A meal may contain starch, proteins and fats, but your cells cannot simply take in most of these large food molecules. Before the nutrients can enter the internal environment of the body, food must be moved, mixed, broken down, absorbed and finally used.
The digestive system carries out these jobs as one connected process. The mouth begins the breakdown, the alimentary canal moves food onwards by peristalsis, digestive enzymes produce absorbable molecules, villi transfer those molecules into blood or lymph, and the liver processes many of the absorbed nutrients. Understanding that journey is much more useful than memorising each organ as an isolated fact.
Try these five questions before reading the chapter. They test the journey of food, digestive processes and the first steps of absorption.
- 5 diagnostic questions
- Core terms and organ functions
- Instant explanation for every answer option
The five processes of nutrition in humans
The journey of food can be organised into five processes. Each describes a different stage, so the terms should not be used interchangeably.
| Process | What it means | Where it fits into the journey |
|---|---|---|
| Ingestion | Taking food and drink into the body through the mouth | The journey begins. |
| Digestion | Breaking large, insoluble food molecules into small, soluble molecules | Physical and chemical breakdown prepare nutrients for absorption. |
| Absorption | Movement of digested nutrients through the wall of the alimentary canal into the blood or lymph | Occurs mainly across villi in the ileum. |
| Assimilation | Use of absorbed nutrients by cells and tissues | Nutrients become part of cells or are used to release energy. |
| Egestion | Removal of undigested and unabsorbed food from the alimentary canal through the anus | The remaining faeces leave the body. |
Egestion is not excretion. Egestion removes material that was never absorbed into the body. Excretion removes metabolic waste made by cells, such as urea and carbon dioxide.
The digestive system and the alimentary canal
The alimentary canal is the continuous muscular tube through which food passes. The digestive system includes this canal as well as associated organs that add secretions or process absorbed nutrients.
Route followed by food:
mouth → oesophagus → stomach → duodenum → ileum → colon → rectum → anus
Associated organs: salivary glands, pancreas, liver and gall bladder. Food does not pass through these associated organs.
Adapted from digestive-system-labeled by Servier Medical Art, licensed under CC BY 3.0 Unported. “Esophagus” was changed to “Oesophagus”.
How peristalsis keeps the contents moving
The wall of the alimentary canal contains muscles. Their rhythmic, wave-like contractions mix the contents and propel them forwards. This process is called peristalsis.
Peristalsis is especially easy to notice in the oesophagus, where a wave of contraction moves a swallowed bolus towards the stomach. It continues along the rest of the canal, mixing food with digestive secretions as well as moving it onwards.
Peristalsis does not depend on gravity. Its muscular waves can propel food even when a person is lying down.
The journey through each part of the digestive system
| Part | Main function | Important detail |
|---|---|---|
| Mouth | Ingests food; teeth cut and grind it; the tongue mixes it with saliva and forms a bolus. | Chewing increases surface area. Salivary amylase begins starch digestion. |
| Salivary glands | Secrete saliva into the mouth. | Saliva moistens food and contains amylase. |
| Oesophagus | Transports the bolus from the mouth to the stomach. | Peristalsis propels the bolus; the oesophagus is not a major site of digestion. |
| Stomach | Stores and churns food, mixing it with gastric juice. | Hydrochloric acid provides an acidic pH and kills many swallowed microorganisms. Protease begins protein digestion. |
| Duodenum | Receives acidic contents from the stomach, bile and pancreatic juice. | Much of the chemical digestion of carbohydrates, proteins and fats occurs here. |
| Pancreas | Produces pancreatic juice containing digestive enzymes. | Pancreatic amylase, protease and lipase are delivered to the duodenum. |
| Liver | Produces bile and processes absorbed nutrients. | It also forms urea and breaks down alcohol and hormones. |
| Gall bladder | Stores and concentrates bile before releasing it into the duodenum. | It does not produce bile. |
| Ileum | Completes digestion and absorbs digested nutrients. | Its lining has many villi, each containing capillaries and a lacteal. |
| Colon | Absorbs much of the remaining water and mineral ions. | Undigested and unabsorbed material becomes more solid as faeces form. |
| Rectum | Temporarily stores faeces. | It is a storage region, not the main site of water absorption. |
| Anus | Controls and allows the egestion of faeces. | Sphincter muscles control the opening. |
Physical digestion prepares food for enzymes
Physical digestion changes the size or arrangement of food without changing the chemical identity of its molecules. It makes chemical digestion more efficient by increasing the surface area on which enzymes can act.
Chewing: teeth cut and grind food into smaller pieces.
Churning: stomach muscles mix food with gastric juice and break it into a semi-liquid mixture.
Emulsification: bile separates large fat globules into many small droplets.
