Peristalsis

Where it happens

Peristalsis happens along the whole length of the alimentary canal, in the muscular wall that surrounds the oesophagus, stomach, small intestine and large intestine. Two layers of muscle, circular and longitudinal, work together to move food forward.

The muscle involved is smooth muscle, which contracts slowly, without tiring, and without conscious control. In the oesophagus, peristalsis carries each swallowed bolus to the stomach in a single wave.

In the stomach the waves mix food with gastric juice into chyme and squeeze it through the pyloric sphincter a little at a time. In the small intestine the waves are slower and are combined with segmentation, a churning movement that mixes chyme with enzymes and brings it into contact with the villi.

In the large intestine, slow waves move the remaining material towards the rectum while water is absorbed.

Inputs and outputs

• Input: a bolus of chewed food, or later chyme, that stretches the gut wall and triggers the reflex.
• Input: nerve impulses from the nerve network in the gut wall, coordinated without conscious control.
• Input: energy from ATP for contraction of the smooth muscle fibres.
• Output: forward movement of the contents in one direction along the alimentary canal.
• Output: mixing of food with saliva, gastric juice, bile and intestinal juice, so enzymes reach more of the food.
• Output: contact between digested food and the villi of the ileum, which supports absorption.

The steps

1. Swallowing pushes a bolus of food into the oesophagus, where it stretches the wall.
2. Stretch receptors in the wall trigger a reflex through the nerve network in the gut wall, without conscious control.
3. The circular muscles immediately behind the bolus contract, narrowing the lumen and squeezing the food forward.
4. At the same time, the longitudinal muscles just ahead of the bolus contract, shortening and widening that section so it can receive the food.
5. The circular muscles ahead of the bolus and the longitudinal muscles behind it relax, so the two layers act antagonistically.
6. The contraction wave moves forward along the gut, each section contracting in turn behind the food as the previous section relaxes.
7. The wave stops at a sphincter, such as the pyloric sphincter, which opens briefly to let the contents pass into the next region before the next wave begins.

Why it matters and how it is controlled

Peristalsis keeps food, and later waste, moving through the alimentary canal in one direction, giving digestive enzymes time to act and giving the small intestine time to absorb the products of digestion. Without peristalsis, food would not move through the gut on its own.

Peristalsis is controlled by the involuntary nervous system and by the network of nerve cells within the gut wall itself, so it continues during sleep and does not need attention. The rate is adjusted to conditions: the presence of food speeds the waves, while fear or strenuous exercise slows them.

Dietary fibre matters because it adds bulk that stretches the gut wall and keeps the waves strong; a diet low in fibre gives a smaller bolus, weaker stimulation and slower movement, which leads to constipation. Reverse peristalsis, moving the contents back up the gut, is the mechanism of vomiting.

How it is examined

You may be asked to describe how circular and longitudinal muscles work together during peristalsis, to explain why peristalsis only moves food in one direction, or to identify peristalsis in a diagram of the gut wall showing muscle contraction.

Diagram questions usually show a section of gut with a bolus and shaded muscle bands and ask you to label which layer is contracted at each point and to describe the movement using the terms antagonistic, circular and longitudinal. Explanation questions link peristalsis to fibre in a balanced diet, asking why constipation follows a low-fibre diet, or to the difference between physical digestion and chemical digestion.

A common error that costs marks is describing the wave without stating which muscle layer contracts where, so always name the layer and its position relative to the food.

Common misconceptions

Worked exam-style question

Question. The diagram shows part of the oesophagus with a bolus of food. Region X is immediately behind the bolus and region Y is immediately in front of it.

(a) State the condition of the circular muscle and the longitudinal muscle in region X. (b) Describe how the muscles in regions X and Y work together to move the bolus.

(c) A patient who ate very little fibre for several weeks reported that food moved slowly through the gut. Explain this observation.

(d) Explain why peristalsis is classed as physical digestion rather than chemical digestion.

Model answer. (a) In region X the circular muscle is contracted and the longitudinal muscle is relaxed. (b) The contracted circular muscle in X narrows the lumen and pushes the bolus forward.

In region Y the longitudinal muscle contracts while the circular muscle relaxes, so the lumen widens and shortens to receive the bolus. The two layers act antagonistically, and the wave of contraction passes along the oesophagus in one direction.

(c) Fibre provides bulk that stretches the gut wall and stimulates stronger peristalsis. With little fibre the contents are smaller and softer, the wall is stretched less, contractions are weaker and the contents move more slowly, which leads to constipation.

(d) Peristalsis moves and mixes food by muscle contraction; it does not break chemical bonds or change large molecules into small ones, which is the work of enzymes in chemical digestion.

Source:SRC-DSKP-EN

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