Lesson 1 of 7 / Nutrition in humans
The journey through the digestive system
Where does food go, and what changes along the way?
In this lesson: Locate the main digestive organs and distinguish ingestion, digestion, absorption, assimilation and egestion.
About 6 min
The key ideaFood travels along the alimentary canal. Digestive glands supply substances into it; small nutrients are absorbed and then used by body cells.
The alimentary canal is the continuous tube from mouth to anus. Accessory organs, such as the pancreas and liver, add secretions; swallowed food does not pass through them.
Explore the idea
Food route or supporting organ?
Mouth: Food is ingested. Teeth break it into smaller pieces and mix it with saliva.
Original route map, not an anatomical drawing. Solid route: food. Dashed routes: secretions. The liver also receives absorbed nutrients through blood, a route explored later.
Explanation
Ingestion is taking food into the mouth. Teeth break it into smaller pieces, increasing the surface available to enzymes. Salivary glands release saliva, which moistens food and contains amylase to begin starch digestion. Swallowing moves the food into the oesophagus, whose muscular wall propels it towards the stomach.
The stomach stores food for a time and churns it with gastric juice. Its acid provides suitable conditions for a stomach protease and helps kill many ingested microorganisms. Protein digestion begins here. The partly digested contents then enter the duodenum, the first part of the small intestine.
The pancreas releases digestive enzymes and alkaline fluid into the duodenum. The liver produces bile, which is stored and concentrated in the gallbladder before release into the duodenum. These secretions support digestion. Food does not travel through the pancreas, liver or gallbladder.
Digestion produces smaller soluble molecules that can be absorbed. The ileum, a later part of the small intestine, has many villi that provide a large surface for absorption. Digested nutrients cross the intestinal wall into blood or lymph. Much water is also absorbed in the small intestine.
The colon absorbs remaining water and mineral ions from the material left in the canal, helping form faeces. The rectum stores faeces until they are expelled through the anus. This removal of undigested material is egestion.
After absorption, nutrients are delivered to cells. Their use or incorporation into the body is assimilation: for example, amino acids build muscle proteins, or glucose is stored as glycogen in the liver. Digestion, absorption and assimilation are different stages, even though they work together to nourish the body.
Step by step
- 1
Follow the canal
Mouth, oesophagus, stomach, small intestine, colon, rectum and anus form the route travelled by food.
- 2
Add the supporting organs
Salivary glands, pancreas, liver and gallbladder supply secretions. They are not stops along the food route.
- 3
Name the process precisely
Crossing the intestine wall is absorption. Building an absorbed nutrient into a body substance is assimilation.
Worked example
An amino acid has two different journeys
An amino acid crosses the small intestine wall into blood, then is used to make a muscle protein. Name each process and explain the difference.
One way to explain it
Crossing the intestinal wall into blood is absorption. Using the amino acid to synthesise muscle protein is assimilation, because it becomes part of a substance needed by the body.
Why this answer works
- Locate the boundary crossed during absorption.
- Identify the use of the absorbed molecule.
- Do not call protein synthesis digestion.
Is this true? "Food enters the liver and pancreas on its way through the digestive system."
These organs send secretions into the canal. The swallowed food itself remains in the canal until nutrients are absorbed or undigested material is egested.