Why bile helps fat digestion
Fats do not mix well with the watery contents of the small intestine. Without bile, they tend to remain as relatively large globules. Bile salts disperse these globules into many smaller droplets. The total surface area of fat exposed to lipase increases, so lipase can digest the fat more rapidly.
Bile is alkaline and also helps neutralise the acidic contents arriving from the stomach. This creates conditions in the duodenum that are more suitable for pancreatic and intestinal enzymes.
Bile is not an enzyme. It emulsifies fat but does not break the chemical bonds in fat molecules. Lipase carries out the chemical digestion of fats into fatty acids and glycerol.
Chemical digestion uses specific enzymes
Chemical digestion changes one kind of molecule into another. Digestive enzymes catalyse hydrolysis reactions, using water to split large molecules into smaller units. Each enzyme acts on a particular substrate because its active site has a complementary shape.
| Enzyme | Substrate | End-product | Main source and site of action |
|---|---|---|---|
| Amylase | Starch | Maltose | Salivary glands: mouth Pancreas: duodenum |
| Maltase | Maltose | Glucose | Small-intestinal lining: small intestine |
| Protease | Proteins and shorter polypeptides | Amino acids after complete digestion | Stomach, pancreas and small-intestinal lining: stomach and small intestine |
| Lipase | Fats | Fatty acids and glycerol | Mainly pancreas: small intestine |
Carbohydrate digestion occurs in two main steps
Starch → maltose → glucose
Amylase catalyses the first step. Maltase catalyses the second.
Salivary amylase begins digesting starch in the mouth. After swallowing, the acidic conditions in the stomach denature salivary amylase, so little or no starch digestion by this enzyme continues there. Pancreatic amylase resumes starch digestion in the duodenum, and maltase in the small intestine converts maltose into absorbable glucose.
Protein digestion begins in the stomach
A stomach protease works in acidic conditions and begins breaking proteins into shorter chains. Proteases from the pancreas and small-intestinal lining continue the process in the small intestine. After complete digestion, the absorbable end-products are amino acids.
Fat digestion occurs mainly in the small intestine
Bile first emulsifies the fat. Lipase then catalyses the chemical breakdown of fat molecules into fatty acids and glycerol. Keeping physical emulsification separate from enzymatic digestion prevents a common examination error.
Absorption across the villi of the ileum
Digestion is useful only if its products can cross the intestinal wall. Most absorption takes place in the ileum, whose inner surface is folded and covered with many finger-like projections called villi. The cells on each villus also bear microscopic projections called microvilli.
| Feature of a villus | How it supports absorption |
|---|---|
| Many villi and microvilli | Provide a very large surface area for absorption. |
| Epithelium one cell thick | Provides a short distance for nutrients to cross. |
| Dense network of blood capillaries | Rapidly carries glucose and amino acids away, helping maintain concentration gradients. |
| Lacteal | Receives absorbed products of fat digestion and carries them into the lymphatic system. |
| Epithelial cells with many mitochondria | Supply energy for active transport when nutrients such as glucose must be absorbed against a concentration gradient. |
Capillaries and lacteals receive different nutrients
Blood capillaries: receive glucose, amino acids, mineral ions and water-soluble vitamins. Blood carrying many of these newly absorbed nutrients then travels to the liver through the hepatic portal vein.
Lacteal: receives the absorbed products of fat digestion. These enter the lymphatic system and later join the bloodstream.
Glucose and amino acids may cross the villus epithelium by diffusion when a favourable concentration gradient exists. They can also be absorbed by active transport when movement against a concentration gradient is required. This is why villus epithelial cells need energy from respiration.
From absorption to assimilation
Absorption moves nutrients into the transport system. Assimilation happens afterwards, when cells take up and use those nutrients.
| Absorbed nutrient | Examples of assimilation |
|---|---|
| Glucose | Used in respiration to release energy; excess may be converted to glycogen for storage. |
| Amino acids | Used to synthesise proteins for growth, repair, enzymes and other cell structures. |
| Fatty acids and glycerol | Used to form lipids for cell structures, insulation and energy storage. |
The hepatic portal vein connects the ileum to the liver
Blood leaving the capillaries of the small intestine does not first distribute its newly absorbed nutrients around the whole body. It travels to the liver through the hepatic portal vein.
Exam statement: The hepatic portal vein transports blood rich in absorbed nutrients from the small intestine to the liver.
This arrangement allows the liver to regulate, store or modify absorbed substances before the blood enters the general circulation. Remember that the hepatic portal vein carries blood to the liver; the hepatic vein carries blood away from it.
