Human Body Systems
Explore the digestive, circulatory, and respiratory systems -- and discover how they work together to keep you alive.
The Digestive System
The digestive system breaks down the food you eat into tiny nutrients your body can use for energy, growth, and repair. It is like a long, winding tube that processes food from start to finish.
The Digestive Pathway
Mouth
Food is chewed and mixed with saliva
Oesophagus
Tube that pushes food to the stomach
Stomach
Acids break food into a mushy liquid
Small Intestine
Nutrients are absorbed into the blood
Large Intestine
Water is absorbed; waste is formed
Waste exits the body
Undigested food leaves as faeces
Amazing fact: Your small intestine is about 6 metres long -- that is longer than most rooms! It is tightly coiled to fit inside your body.
The Circulatory System
The circulatory system is your body's transport network. It moves blood around your body, delivering oxygen and nutrients to every cell and collecting waste to be removed.
Heart
A muscular pump about the size of your fist. It beats around 100,000 times per day, pushing blood through your body.
Blood Vessels
Arteries carry blood away from the heart. Veins carry blood back. Capillaries are tiny vessels connecting them.
Blood
Carries oxygen, nutrients, and waste products. Red blood cells carry oxygen; white blood cells fight germs.
How Blood Flows
Heart pumps
Arteries
(blood to body)
Capillaries
(exchange happens)
Veins
(blood to heart)
Back to heart
The Respiratory System
The respiratory system is responsible for breathing. It brings oxygen into your body (which your cells need) and removes carbon dioxide (a waste gas).
How Breathing Works
Breathing In (Inhale)
- 1. Air enters through your nose/mouth
- 2. Travels down the windpipe (trachea)
- 3. Enters the lungs through bronchi (tubes)
- 4. Reaches tiny air sacs (alveoli)
- 5. Oxygen passes into the blood
Breathing Out (Exhale)
- 1. Carbon dioxide passes from blood to alveoli
- 2. Travels back up through bronchi
- 3. Up the windpipe
- 4. Out through nose/mouth
- 5. Waste gas is removed!
How These Systems Work Together
Digestive + Circulatory: The digestive system breaks food into nutrients. The circulatory system picks up these nutrients from the small intestine and delivers them to every cell in the body.
Respiratory + Circulatory: The respiratory system gets oxygen from the air. The circulatory system carries that oxygen to all your cells and brings back carbon dioxide to be breathed out.
All three together: Your cells need oxygen (from breathing) and nutrients (from food) to produce energy. The blood carries both to where they are needed. Without any one system, the others cannot function!
Key Vocabulary
Organ
A part of the body made of tissues that performs a specific job (e.g. heart, lungs, stomach).
Nutrient
A substance in food that the body needs for energy, growth, and repair.
Artery
A blood vessel that carries blood away from the heart to the body.
Vein
A blood vessel that carries blood back to the heart from the body.
Lungs
Two spongy organs in your chest where oxygen enters the blood and carbon dioxide is removed.
Diaphragm
A dome-shaped muscle under the lungs that contracts to help you breathe in and out.
Worked Examples
Trace the journey of a bite of apple through the digestive system.
Step 1: In the mouth, the apple is chewed into smaller pieces and mixed with saliva.
Step 2: It travels down the oesophagus to the stomach, where acids break it down further.
Step 3: In the small intestine, nutrients (vitamins, sugars) are absorbed into the blood.
Step 4: In the large intestine, water is absorbed. Any leftover waste exits the body.
Why does your heart beat faster when you exercise?
Step 1: When you exercise, your muscles work harder and need more oxygen and energy.
Step 2: Your heart beats faster to pump blood more quickly, delivering more oxygen to your muscles.
Step 3: You also breathe faster to get more oxygen into your lungs. The circulatory and respiratory systems work together!
What is the difference between arteries and veins?
Arteries: Carry blood away from the heart. They carry oxygen-rich blood (usually). They have thick, muscular walls.
Veins: Carry blood back to the heart. They carry blood with less oxygen. They have thinner walls and valves to stop blood flowing backwards.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
Where are nutrients absorbed into the blood in the digestive system?
Question 2
What is the main job of the heart?
Question 3
Which gas do we breathe in that our bodies need?
Question 4
Blood vessels that carry blood away from the heart are called:
Question 5
Which TWO body systems work together when you eat food and the nutrients are delivered to your cells?
Key Concepts Summary
- ●The digestive system breaks down food into nutrients: mouth → oesophagus → stomach → small intestine → large intestine.
- ●The circulatory system (heart, blood vessels, blood) transports oxygen and nutrients to every cell.
- ●The respiratory system (lungs, windpipe) brings oxygen in and removes carbon dioxide.
- ●Arteries carry blood away from the heart; veins carry blood back to the heart.
- ●All three systems work together to keep your body alive and functioning.