Topic 7.2 · Core
The Digestive System
Picture the digestive system as one continuous tube running from mouth to anus, with four separate glands piping secretions onto that tube from outside it rather than joining it. Keep that picture in mind and most of this section is really just naming the tube’s regions and the glands that feed into it, in the right order.
The alimentary canal
Food travels through the alimentary canal in a fixed order: mouth, oesophagus, stomach, small intestine — made up of the duodenum and then the ileum — and large intestine, made up of the colon, rectum and anus. Each region has its own job.
| Region | What happens there |
|---|---|
| Mouth | food is ingested; teeth physically break it up; saliva, containing amylase, begins starch digestion |
| Oesophagus | carries swallowed food down to the stomach — no digestion occurs here |
| Stomach | a muscular, acidic organ that churns food and begins protein digestion |
| Duodenum | the first, short part of the small intestine; receives acidic stomach contents together with bile and pancreatic secretions |
| Ileum | the later, longer part of the small intestine; the main site of absorption |
| Colon | absorbs some of the water that remains |
| Rectum | stores faeces before they are egested |
| Anus | the opening through which faeces leave the body |
The duodenum and the ileum are both part of the small intestine, but they are not interchangeable: the duodenum is where bile and pancreatic secretions arrive and important chemical digestion happens, while the ileum is the region doing most of the absorbing. In the same way, the colon and rectum are both part of the large intestine but do different jobs — the colon absorbs remaining water, the rectum only stores what is left before it leaves the body.
The four associated organs
The salivary glands, pancreas, liver and gall bladder are described as associated organsrather than part of the alimentary canal, and that distinction is worth taking literally: food does not normally pass through any of them. Instead, each one delivers a secretion into the canal from outside it. The salivary glands secrete saliva containing amylase into the mouth. The pancreas secretes digestive enzymes — amylase, protease and lipase — into the duodenum. The liver produces bile. The gall bladder stores that bile and releases it into the duodenum when it is needed. A common error worth avoiding here: bile is made in the liver, not the gall bladder — the gall bladder only stores and releases what the liver has already produced.
Ingestion, digestion, absorption, assimilation, egestion
These five words describe five separate stages, and Cambridge examiners test the boundaries between them specifically, so it is worth learning each definition on its own rather than as a vague synonym for “processing food.” Ingestion is the taking of substances such as food and drink into the body through the mouth. Digestion is the physical and chemical breakdown of food so that useful molecules can eventually be absorbed. Absorption is the movement of small food molecules and ions through the wall of the intestine into the blood or lymph. Assimilation is the uptake and use of those absorbed nutrients by cells and tissues — absorbed glucose used in respiration, or absorbed amino acids used to build new proteins, for example. Egestion is the passing out of food that was never digested or absorbed, as faeces, through the anus.
These are a sequence, not synonyms, and each stage only begins where the last one has finished. A molecule that has been digested but has not yet crossed the gut wall has not been absorbed. A molecule that has reached the blood has been absorbed, but it has not yet been assimilated until a cell actually takes it up and uses it. Answering “digested” when a question is really asking about absorption — or the reverse — is a precise and avoidable way to lose a mark.
The most consequential mix-up, though, is between egestion and excretion, and it is worth stating plainly: egestion is not excretion. Egestion removes material that was never absorbed into the body in the first place — food that passed straight through the gut undigested. Excretion removes waste substances that the body’s own metabolism actually produced. Because of that difference, faeces are egested, not excreted, even though it is common to hear the word used loosely — in an exam answer, that looseness costs the mark.
With the organs and the vocabulary in place, the next question is how food actually gets broken down along this route — starting with physical digestion and, in more mechanical detail, human teeth, before moving on to the chemistry in chemical digestion and where the products of all of it end up in absorption.