Digestion and Absorption
1. The Digestive System
Learning outcomes
- I can identify the major organs of the human digestive system.
- I can describe the function of each digestive organ.
- I can trace the path of food through the digestive tract.
- I can explain how the digestive system works as an organ system.
- I can relate digestive structures to their functions.
What Is the Digestive System?
The digestive system is the organ system responsible for breaking food down into small, soluble molecules that can be absorbed into the body and used by cells.
Food contains important nutrients such as:
- Carbohydrates
- Proteins
- Fats
- Vitamins
- Minerals
- Water
Many nutrient molecules in food are too large to pass through the wall of the digestive system. Digestion breaks large molecules into smaller molecules that can be absorbed into the blood or lymph and transported around the body.
The digestive system therefore performs several connected functions:
ingestion → digestion → absorption → assimilation → egestion
The Digestive Tract
The main part of the digestive system is a long muscular tube called the digestive tract, or alimentary canal.
Food travels through this tube.
The main route is:
Mouth → oesophagus → stomach → small intestine → large intestine → rectum → anus
Different sections of the digestive tract are specialised for different functions.
Other organs, including the liver, gall bladder and pancreas, help digestion even though food does not actually pass through them.
The Mouth
Digestion begins in the mouth.
The mouth carries out both mechanical digestion and chemical digestion.
Mechanical Digestion
The teeth physically break food into smaller pieces.
This is called mastication, or chewing.
Breaking food into smaller pieces:
- Makes it easier to swallow.
- Increases its surface area.
- Allows digestive enzymes to act more effectively.
Chemical Digestion
Food mixes with saliva, which is produced by the salivary glands.
Saliva contains the enzyme amylase.
Amylase begins the digestion of starch into smaller sugars.
The tongue mixes the food with saliva and forms it into a soft mass called a bolus.
Structure and Function in the Mouth
The structures of the mouth are suited to their functions.
Teeth are hard and strong, allowing them to cut, tear and grind food.
Salivary glands release saliva containing water and digestive enzymes.
The tongue is muscular, allowing it to move food, mix it with saliva and push the bolus toward the back of the mouth.
These structures work together rather than functioning independently.
Swallowing
After food has been chewed and mixed with saliva, the tongue pushes the bolus toward the back of the mouth.
During swallowing, food enters the oesophagus.
A flap of tissue called the epiglottis helps prevent food from entering the trachea and respiratory system.
Food then travels toward the stomach.
The Oesophagus
The oesophagus is a muscular tube connecting the mouth to the stomach.
Food does not simply fall through the oesophagus because of gravity.
Instead, the walls of the oesophagus contain muscles that produce waves of contraction called peristalsis.
During peristalsis:
- Circular muscles contract behind the food.
- Muscles ahead of the food relax.
- The bolus is pushed along the digestive tract.
Peristalsis occurs throughout much of the alimentary canal.
The Stomach
The stomach is a muscular sac that temporarily stores food and continues digestion.
Its muscular walls contract and relax, mixing food with gastric juice.
The stomach therefore performs both:
- Mechanical digestion through churning.
- Chemical digestion through enzymes and acid.
Food can remain in the stomach while it is gradually processed into a semi-liquid mixture.
Stomach Acid
The stomach produces hydrochloric acid.
This creates strongly acidic conditions.
The acidic environment:
- Helps kill many microorganisms swallowed with food.
- Provides suitable conditions for stomach protease enzymes.
A common misconception is that stomach acid itself performs all digestion in the stomach.
The acid is important, but much of the chemical breakdown of food is carried out by enzymes.
Protein Digestion in the Stomach
The stomach produces protease enzymes, including pepsin.
Proteases break large protein molecules into smaller peptides.
The stomach lining also produces mucus.
This mucus helps protect the stomach wall from:
- Acid
- Digestive enzymes
- Mechanical damage
The stomach therefore needs both powerful digestive conditions and mechanisms that protect its own tissues.