The liver processes nutrients and other substances
The liver has several roles in nutrition and regulation. These roles are linked: the liver receives absorbed nutrients, adjusts their concentrations, converts some molecules into other forms and removes potentially harmful substances.
| Role of the liver | What happens |
|---|---|
| Blood glucose regulation | Excess glucose can be converted to glycogen for storage. When more glucose is needed, glycogen can be converted back to glucose. |
| Fat digestion | The liver produces bile, which emulsifies fats and helps neutralise acidic contents in the duodenum. |
| Amino acid metabolism and urea formation | Excess amino acids are deaminated. The nitrogen-containing part ultimately forms urea, which is carried in the blood to the kidneys for excretion. |
| Breakdown of alcohol | The liver breaks down much of the alcohol that enters the blood, but excessive consumption can damage liver cells. |
| Breakdown of hormones | The liver helps remove hormones after their signals are no longer required. |
Why excess amino acids form urea
The body cannot store excess amino acids in the same way that it stores glycogen or fat. In the liver, the amino group is removed by deamination. This produces a nitrogen-containing substance that is converted into urea. Urea is less toxic, travels in the blood and is removed from the body by the kidneys in urine.
Alcohol affects the brain, liver and society
Alcohol is a depressant drug that affects brain function. Even before obvious drunkenness, it can slow information processing and weaken judgement.
| Type of effect | Examples | Why it matters |
|---|---|---|
| Short-term effects on the brain | Increased reaction time, reduced self-control, impaired judgement and poorer coordination | A longer reaction time means a person responds more slowly to danger. |
| Long-term excessive consumption | Liver disease and brain damage | Repeated exposure can cause lasting damage and loss of normal function. |
| Social implications | Road accidents, unsafe decisions, conflict, financial strain and reduced performance at school or work | The effects extend beyond the drinker and may harm families and the wider community. |
Do not misread “increased reaction time”. It means the interval between a stimulus and the response becomes longer, so the response is slower.
What happens to material that is not absorbed?
By the time the contents reach the colon, most useful nutrients have been absorbed. The colon absorbs much of the remaining water and mineral ions. Undigested and unabsorbed material becomes faeces, which are stored temporarily in the rectum and egested through the anus.
This final stage completes the journey through the alimentary canal, but it is not the end of nutrition. The absorbed nutrients are still being transported, regulated and assimilated throughout the body.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Calling egestion “excretion” | Faeces contain material that was not absorbed, whereas excretory products are metabolic wastes made by cells. | Use egestion for undigested food leaving the anus and excretion for wastes such as urea. |
| Saying bile is an enzyme | Bile does not catalyse the hydrolysis of fat molecules. | State that bile emulsifies fats and increases the surface area for lipase. |
| Saying the gall bladder produces bile | Bile is produced by the liver. | The gall bladder stores, concentrates and releases bile. |
| Using physical and chemical digestion as if they mean the same thing | Physical digestion does not change the food molecules; chemical digestion does. | Link chewing and emulsification to increased surface area, and link enzymes to molecular breakdown. |
| Saying all digested nutrients enter blood capillaries | The products of fat digestion enter the lacteal. | Glucose and amino acids → capillaries; fat-digestion products → lacteal. |
| Saying the villus wall is “thin” without explaining why | The description alone does not connect structure to function. | State that an epithelium one cell thick provides a short distance for absorption. |
| Saying absorption and assimilation both mean nutrients entering the blood | Assimilation happens after absorption. | Absorption moves nutrients into blood or lymph; assimilation is their use by cells. |
| Reversing the direction of the hepatic portal vein | It does not carry blood away from the liver. | State that it carries nutrient-rich blood from the small intestine to the liver. |
| Saying increased reaction time means a faster reaction | A longer reaction time means a longer delay before responding. | Write that alcohol increases reaction time, so responses become slower. |
What you should be able to do
Use the five nutrition terms
Distinguish ingestion, digestion, absorption, assimilation and egestion, and separate egestion from excretion.
Trace the route of food
Place the mouth, oesophagus, stomach, duodenum, ileum, colon, rectum and anus in the correct order.
Explain organ functions
Describe the roles of every named part of the digestive system, including the pancreas, liver and gall bladder.
Describe peristalsis
Refer to rhythmic, wave-like muscular contractions that mix and propel the contents of the alimentary canal.
Compare physical and chemical digestion
Explain how chewing, churning and emulsification assist enzyme action without confusing bile with an enzyme.
Match enzymes to their reactions
State the substrates and end-products of amylase, maltase, protease and lipase.
Relate villus structure to absorption
Explain the value of a large surface area, a one-cell-thick epithelium, capillaries, a lacteal and mitochondria.
Explain nutrient transport
Distinguish the capillary and lacteal routes and state the function of the hepatic portal vein.
