Peristalsis

Once you swallow, food does not simply fall down to your stomach under gravity. It is pushed along the gut by peristalsis.

• Peristalsis is a series of waves of muscle contraction that move along the wall of the gut.
• Each wave squeezes the food behind it, pushing it forward — like squeezing a tube of toothpaste from the bottom up.
• It is so effective that you can swallow even when upside down, because it does not rely on gravity.
• Peristalsis occurs all the way along the gut — the oesophagus, stomach, small intestine and large intestine.

The lump of chewed food being pushed along is called the bolus.

Exam tip. A common 2-mark question is "explain how food is moved along the gut". The keywords are waves, muscle contraction and peristalsis — name all three.

The wall of the gut contains two layers of muscle arranged in different directions:

• Circular muscle — runs around the gut (like rings or hoops). When it contracts, the gut narrows (gets thinner) at that point.
• Longitudinal muscle — runs along the gut (lengthways). When it contracts, that part of the gut shortens and widens.

These two layers are antagonistic — at any one point they do opposite jobs: when the circular muscle is contracted the longitudinal muscle is relaxed, and vice versa.

Exam tip. Don't muddle the two. Circular = wraps around → squeezes narrow. Longitudinal = runs along → makes the gut shorter.

Peristalsis works because the muscles contract behind the food and relax in front of it:

1. Behind the bolus, the circular muscle contracts → the gut narrows here and squeezes the bolus forward.
2. In front of the bolus, the circular muscle relaxes → the gut widens here, leaving space for the food to move into.
3. This pattern of contract-behind / relax-in-front travels along the gut as a wave, pushing the bolus steadily forward.

So a contracted ring of circular muscle moves along behind the food, forcing it onward — much like squeezing a ball of food along a soft rubber tube.

Exam tip. The mark-scheme phrase examiners want is "muscle contracts behind the bolus to push it along". Saying simply "muscles squeeze the food" is too vague for full marks.

Fibre (also called roughage) is the indigestible part of plant food — for example the cellulose in vegetables, fruit skins and wholemeal bread. We cannot digest or absorb it, so why does it matter?

• Fibre adds bulk to the food in the gut, giving the muscles something firm to push against.
• This makes peristalsis more effective, so food (and later waste) is moved along more easily.
• A diet low in fibre means there is less bulk, peristalsis is weaker/slower, and this can lead to constipation (the gut contents move too slowly).

So fibre is important even though it is never absorbed — its job is mechanical, helping peristalsis keep things moving.

Exam tip. The key idea is "bulk for the muscles to push against". Link this to constipation if a low-fibre diet is mentioned.

Here is the whole process in one flow:

1. You chew and swallow food, forming a bolus.
2. In the gut wall, circular and longitudinal muscle layers work antagonistically.
3. Circular muscle contracts behind the bolus (gut narrows, squeezing it forward) and relaxes in front (gut widens, making room).
4. These contractions pass along as waves — this is peristalsis — pushing the bolus from the oesophagus all the way to the large intestine.
5. Fibre (roughage) in the diet adds bulk, giving the muscles something to push against so peristalsis works well; a low-fibre diet can cause constipation.

Exam tip. For longer (4–6 mark) answers, write this as an ordered sequence and make sure you include the names circular muscle, bolus, contracts behind / relaxes in front, wave and peristalsis.