PSLE Science · Primary 5 and 6
Human Body Systems
The human body is made up of systems that each do an important job, and the systems work together to keep us alive. In PSLE Science this topic belongs to the Systems theme. You need to know the organs of the digestive, respiratory and circulatory systems, what the skeletal and muscular systems do, and, most importantly, how the systems work together, for example when you exercise. Questions often give data on heart rate or breathing rate and ask you to explain it, so this guide shows how.
What the syllabus expects
- State the functions of the skeletal and muscular systems
- Identify the organs of the digestive system and describe what happens to food in each
- Describe how the respiratory system takes in oxygen and removes carbon dioxide
- Describe how the circulatory system transports oxygen, digested food and carbon dioxide
- Explain how the systems work together, for example during exercise
- Calculate and interpret heart rate and breathing rate data
Organs, systems, bones and muscles
The body is made up of cells. Organs, such as the heart and the stomach, are made of cells, and a system is a group of organs working together to perform a function. The skeletal system is made up of bones. It supports the body, gives it shape, protects soft organs (the skull protects the brain, and the rib cage protects the heart and lungs) and allows movement. The muscular system is made up of muscles. Muscles contract and pull on the bones they are attached to, which moves the bones at joints. Muscles can only pull, not push, so they work in pairs. The heart is also a muscle, and it contracts to pump blood all the time.
The digestive system
Digestion is the breaking down of food into simpler substances that can be absorbed into the blood. Food enters the mouth, where the teeth cut and grind it into smaller pieces and saliva, a digestive juice, starts to digest it. The gullet carries the food down to the stomach. The stomach walls churn the food and mix it with digestive juices, so more digestion takes place. In the small intestine, more digestive juices are added and digestion is completed. The digested food is absorbed through the walls of the small intestine into the bloodstream. The undigested food passes into the large intestine, where water is absorbed, and the remaining waste leaves the body through the anus. The small intestine is long, which gives more time and more surface for digested food to be absorbed.
The respiratory and circulatory systems
The respiratory system includes the nose, the windpipe and the lungs. When we breathe in, air enters through the nose and travels down the windpipe to the lungs. In the lungs, oxygen from the air passes into the blood, and carbon dioxide passes from the blood into the lungs, to be breathed out. Air we breathe out contains less oxygen and more carbon dioxide than the air we breathe in. The circulatory system is made up of the heart, the blood vessels and the blood. The heart pumps blood to all parts of the body through the blood vessels. Blood carries oxygen from the lungs and digested food from the small intestine to every part of the body. It carries carbon dioxide from all parts of the body back to the lungs. Each heartbeat pushes blood through the blood vessels, which you can feel as your pulse at the wrist.
Systems working together
Cells need energy to work. They get it by using oxygen to release energy from digested food, and this produces carbon dioxide as a waste. So the digestive system supplies digested food, the respiratory system supplies oxygen and removes carbon dioxide, and the circulatory system carries all of these between the organs and the cells. During exercise, the muscles work harder and need more energy. They need more oxygen and digested food, and they produce more carbon dioxide. The breathing rate increases, so more oxygen is taken in and more carbon dioxide is removed. The heart beats faster, so blood carries oxygen and digested food to the muscles, and carbon dioxide away from them, more quickly. After exercise, both rates gradually return to their resting values.
Worked example
Jun Wei measured his heart rate and breathing rate at rest and again after running for 5 minutes. At rest: heart rate 75 beats per minute, breathing rate 16 breaths per minute. After running: heart rate 120 beats per minute, breathing rate 30 breaths per minute. (a) By how much did his heart rate increase? (b) Explain why his breathing rate and heart rate both increased. (c) Name another system that supplies what his muscles need, and state what it supplies.
- For (a), subtract: 120 − 75 = 45 beats per minute.
- For (b), start with the muscles: when running, his leg muscles work harder and need more energy.
- To release more energy, the muscles need more oxygen and digested food, and they produce more carbon dioxide.
- Breathing rate increased so that more oxygen could be taken into the lungs and more carbon dioxide removed from the body.
- Heart rate increased so that blood transported oxygen and digested food to the muscles, and carbon dioxide away from them, faster.
- For (c), the digestive system supplies the digested food that is absorbed into the blood in the small intestine.
Answer: (a) 45 beats per minute. (b) His muscles needed more energy, so they needed more oxygen and digested food and produced more carbon dioxide. He breathed faster to take in more oxygen and remove more carbon dioxide, and his heart beat faster to transport these substances to and from the muscles more quickly. (c) The digestive system, which supplies digested food.
Common mistakes
- Saying digestion only happens in the stomach. It starts in the mouth, continues in the stomach and is completed in the small intestine.
- Saying digested food is absorbed in the large intestine. Digested food is absorbed in the small intestine. The large intestine absorbs water.
- Saying we breathe in only oxygen and breathe out only carbon dioxide. We breathe in air. The air we breathe out has less oxygen and more carbon dioxide.
- Saying the heart makes or cleans blood. The heart pumps blood around the body through the blood vessels.
- Explaining a faster heart rate with 'the body needs more oxygen' only. Say that the muscles need more energy, so more oxygen and digested food must be carried to them faster.
Exam tips
- When asked how two systems work together, name both systems and say what each one does in that situation.
- In heart rate questions, show how you change the count to one minute, and give the unit, beats per minute.
- Use the phrases 'absorbed into the bloodstream in the small intestine' and 'oxygen passes into the blood in the lungs'. They match what markers look for.
Practice questions
1. Easy: Put these parts in the order that food passes through them: stomach, mouth, large intestine, gullet, small intestine.
Show answer
Mouth, gullet, stomach, small intestine, large intestine. The undigested waste then leaves the body through the anus.
2. Medium: Siti counted 21 pulse beats in 15 seconds. Calculate her heart rate in beats per minute.
Show answer
84 beats per minute. One minute is 60 seconds, which is 4 lots of 15 seconds, so 21 × 4 = 84.
3. Harder: A man's lungs are damaged by disease, so less oxygen can pass into his blood. Explain why he gets tired easily.
Show answer
Less oxygen passes into his blood in the lungs, so less oxygen is carried to his muscles and other cells. With less oxygen, his cells cannot release as much energy from digested food, so he gets tired easily.
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