Human Digestive System
Explore how your body breaks down food into nutrients that give you energy, help you grow, and keep you healthy.
What Is Digestion?
Digestion is the process of breaking down food into tiny pieces called nutrients that your body can absorb and use. Your digestive system is like a long tube that runs from your mouth to your bottom, and it is about 9 metres long in total!
There are two types of digestion:
Mechanical Digestion
Physically breaking food into smaller pieces -- like chewing with your teeth or the churning action of your stomach.
Chemical Digestion
Using special chemicals called enzymes to break food down into even smaller molecules that can be absorbed.
The Journey of Food
Follow the path food takes through your digestive system:
Mouth
Your teeth chew food into smaller pieces (mechanical). Saliva contains enzymes that start breaking down starches (chemical). Your tongue helps mix and push food to the back of your throat.
Oesophagus
A muscular tube that pushes food from the mouth to the stomach using wave-like muscle contractions called peristalsis. It works even if you are upside down!
Stomach
A muscular bag that churns food and mixes it with stomach acid and enzymes. Food stays here for 2--4 hours, turning into a thick liquid called chyme.
Small Intestine
About 6 metres long! This is where most nutrients are absorbed into the blood. The walls are covered in tiny finger-like projections called villi to increase the surface area for absorption.
Large Intestine
About 1.5 metres long but much wider. Absorbs water from undigested food. The remaining waste becomes solid faeces, stored in the rectum until it leaves the body.
Helper Organs
Some organs are not part of the digestive tube but play important roles in digestion:
Liver
Produces bile, which helps break down fats. Also processes nutrients from the blood and removes toxins.
Gallbladder
Stores bile made by the liver and releases it into the small intestine when fatty food needs to be digested.
Pancreas
Produces powerful digestive enzymes and releases them into the small intestine. Also controls blood sugar levels.
Nutrients Your Body Needs
The food you eat contains different types of nutrients that your body needs to work properly:
- ●Carbohydrates -- Your main source of energy (bread, rice, pasta, fruit)
- ●Proteins -- Help build and repair muscles and cells (meat, eggs, beans, nuts)
- ●Fats -- Provide long-lasting energy and protect organs (butter, oils, avocado)
- ●Vitamins & Minerals -- Keep your body healthy and fight disease (fruit, vegetables)
- ●Water -- Essential for every body function. You should drink about 6--8 glasses a day.
Key Vocabulary
Enzyme
A special chemical that speeds up the breakdown of food into smaller molecules.
Peristalsis
Wave-like muscle contractions that push food through the digestive system.
Villi
Tiny finger-like projections in the small intestine that absorb nutrients into the blood.
Nutrients
Substances in food that the body needs for energy, growth, and repair.
Worked Examples
What happens when you eat a sandwich?
Step 1: Your teeth chew the bread and fillings into smaller pieces (mechanical digestion). Saliva starts breaking down the starch in the bread (chemical digestion).
Step 2: You swallow and peristalsis pushes the food down the oesophagus to the stomach.
Step 3: Stomach acid and enzymes break it into chyme. It then moves to the small intestine where nutrients are absorbed.
Answer: The sandwich goes through mechanical and chemical digestion, with nutrients absorbed in the small intestine and water absorbed in the large intestine.
Why is the small intestine so long?
Step 1: The small intestine is where most nutrients are absorbed into the blood.
Step 2: A longer tube means more surface area for absorption. The villi further increase this surface area.
Answer: The small intestine is long (about 6 m) and has villi to maximise the surface area available for absorbing nutrients from digested food.
Why does your stomach not digest itself?
Step 1: The stomach produces strong acid and enzymes to break down food.
Step 2: The stomach wall is lined with a thick layer of mucus that protects it.
Answer: A protective layer of mucus coats the inside of the stomach, creating a barrier between the stomach wall and the acid.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
Where does most nutrient absorption take place?
Question 2
What is the wave-like muscle movement that pushes food through the digestive system called?
Question 3
Which organ produces bile to help break down fats?
Question 4
What is the main role of the large intestine?
Question 5
Which nutrient group is your body's main source of energy?
Key Concepts Summary
- ●Digestion breaks food down into nutrients using mechanical and chemical processes.
- ●Food travels: mouth → oesophagus → stomach → small intestine → large intestine.
- ●The small intestine is where most nutrients are absorbed, helped by villi.
- ●The liver, gallbladder, and pancreas are helper organs that produce digestive chemicals.
- ●Your body needs carbohydrates, proteins, fats, vitamins, minerals, and water to stay healthy.