Describe the liver's roles
Cover glucose–glycogen conversion, fat digestion, amino acid metabolism and urea formation, alcohol breakdown and hormone breakdown.
Outline the effects of alcohol
Explain short-term effects on the brain, long-term excessive consumption and wider social implications.
These ten questions test whether you can connect organ functions, enzymes, absorption routes and liver processes in unfamiliar situations.
- 10 mastery questions
- Linked processes and exam-style reasoning
- Instant explanation for every answer option
Frequently asked questions
Human digestion: questions students often ask
Once the journey of food is clear, the remaining confusion usually comes from pairs of terms that sound similar: digestion and absorption, absorption and assimilation, bile and lipase, capillaries and lacteals, or egestion and excretion.
What is the difference between the alimentary canal and the digestive system?
The alimentary canal is the continuous tube through which food passes, from the mouth to the anus. The digestive system includes the alimentary canal and associated organs such as the salivary glands, pancreas, liver and gall bladder. These associated organs assist digestion, but food does not pass through them.
What is the difference between physical and chemical digestion?
Physical digestion breaks food into smaller pieces or droplets without changing the food molecules chemically. Chemical digestion uses enzymes to split large molecules into smaller molecules. Chewing is physical digestion; amylase breaking starch into maltose is chemical digestion.
Why does chewing make chemical digestion faster?
Chewing produces many smaller pieces with a larger total surface area. More enzyme molecules can therefore contact the food at the same time, increasing the rate of chemical digestion.
Is bile a digestive enzyme?
No. Bile emulsifies fats into smaller droplets and helps neutralise acidic contents from the stomach. It does not hydrolyse fat molecules. Lipase is the enzyme that breaks fats into fatty acids and glycerol.
If the liver makes bile, why is the gall bladder needed?
The liver produces bile continuously. The gall bladder stores and concentrates it, then releases it into the duodenum when it is needed. The gall bladder therefore manages the supply of bile but does not manufacture it.
Why does starch digestion stop in the stomach?
The acidic conditions in the stomach denature salivary amylase by changing the shape of its active site. Pancreatic amylase later continues starch digestion in the duodenum, where bile and pancreatic secretions help create a more suitable pH.
Where does most digestion occur, and where does most absorption occur?
Much of chemical digestion occurs in the duodenum, which receives pancreatic juice and bile. Most absorption of digested nutrients occurs in the ileum, whose lining bears many villi and microvilli.
Why does a villus need both capillaries and a lacteal?
They provide different transport routes. Glucose and amino acids enter the blood capillaries. The products of fat digestion enter the lacteal and are carried through the lymphatic system before joining the bloodstream.
Can glucose enter a villus by both diffusion and active transport?
Yes. Glucose can move down a favourable concentration gradient, and active transport can absorb glucose against a concentration gradient. Active transport requires energy supplied by respiration, which is why absorbing epithelial cells contain many mitochondria.
What is the difference between absorption and assimilation?
Absorption is the movement of digested nutrients through the intestinal wall into blood or lymph. Assimilation is their subsequent use by cells. For example, glucose entering a capillary is absorption; a muscle cell using glucose in respiration is assimilation.
What is special about the hepatic portal vein?
It transports blood rich in absorbed nutrients directly from the small intestine to the liver. This allows the liver to process, store or regulate many substances before the blood enters the general circulation.
Why does the liver convert glucose to glycogen and back again?
Converting excess glucose to glycogen provides a compact storage form and helps prevent blood glucose concentration from rising too far. When blood glucose is needed, glycogen can be converted back to glucose. This contributes to blood glucose regulation.
Why is urea formation linked to digestion even though urea leaves in urine?
Digestion produces amino acids, and absorbed amino acids are used for protein synthesis. If there are more amino acids than the body needs, the liver deaminates them and forms urea. The kidneys then excrete that urea in urine. The process links nutrition, liver metabolism and excretion.
Why are faeces not described as an excretory product?
Most of the material in faeces was never absorbed into the internal environment and was not produced as metabolic waste by cells. Its removal from the alimentary canal is therefore egestion. Urea and carbon dioxide are excretory products because they are made by metabolic reactions in the body.
Does “increased reaction time” mean alcohol makes a person react faster?
No. Reaction time is the delay between a stimulus and the response. If that time increases, the delay becomes longer and the person reacts more slowly. Together with poorer judgement and coordination, this raises the risk of accidents.
The digestive system is easiest to understand as a connected journey. Food is ingested, propelled by peristalsis, physically prepared and chemically digested; the products are absorbed through villi, transported by blood or lymph and assimilated by cells. The liver then helps regulate and process what has entered the body, while the colon, rectum and anus deal with what was not absorbed. Once those links are clear, the many organ names become parts of one coherent process rather than a list to memorise.