Chapter 2 · P4 Science
🫀 Human Systems
📚 Section A — Revision Notes
What I Will Learn
- Identify some human body systems and describe what each one does
- Name the parts of the digestive system and explain their roles
- Explain how the parts of the digestive system work as a team
1
Human Body Systems and Their Functions
- Our body is built from many systems, all running simultaneously to keep us strong and healthy.
- Each system is a team of organs — specialised body parts that each carry out a particular job. The system runs properly only when every organ plays its part.
- Scientists use models to represent human systems. A model captures how something looks or works in a simpler, easier-to-study form.
▲ The five human body systems
🫁 Respiratory System
- Made up of the nose, windpipe and lungs.
- Its job is to allow the exchange of gases between our body and the surrounding air.
💡 Did You Know?
When you breathe in, the respiratory system pulls in oxygen — a gas our body cannot survive without. It also flushes out carbon dioxide, a waste gas. The full story of how this works is covered in Primary 5.
🫀 Circulatory System
- Made up of the heart and blood vessels carrying blood.
- Acts as the body’s delivery network — shipping digested food and gases to every part of the body, and hauling waste materials away.
💡 Did You Know?
Blood vessels thread through every single part of your body — from the tips of your toes to the top of your head. You’ll explore the circulatory system in greater depth in Primary 5.
🦴 Skeletal System
- Built entirely from bones. Key bony structures include the skull, rib cage, backbone and many other bones.
- Gives our body its shape and acts as a shield that protects vital organs such as the heart and lungs.
- Teams up with the muscular system to power every movement our body makes.
💡 Did You Know?
The skull is a dome of fused bones that guards your brain. The rib cage forms a bony cage around your heart and lungs, keeping them safe from knocks and impacts.
💪 Muscular System
- Made entirely of muscles.
- Partners with the skeletal system to move the body.
- Some muscles, like those in our arms and legs, are under our voluntary control — we choose when to move them. Others, like the muscles of the heart, work on their own without us thinking about it.
💡 Did You Know?
The heart is one big muscle — and it never clocks off. It keeps beating throughout your entire life without ever taking a break.
🍽️ Digestive System
- Made up of the mouth, gullet, stomach, small intestine and large intestine.
- Its two main jobs are to break food down into simpler substances and to absorb those substances — along with water and minerals — so the body can use them.
📋 Summary — Human Systems at a Glance
| System | Key Parts | Function(s) |
|---|---|---|
| Respiratory |
|
Exchanges gases between our body and the surroundings |
| Circulatory |
|
|
| Skeletal |
|
|
| Muscular |
|
Works with the skeletal system to move the body |
| Digestive |
|
|
📋 Checkpoint 2.1
1. Name three human body systems and state the function of each.
- Respiratory system — exchanges gases between the body and the surroundings
- Circulatory system — carries digested food and gases to all parts of the body; removes waste
- Skeletal system — gives the body shape; protects organs; works with muscles to allow movement
- (Accept any three from the five systems above)
2
Parts of the Digestive System and Their Functions
- Our body runs on food — it supplies the energy we need and keeps all our systems functioning.
- Before food can be used by the body, it must be converted into simpler substances that can be absorbed into the blood.
Digestion — the process that transforms food into simpler substances small enough to be absorbed into the bloodstream and used by the body.
🎯 Get It Right!
Wrong concept: Digestion just breaks food into smaller pieces.
Correct concept: Digestion breaks food into simpler substances — not just smaller pieces. Smaller pieces alone cannot be absorbed into the blood. Only simpler substances can enter the bloodstream.
- Each part of the digestive system plays a role in digestion or absorption.
- Several parts produce digestive juices — liquids that chemically break food down.
▲ The digestive system
👄 Mouth — Where Digestion Begins
- Food enters through the mouth. The mouth produces saliva, which softens food and contains digestive juice — so digestion kicks off right here.
- Teeth chew food into tinier pieces, making it easier to swallow. Smaller pieces also expose more surface area to digestive juices, which speeds up digestion.
- The tongue tastes the food and rolls it into a ball shape so it can slide down more easily when swallowed.
💡 Did You Know?
The tongue is actually a sense organ — one of five. Our other sense organs are the eyes, ears, nose and skin.
▲ Chewing creates smaller pieces with a larger surface area, so digestive juice works faster
🔀 Gullet — The Food Slide
- Once swallowed, food enters the gullet — a muscular tube whose walls squeeze and push food from the mouth down into the stomach.
- No digestion takes place in the gullet. It is purely a passageway.
🫙 Stomach — The Mixing Chamber
- The stomach is a muscular pouch that churns food and drenches it with digestive juices it produces itself.
- This combination of churning and juice breaks food down further, continuing the digestion process.
