Introduction

Every day, your body needs energy and nutrients to grow, repair tissues, and carry out thousands of chemical reactions. However, the food you eat cannot be used by your cells immediately. It must first be broken down into smaller molecules that can be absorbed into the bloodstream. This is the job of the digestive system.

The digestive system is a series of organs that work together to digest food, absorb nutrients, and remove undigested waste. Along the way, several accessory organs—including the liver, pancreas, and gallbladder—produce substances that help make digestion more efficient. Without the digestive system, your body could not obtain the nutrients needed to survive.

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What Is the Digestive System?

The digestive system is an organ system that:

• Breaks food into smaller molecules.
• Absorbs nutrients into the bloodstream.
• Eliminates undigested waste.

Digestion involves two main processes:

• Mechanical digestion – Physically breaking food into smaller pieces.
• Chemical digestion – Using enzymes and digestive juices to break large molecules into smaller ones.

Together, these processes make nutrients small enough to be absorbed by the body.

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Figure 1. The digestive system breaks down food, absorbs nutrients, and removes waste.

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Major Organs of the Digestive System

The digestive tract consists of:

• Mouth
• Esophagus
• Stomach
• Small intestine
• Large intestine
• Rectum
• Anus

Three important accessory organs assist digestion:

• Liver
• Pancreas
• Gallbladder

Each organ has a specialised role.

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The Mouth

Digestion begins in the mouth.

The mouth performs both mechanical and chemical digestion.

Functions

• Teeth chew food into smaller pieces.
• The tongue mixes food with saliva.
• Salivary glands produce saliva.
• Saliva contains the enzyme amylase, which begins the digestion of starch.

Chewing increases the surface area of food, making chemical digestion more efficient.

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The Esophagus

The esophagus is a muscular tube connecting the mouth to the stomach.

Food is moved by peristalsis—wave-like contractions of smooth muscles.

Peristalsis pushes food toward the stomach even when a person is lying down.

The esophagus does not digest food but transports it safely to the stomach.

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Figure 2. Food travels from the mouth through the esophagus to the stomach by peristalsis.

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The Stomach

The stomach is a muscular organ that stores and digests food.

Functions

• Churns food to mix it with digestive juices.
• Produces hydrochloric acid (HCl).
• Produces enzymes such as pepsin, which digests proteins.
• Kills many harmful microorganisms.

The stomach turns food into a thick liquid called chyme, which is gradually released into the small intestine.

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The Small Intestine

The small intestine is the main site of digestion and nutrient absorption.

It is about 6 metres long in adults.

Functions

• Completes chemical digestion.
• Absorbs nutrients into the bloodstream.
• Contains tiny finger-like projections called villi, which greatly increase the surface area for absorption.

Nutrients such as glucose, amino acids, fatty acids, vitamins, and minerals pass through the villi into the blood.

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Figure 3. Villi provide a large surface area for absorbing nutrients into the bloodstream.

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The Large Intestine

The large intestine absorbs:

• Water
• Mineral salts

from undigested food.

Helpful bacteria living in the large intestine:

• Produce some vitamins.
• Help break down certain materials.

The remaining waste becomes faeces and is stored in the rectum before leaving the body through the anus.

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The Pathway of Food

Food follows this route through the digestive system:

Mouth → Esophagus → Stomach → Small Intestine → Large Intestine → Rectum → Anus

Each organ performs a specialised function before food moves to the next stage.

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Figure 4. Food passes through each organ in a specific order during digestion.

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Accessory Organs

Although food does not pass through them, these organs are essential for digestion.

Liver

The liver is the largest internal organ.

Functions include:

• Producing bile.
• Processing absorbed nutrients.
• Storing glycogen.
• Detoxifying harmful substances.

Bile helps break large fat droplets into smaller ones, making them easier for enzymes to digest.

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Gallbladder

The gallbladder stores bile produced by the liver.

When fatty food enters the small intestine, the gallbladder releases bile into the digestive tract.

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Pancreas

The pancreas produces:

• Digestive enzymes.
• Bicarbonate solution.

The enzymes digest carbohydrates, proteins, and fats.

Bicarbonate neutralises the acidic chyme entering from the stomach, protecting the lining of the small intestine.

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Figure 5. The liver, pancreas, and gallbladder are accessory organs that assist digestion.

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How the Digestive System Maintains Life

The digestive system supplies the body with:

• Glucose for energy.
• Amino acids for growth and repair.
• Fatty acids for energy storage and cell membranes.
• Vitamins and minerals for healthy body functions.
• Water for cellular processes.

Without digestion and absorption, cells would not receive the nutrients needed to survive.

The digestive system also works closely with the:

• Circulatory system, which transports absorbed nutrients.
• Muscular system, which moves food through the digestive tract.
• Nervous system, which helps regulate digestion.
• Endocrine system, which releases hormones that control digestive processes.

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Worked Example

Question

Complete the pathway of food through the digestive system.

Mouth → ________ → Stomach → ________ → Large intestine → Rectum → Anus

Solution

Mouth → Esophagus → Stomach → Small intestine → Large intestine → Rectum → Anus

Food is digested and nutrients are mainly absorbed in the small intestine.

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Real-World Connection

After eating a meal, carbohydrates are broken down into glucose, which is absorbed through the villi of the small intestine into the bloodstream. The circulatory system carries this glucose to body cells, where it is used in cellular respiration to release energy. This energy powers everything from thinking and walking to growing and repairing tissues.

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Did You Know?

The small intestine is about 6 metres long, but it fits comfortably inside your abdomen because it is tightly coiled. Its lining contains millions of villi and microscopic microvilli, creating an enormous surface area—about the size of a tennis court—for absorbing nutrients efficiently.

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Key Terms

Amylase – An enzyme in saliva that begins the digestion of starch.

Bile – A digestive fluid produced by the liver that helps break large fat droplets into smaller ones.

Chyme – The semi-liquid mixture of partially digested food and digestive juices that leaves the stomach.

Digestion – The mechanical and chemical breakdown of food into smaller molecules.

Esophagus – The muscular tube that carries food from the mouth to the stomach.

Gallbladder – An organ that stores and releases bile.

Peristalsis – Wave-like muscle contractions that move food through the digestive tract.

Small intestine – The main site of digestion and nutrient absorption.

Villi – Tiny finger-like projections that increase the surface area of the small intestine for absorption.

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Key Takeaways

• The digestive system breaks down food, absorbs nutrients, and removes undigested waste.
• Food follows the pathway: mouth → esophagus → stomach → small intestine → large intestine → rectum → anus.
• The mouth, stomach, and small intestine perform both mechanical and chemical digestion.
• Most nutrient absorption occurs in the small intestine, while the large intestine mainly absorbs water and salts.
• The liver, pancreas, and gallbladder are accessory organs that produce or store substances needed for digestion.
• The digestive system works with other organ systems to provide the nutrients and energy needed to maintain life.