From the Stomach to the Small Intestine
Food leaving the stomach is a partially digested mixture.
It enters the first section of the small intestine, called the duodenum.
Here, digestive substances from several organs enter the digestive tract.
These include substances produced by the:
- Pancreas
- Liver
- Gall bladder
The small intestine is therefore a major site of both digestion and absorption.
The Liver
The liver is a large organ with many functions.
Its role in digestion includes producing bile.
Bile helps with the digestion of fats.
It does this partly by breaking large fat droplets into many smaller droplets. This process is called emulsification.
Emulsification increases the surface area available for lipase enzymes.
The liver also performs many functions after nutrients have been absorbed, including processing and storing nutrients.
The Gall Bladder
The gall bladder is a small organ located beneath the liver.
Its main digestive role is to:
- Store bile produced by the liver.
- Concentrate bile.
- Release bile into the small intestine when needed.
An important distinction is:
The liver produces bile.
The gall bladder stores and releases bile.
The Pancreas
The pancreas produces digestive enzymes and releases them into the small intestine.
These include:
- Amylase for carbohydrate digestion.
- Proteases for protein digestion.
- Lipase for fat digestion.
The pancreas also releases bicarbonate-rich fluid that helps neutralise the acidic material arriving from the stomach.
This is important because enzymes in the small intestine generally work best under conditions that are less acidic than those in the stomach.
The Small Intestine
The small intestine is a long, narrow tube where most chemical digestion is completed and most nutrient absorption occurs.
Despite its name, the small intestine is much longer than the large intestine. It is called "small" because its diameter is smaller.
The small intestine is divided into sections including the:
- Duodenum
- Jejunum
- Ileum
Digestive enzymes break large molecules into small soluble molecules that can be absorbed.
Digestion of the Main Nutrients
Different nutrients require different enzymes.
Carbohydrates
Large carbohydrates such as starch are eventually broken into simple sugars such as glucose.
Proteins
Proteins are broken into amino acids.
Fats
Fats are broken into fatty acids and glycerol.
These smaller products can then be absorbed through the wall of the small intestine.
Absorption in the Small Intestine
The inner surface of the small intestine contains millions of tiny finger-like projections called villi.
Villi greatly increase the surface area available for absorption.
Each villus has:
- A thin surface.
- A rich network of blood capillaries.
- A structure called a lacteal associated with fat absorption.
These adaptations allow digested nutrients to be absorbed efficiently.
Why Villi Are Effective
The structure of villi is closely related to their function.
Large Surface Area
Millions of villi provide a very large surface for absorption.
Thin Surface
The surface is very thin, reducing the distance that substances must travel.
Good Blood Supply
Capillaries carry absorbed nutrients away, helping maintain concentration gradients.
Lacteals
Products of fat digestion can enter the lymphatic system through lacteals.
This is an excellent example of an important biological principle:
Structure is adapted to function.
What Happens to Absorbed Nutrients?
After absorption, nutrients are transported to other parts of the body.
For example:
Glucose can be transported in the blood and used in cellular respiration.
Amino acids can be used to make proteins.
Fatty acids and glycerol can be used to build lipids or provide energy.
The use of absorbed nutrients by cells is called assimilation.
Digestion and absorption therefore allow food molecules to become useful materials for the body's cells.
The Large Intestine
Material that cannot be digested or absorbed in the small intestine passes into the large intestine.
The large intestine includes the colon.
One of its major functions is the absorption of water and mineral ions from the remaining material.
This is important because large quantities of water enter the digestive system through:
- Food and drinks.
- Saliva.
- Gastric juice.
- Intestinal secretions.
Much of this water needs to be recovered.
Gut Microorganisms
The large intestine also contains a large community of microorganisms known collectively as the gut microbiota.
These microorganisms can:
- Break down some substances that human digestive enzymes cannot.
- Produce certain useful compounds.
- Interact with the immune system.
- Compete with some harmful microorganisms.
The digestive system therefore contains an entire biological community as well as human tissues.