🌀 Small Intestine — The Main Event
- Part-digested food arriving from the stomach meets even more digestive juices here. Digestion is completed in the small intestine.
- The digested substances are then absorbed through the intestine walls into the blood, which carries them to the rest of the body.
💡 Did You Know?
The inner lining of the small intestine is covered in tiny folds. These folds dramatically increase the surface area available for absorption — the more surface area, the faster digested food passes into the blood.
🔄 Large Intestine — Water Recovery
- Whatever the small intestine couldn’t digest moves on to the large intestine.
- The large intestine’s key job is to absorb water from this undigested material back into the blood.
💡 Did You Know?
Not everything we eat gets digested. What’s left is stored in a section called the rectum and eventually passes out of the body as waste through the anus.
📋 Checkpoint 2.2
1. Name two organs in the digestive system where digestion occurs.
- Digestion occurs in the mouth, the stomach and the small intestine (any two of these three)
- It does not occur in the gullet or the large intestine
💡 Exam Tip — Reading Digestion Graphs
Bar graphs showing the amount of digested or undigested food in each organ appear frequently in exams. Study them carefully — the key rules to remember are:
- Digestion starts in the mouth and ends in the small intestine — so digested food increases as food travels through.
- The gullet has the same amount of digested food as the mouth — because no digestion happens there.
- The large intestine has zero digested food — all digested food was already absorbed in the small intestine.
- Undigested food decreases as it moves through — the digestive system keeps breaking more down.
- Mouth and gullet are the same height — no digestion in the gullet, so the same amount of undigested food leaves.
- Small intestine and large intestine are the same height — no digestion in the large intestine, so the same amount of undigested food passes through.
🎯 Get It Right!
Wrong concept: Digestion only happens in the stomach.
Correct concept: Digestive juices are made in the mouth, stomach and small intestine — so digestion occurs in all three. The gullet and large intestine produce no digestive juices, so no digestion takes place there.
3
How the Digestive System Works as a Team
- Every part of the digestive system must function well together for food to be properly digested. If any one part fails, digestion is disrupted and health problems can follow.
- The digestive system also relies on other body systems:
- The muscular system pushes food along the digestive tract through muscle contractions.
- The circulatory system collects the digested nutrients and ferries them — via the blood — to every corner of the body.
📋 Checkpoint 2.3
1. If part of the small intestine is removed, will the rate of digestion of food increase, decrease or remain the same? Explain your answer.
- The rate of digestion will decrease.
- The small intestine is where digestion is completed. Removing part of it reduces the amount of digestive juice produced and the surface area available for absorption, so food cannot be fully digested or absorbed as efficiently.
⚙️ Worked Example 2.1
Zhi Han wants to find out whether the size of food affects how quickly it is digested. He uses two containers with the same amount of food and the same digestive juice. Container A has one large chunk of food. Container B has the same food broken into smaller pieces.
▲ Container A (large piece) vs Container B (smaller pieces) — same total amount of food and digestive juice
(a) Name two other variables that should be kept the same for a fair test. [1]
(b) Container B’s food was digested faster. Based on this result, explain why chewing helps with faster digestion. [2]
(a) Any two of: the type of food used, the amount and type of digestive juice, the temperature, the location of the containers, the size and type of containers.
(b) When we chew, teeth break food into smaller pieces. Smaller pieces have a larger surface area in contact with digestive juice. A larger surface area means the digestive juice can act on more of the food at once — this speeds up digestion.
💡 Exam Tip — Watch the Word “Other”
When a question asks you to “Name two other variables to keep the same”, underline the word other. It means you must not repeat any variable already mentioned in the question. In this case, the amount of food is already stated — so do not write that down again.
📌 Chapter Summary
- The body runs on five key systems: respiratory, circulatory, skeletal, muscular and digestive — each made of organs with specific roles.
- The respiratory system (nose, windpipe, lungs) swaps gases between body and surroundings.
- The circulatory system (heart, blood vessels) delivers nutrients and gases; clears waste.
- The skeletal system (bones) shapes the body, shields organs, and teams with muscles to enable movement.
- The muscular system drives movement alongside bones; some muscles (like the heart) work involuntarily.
- Digestion = breaking food into simpler substances that can enter the blood — not just smaller pieces.
- Digestion starts in the mouth (saliva + teeth + tongue) and is completed in the small intestine.
- The gullet is a passageway only — no digestion. The large intestine absorbs water from undigested food.
- On graphs: mouth and gullet have the same digested-food level; large intestine has zero digested food.
- Chewing works because smaller food pieces have greater surface area — more contact with digestive juice = faster digestion.
- The digestive system links with the muscular (moves food along) and circulatory (transports nutrients) systems.
