Small bowel and colon motility !
- Mechanism
- Exam priority: medium
The gut mixes its contents with segmenting contractions and moves them on with peristalsis; the colon dries them and pushes them with mass movements.
Updated 2026-11-01
Also called: Segmentation and peristalsis, Mass movements
See it in 3D: Duodenum · Jejunum · Ileum · Cecum · Ascending colon · Transverse colon · Descending colon · Sigmoid colon · Taeniae coli
What it is
In the small intestine, segmenting contractions dominate: they mix chyme with enzymes and bring it into contact with the mucosa, while short peristaltic waves move it slowly forward (≈3–5 h to the cecum). The ileocecal valve controls entry into the colon. The proximal colon absorbs water with slow haustral contractions, and a few times a day a mass movement pushes stool into the distal colon and rectum.
Key facts
- Mouth-to-cecum transit is ≈3–5 h and colonic transit ≈1–3 days, with wide individual variation.
- Irritable bowel syndrome changes the motor pattern and visceral sensitivity without a structural lesion; the Rome IV criteria rest on pain related to defecation and changes in bowel habit.
- Opioids increase non-propulsive segmentation and sphincter tone, which is why they constipate.
How it works, step by step
- Chyme entering the duodenum stretches the wall and triggers the local peristaltic reflex.
- Segmenting contractions, paced by slow waves, split and rejoin the contents many times a minute.
- Short peristaltic waves move chyme a few centimetres at a time; transit is slow to give time for absorption.
- Ileal distension and cecal pressure keep the ileocecal valve closed; eating opens it through the gastroileal reflex.
- In the proximal colon, haustral contractions mix and dry the contents.
- A mass movement contracts a long segment of colon as a unit and carries stool into the sigmoid and rectum.
- Rectal distension starts the defecation reflex.
Clinical notes
- Bowel obstruction: the commonest small-bowel cause is post-operative adhesions; in the colon, cancer and sigmoid volvulus.
- Paralytic ileus: diffuse distension without bowel sounds or colicky pain, from surgery, hypokalemia, opioids or sepsis.
- Chagasic megacolon: dilation and chronic constipation from loss of myenteric neurons, known in endemic areas of Colombia.
- Intussusception in children: one segment telescopes into the next, with intermittent colicky pain and 'redcurrant jelly' stools.
Test yourself
Think of the answer, then tap the question to see it.
Which type of contraction mixes chyme in the small intestine, and which moves it forward?
Segmentation (alternating circular contractions that split and rejoin the contents) mixes. Peristalsis (contraction above and relaxation below) moves it forward.
Why do bowel sounds change over time in small bowel obstruction?
At first the bowel above the block contracts hard against it (rushes). With distension, ischemia and muscle fatigue, motility falls and the sounds fade.
Which reflex explains why many people need to defecate after breakfast?
The gastrocolic reflex: stomach distension and the arrival of food trigger mass movements in the colon that carry stool into the rectum.
Why do small-bowel contents move in the aboral direction?
Slow-wave frequency falls from the duodenum to the ileum, and the peristaltic reflex contracts above and relaxes below the bolus.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- The enteric nervous system, slow waves and GI control
- Gastric motility and emptying
- Defecation and continence
- Water and electrolyte absorption and how diarrhea happens
- Gastrointestinal hormones
Related conditions
- Small bowel obstruction
- Large bowel obstruction
- Irritable bowel syndrome
- Sigmoid volvulus
- Intussusception (children)
- Chagas disease
In Pharmacology
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: 23.5 The Small and Large Intestines (OpenStax (Rice University), 2022); Small Bowel Obstruction (StatPearls (NCBI Bookshelf), 2024); Irritable Bowel Syndrome (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 64). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.