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Key concept
Peristalsis is the wave-like contraction of muscles in the wall of the alimentary canal that pushes food along. Circular muscles contract behind the food bolus to squeeze it forward, while longitudinal muscles contract ahead to widen the gut and allow the food to move into that section.
Food does not simply fall through the digestive system by gravity: it is actively pushed along by waves of muscular contraction called peristalsis. This process works even if you eat upside down. Understanding peristalsis is important for explaining how food moves through each section of the alimentary canal.
How Peristalsis Works
The wall of the alimentary canal contains two layers of muscle:
Circular muscle: runs around the gut (like rings). When it contracts, it narrows the gut, squeezing food forward.
Longitudinal muscle: runs along the length of the gut. When it contracts, it shortens and widens the section of gut.
The peristalsis sequence:
Circular muscle contracts behind the bolus of food.
This squeezes the food forward.
Longitudinal muscle contracts ahead of the bolus.
This shortens and widens the gut ahead, creating space for the food to move into.
These contractions occur in waves, pushing food continuously along the alimentary canal.
Key point: Peristalsis is involuntary: you cannot consciously control it. It is controlled by the autonomic nervous system.
Where Peristalsis Occurs
Oesophagus: pushes the bolus from the mouth to the stomach (takes about 6 seconds). Works against gravity when lying down or standing on your head.
Stomach: churns food and mixes it with gastric juice. Particularly strong contractions break food into smaller pieces (mechanical digestion).
Small intestine: moves chyme along the intestine, ensuring it contacts the intestinal wall for enzyme action and absorption.
Large intestine: moves contents slowly, allowing time for water absorption. Mass peristalsis pushes faeces towards the rectum.
The Role of Dietary Fibre
Dietary fibre (roughage) is important for peristalsis because:
Fibre cannot be digested by human enzymes: it passes through the gut intact.
It absorbs water and adds bulk to the food in the gut.
This bulk gives the muscles of the gut wall something to push against, stimulating more effective peristalsis.
Without enough fibre, peristalsis is sluggish, food moves slowly, too much water is absorbed → constipation.
A high-fibre diet prevents constipation and may reduce the risk of bowel cancer and diverticular disease.
Try a focused question (3 marks)
Describe how food moves from the mouth to the stomach through the oesophagus. (3 marks)
Show the answer and marking guidance
Food is formed into a bolus by the tongue and swallowed.
Circular muscles in the oesophagus wall contract behind the bolus, pushing it downward.
Waves of muscular contraction (peristalsis) push the bolus along the oesophagus to the stomach.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying food moves through the gut by gravity: peristalsis actively pushes food; it works even against gravity.
Confusing circular and longitudinal muscle functions: circular muscle narrows the gut; longitudinal muscle widens it.
Not mentioning that peristalsis is involuntary: you do not consciously control it.