✏️ Practice Questions — MCQ
1. Which system transports digested food to all parts of the body?
✅ Correct! The circulatory system — through the heart and blood vessels — is the body’s delivery network, shipping digested food to every part of the body.
❌ The circulatory system (heart + blood vessels) carries digested food around the body. The digestive system breaks food down; it doesn’t transport it.
2. In which of the following organs does digestion NOT occur?
✅ Correct! The gullet is purely a passageway — it has no digestive juice and carries out no digestion at all.
❌ The gullet produces no digestive juice, so no digestion occurs there. It simply pushes food from the mouth to the stomach.
3. A student says “digestion breaks food into smaller pieces”. What is wrong with this statement?
✅ Correct! Digestion converts food into chemically simpler substances that can be absorbed into the blood. Smaller pieces alone cannot pass through the intestine walls.
❌ The key error is the word “pieces”. Digestion produces simpler substances — a chemical change — not just physically smaller bits. Only simpler substances can enter the bloodstream.
4. Where is water absorbed back into the blood?
✅ Correct! The large intestine’s main role is to recover water from undigested material and return it to the bloodstream.
❌ It’s the large intestine that absorbs water. The small intestine absorbs digested food — but water recovery happens further along in the large intestine.
5. Why does chewing food help it to be digested faster?
✅ Correct! Smaller pieces from chewing have a greater total surface area, giving digestive juice more food to act on at once — which speeds up digestion.
❌ Chewing creates smaller pieces with a larger combined surface area. More surface area means more contact with digestive juice, so food is broken down faster.
6. Which body system protects the heart and lungs?
✅ Correct! The rib cage (part of the skeletal system) wraps around and shields the heart and lungs from injury.
❌ The skeletal system protects vital organs. The rib cage specifically shields the heart and lungs; the skull shields the brain.
7. The diagram shows the amount of digested food in each organ of the digestive system. Which organ is labelled X on the graph if it shows the greatest amount of digested food?
✅ Correct! Digestion is completed in the small intestine, so it contains the greatest amount of digested food before that food is absorbed into the blood.
❌ The small intestine is where digestion ends, so it has the highest amount of digested food. After that, all digested food is absorbed — leaving none in the large intestine.
8. If a person’s stomach stops producing digestive juice, what is the most likely effect?
✅ Correct! All parts must work together. If the stomach produces no digestive juice, food arriving at the small intestine is insufficiently broken down — the whole digestion process is disrupted.
❌ When one part of the digestive system fails, the whole system is affected. Without stomach juice, food isn’t properly broken down and can’t be absorbed, leading to health problems.
9. Some muscles in our body work without us thinking about them. Which of the following is an example of such a muscle?
✅ Correct! The heart muscle works involuntarily — it beats continuously throughout our lives without us needing to consciously control it.
❌ The heart is an involuntary muscle — it keeps beating on its own, without any conscious effort from us. Arm, leg and jaw muscles are all under our voluntary control.
10. In a fair test to investigate how food size affects digestion, which variable should be CHANGED?
✅ Correct! In a fair test, only the variable being investigated is changed. Here, we are testing food size — so only the size of food pieces should differ between set-ups.
❌ In this experiment, food size is what’s being tested — so that’s the changed variable. Everything else (amount of juice, type of food, temperature) must stay the same to make it a fair test.
✏️ Practice Questions — Open-Ended
1. Explain why digestion is important to the human body. [2]
Food provides the body with energy and the materials needed to function properly. However, food must first be broken down into simpler substances through digestion before the body can absorb and use them.
2. A student claims that the gullet is where most digestion takes place. Do you agree? Explain your answer. [2]
I disagree. No digestion occurs in the gullet at all — it produces no digestive juice and simply pushes food from the mouth into the stomach. The organ where the most digestion occurs is the small intestine, where digestion is completed.
3. Explain why the small intestine is important in digestion. Give TWO reasons. [2]
- The small intestine is where digestion is completed — it produces digestive juice that breaks food down into simpler substances.
- It is also where digested food is absorbed into the blood, through the walls of the small intestine, so nutrients can be transported to the rest of the body.
4. How do the muscular and circulatory systems support the digestive system? [2]
The muscular system provides the muscle contractions that push food along the digestive tract. The circulatory system then carries the digested substances — via the blood — to all parts of the body so they can be used.
5. Priya carried out an experiment to see if temperature affects how fast food is digested. She used two containers with the same amount of food and the same type and amount of digestive juice. Container A was kept at 20°C and Container B at 37°C. State the changed variable and name ONE other variable that should be kept the same. [2]
- Changed variable: the temperature of the containers (20°C vs 37°C)
- One variable to keep the same (any one of): the type of food used / the size of food pieces / the size and type of containers / the location where the containers are kept