (National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), 2026); DailyMed - DOXYLAMINE SUCCINATE AND PYRIDOXINE HYDROCHLORIDE tablet, delayed release (DailyMed (U.S. National Library of Medicine), 2026).
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.