Formation of Faeces
After much of the water has been absorbed, the remaining material becomes more solid.
Faeces contain:
- Undigested food material
- Fibre
- Bacteria
- Dead cells
- Water
- Other waste material
The faeces move toward the rectum.
The Rectum and Anus
The rectum stores faeces before they leave the body.
The anus is the opening through which faeces are expelled.
Muscular rings called sphincters help control the opening of the anus.
The removal of undigested material from the digestive tract is called egestion.
Egestion should not be confused with excretion.
Egestion removes undigested material from the digestive tract.
Excretion removes metabolic waste produced by cells, such as carbon dioxide or urea.
Tracing the Path of Food
The complete route travelled by food can be summarised as:
Mouth
↓
Oesophagus
↓
Stomach
↓
Small intestine
↓
Large intestine
↓
Rectum
↓
Anus
The liver, gall bladder and pancreas support digestion, but food does not pass through these organs.
The Digestive System as an Organ System
An organ system is a group of organs that work together to perform major functions.
The digestive system demonstrates this clearly.
The mouth prepares food and begins digestion.
The oesophagus transports food.
The stomach stores, mixes and chemically digests food.
The liver produces bile.
The gall bladder stores and releases bile.
The pancreas supplies digestive enzymes and bicarbonate.
The small intestine completes much of digestion and absorbs nutrients.
The large intestine absorbs water and forms faeces.
The rectum stores faeces.
The anus controls their removal.
No single organ could perform the entire process effectively by itself.
How the Digestive System Connects to Other Organ Systems
The digestive system does not work in isolation.
Circulatory System
Absorbed nutrients enter the blood and are transported to cells throughout the body.
Muscular System
Smooth muscles produce peristalsis and move food through the digestive tract.
Nervous System
Nerves help regulate digestive movements and secretions.
Endocrine System
Hormones help coordinate digestive processes and regulate the release of digestive substances.
Lymphatic System
Products of fat digestion enter lymphatic vessels within the intestinal villi.
The digestive system is therefore part of a much larger network of interacting organ systems.
Structure and Function
Digestive organs have structures suited to their particular functions.
| Structure | Feature | Function |
|---|---|---|
| Teeth | Hard surfaces and different shapes | Mechanical breakdown of food |
| Oesophagus | Muscular wall | Moves food by peristalsis |
| Stomach | Thick muscular wall | Churns and mixes food |
| Stomach lining | Mucus-producing cells | Protects tissues |
| Small intestine | Long and highly folded | Provides large area for digestion and absorption |
| Villi | Numerous finger-like projections | Increase absorption surface area |
| Capillaries in villi | Rich blood supply | Carry absorbed nutrients away |
| Large intestine | Long absorptive surface | Recovers water and ions |
| Rectum | Expandable muscular region | Temporarily stores faeces |
A Worked Example: Following a Sandwich
Imagine eating a sandwich containing bread, meat and some fat.
Mouth
The sandwich is chewed.
Amylase begins breaking down starch in the bread.
Oesophagus
Peristalsis moves the bolus toward the stomach.
Stomach
The food is churned.
Acid creates suitable conditions for stomach proteases, which begin significant protein digestion.
Small Intestine
Pancreatic enzymes continue digestion.
Bile helps emulsify fats.
Carbohydrates are broken into simple sugars, proteins into amino acids, and fats into fatty acids and glycerol.
Villi
Digested nutrients are absorbed.
Large Intestine
Water is absorbed from the remaining material.
Rectum and Anus
Undigested material is eventually removed from the body.
This example demonstrates how several organs cooperate to process a single meal.
Mechanical and Chemical Digestion
These two processes should be distinguished.
Mechanical Digestion
Mechanical digestion physically breaks food into smaller pieces without changing the chemical identity of its molecules.
Examples include:
- Chewing
- Stomach churning
- Emulsification of fats by bile
Chemical Digestion
Chemical digestion breaks large molecules into smaller molecules through chemical reactions, usually catalysed by enzymes.
