Lesson 2 of 7 / Nutrition in humans
Moving food by peristalsis
Why can food move along the gut without relying on gravity?
I can explain how coordinated waves of muscle contraction mix and propel food.
I can explain peristalsis as muscular waves that mix and propel contents.Syllabus
Syllabus K325, Topic 4(b). Explain peristalsis as muscular waves that mix and propel contents.
About 4 min
Make a guess. You are not marked.
Astronauts in orbit, with almost no gravity, can still swallow food. What moves it to the stomach?
Show the answer
Waves of muscle contraction in the oesophagus wall.
Peristalsis squeezes food along. Muscle behind the food contracts while the region ahead relaxes.
The key idea
Peristalsis is a rhythmic wave of muscle contraction along the alimentary canal that mixes its contents and moves them forwards.
A bolus is a portion of swallowed food. The gut wall contains muscle; the wave moves along the wall, while the bolus is pushed through the tube.
Explore the idea
The contraction travels behind the food
Stage 1: Circular muscle contracts behind the bolus. The narrowing pushes it towards the relaxed region ahead.
Original sectional model with exaggerated narrowing. This isolates the circular-muscle action; real gut movement also involves longitudinal muscles and more complex mixing patterns.
Explanation
The oesophagus is a muscular tube, not a passive chute. Its wall contracts in a coordinated pattern to move swallowed food towards the stomach. Peristalsis continues in other parts of the alimentary canal.
Circular muscles contract behind a bolus, narrowing the tube and squeezing its contents forwards. The region ahead relaxes to receive the food. Longitudinal muscles help change the length and shape of the wall. Coordinated patterns of contraction travel along the canal as a wave.
Peristaltic activity also mixes the contents. Mixing brings food into contact with digestive juices and helps distribute enzymes through it. This supports digestion but does not itself break chemical bonds in large food molecules.
Gravity can influence movement, but peristalsis supplies the muscular force. An explanation should state the direction of the moving contraction and how it pushes the contents. Saying only that the food falls or that the muscles contract misses the coordinated wave.
Step by step
- 1
Locate the contraction
Circular muscle tightens behind the bolus, reducing the width of the canal there.
- 2
Follow the wave
The narrowing advances along the canal while the region ahead can receive the contents.
- 3
State both effects
The coordinated muscular activity propels contents and mixes them with digestive secretions.
Worked example
Moving a bolus through a horizontal tube
A section of oesophagus is almost horizontal. Explain how a bolus can still move towards the stomach.
One way to explain it
A wave of coordinated muscle contraction moves along the oesophagus. Circular muscles narrow the canal behind the bolus while the region ahead relaxes, pushing the bolus forwards. Movement therefore does not depend only on gravity.
Why this answer works
- Identify muscular action as the driving force.
- Describe the difference behind and ahead of the bolus.
- Connect the travelling wave to forward movement.
Watch out for this
A student says: "Peristalsis chemically digests food because it breaks food into pieces." What is wrong with this?
Show the answer
Muscle movement propels and mixes food. Chemical digestion requires enzymes to break large molecules into smaller soluble products.
Check your understanding
What best explains forward movement during peristalsis?
Show the answer
A travelling contraction narrows the canal behind the food while the region ahead relaxes.
The coordinated wave pushes the contents forwards rather than simply squeezing the whole tube shut at once.
Quick recall
Why can food move along the gut without relying on gravity?