Examples include:
- Starch → smaller sugars
- Proteins → amino acids
- Fats → fatty acids and glycerol
Mechanical digestion often makes chemical digestion more efficient by increasing surface area.
Common Mistakes
Saying Digestion Occurs Only in the Stomach
Digestion begins in the mouth and continues mainly in the stomach and small intestine.
Saying Food Passes Through the Liver or Pancreas
Food does not travel through these organs.
They produce substances that enter the digestive tract.
Saying the Gall Bladder Produces Bile
The liver produces bile.
The gall bladder stores and releases it.
Saying the Large Intestine Is Longer Than the Small Intestine
The small intestine is much longer. "Large" refers mainly to the greater diameter.
Confusing Digestion with Absorption
Digestion breaks large molecules into smaller molecules.
Absorption moves digested nutrients across the intestinal wall into the body's transport systems.
Confusing Egestion and Excretion
Egestion removes undigested material.
Excretion removes metabolic waste produced by cells.
Saying Stomach Acid Digests Everything
Stomach acid creates important conditions, but enzymes carry out much of the chemical digestion.
Check Your Understanding
1. What is the main function of the digestive system?
2. List the organs food passes through, in the correct order.
3. Explain two functions of the mouth during digestion.
4. What is peristalsis?
5. Explain how the structure of the oesophagus helps it perform its function.
6. Give two functions of the stomach.
7. What is the role of hydrochloric acid in the stomach?
8. Which organ produces bile?
9. What is the role of the gall bladder?
10. Give two digestive functions of the pancreas.
11. Why is the small intestine the main site of absorption?
12. Explain how villi are adapted for absorption.
13. What happens to glucose after it is absorbed?
14. What is an important function of the large intestine?
15. Explain why the digestive system is described as an organ system rather than simply a collection of separate organs.
Key Terms
- Digestive system – the organ system responsible for digestion and absorption of nutrients.
- Alimentary canal – the continuous digestive tube through which food travels.
- Digestion – the breakdown of large food molecules into smaller molecules.
- Mechanical digestion – physical breakdown of food into smaller pieces.
- Chemical digestion – chemical breakdown of large molecules, usually using enzymes.
- Peristalsis – waves of muscular contraction that move material through the digestive tract.
- Oesophagus – the muscular tube carrying food from the mouth to the stomach.
- Stomach – muscular organ that stores, churns and chemically digests food.
- Liver – organ that produces bile and performs many metabolic functions.
- Gall bladder – organ that stores and releases bile.
- Pancreas – organ that releases digestive enzymes and bicarbonate into the small intestine.
- Small intestine – the major site of chemical digestion and nutrient absorption.
- Villus – finger-like projection that increases the absorptive surface of the small intestine.
- Large intestine – region where water and ions are absorbed from remaining digestive material.
- Rectum – region that stores faeces before egestion.
- Egestion – removal of undigested material from the digestive tract.
- Absorption – movement of digested nutrients across the intestinal wall.
- Assimilation – incorporation and use of absorbed nutrients by cells and tissues.
Key Takeaways
- The digestive system breaks food into small, absorbable molecules.
- Food travels through the mouth → oesophagus → stomach → small intestine → large intestine → rectum → anus.
- Digestion begins in the mouth with chewing and the action of saliva.
- Peristalsis moves food through the digestive tract.
- The stomach stores, churns and chemically digests food.
- The liver produces bile, while the gall bladder stores and releases it.
- The pancreas supplies digestive enzymes and bicarbonate to the small intestine.
- Most chemical digestion is completed in the small intestine.
- Most nutrient absorption also occurs in the small intestine.
- Villi provide a large surface area and other adaptations for efficient absorption.
- The large intestine absorbs water and mineral ions from the remaining material.
- The rectum stores faeces before they are removed through the anus.
- Digestive structures are specialised for their functions.
- The digestive system works as an integrated organ system, with different organs performing complementary roles.