Primary Science

PSLE Science Handbook: Respiratory and Circulatory System

Respiratory and Circulatory System

Hey there, young scientists and curious parents! Are you ready to dive into the fascinating world of the respiratory and circulatory system? Buckle up because we’re about to embark on an adventure that will make your heart race and your lungs inflate with excitement! We’re here to demystify these crucial systems and help you conquer your science exams like a pro. So, take a deep breath (quite literally!) and get ready to uncover the secrets of how your body breathes and circulates blood. Don’t worry, no lab coats required – just a curious mind and a sprinkle of humor! Let’s dive in and explore the magnificent world within us.

Let’s check out the Respiratory System

Take a breath, hold it, and let it out.

Did you notice your ribcage moving up and out? 


  • With every inhalation, the diaphragm moves downwards, and air fills the lungs.
  • With every exhalation, the diaphragm moves upwards, and air rushes back out. 

This is breathing. 

When we hiccup, it is our diaphragms spasming and forcing us to suddenly suck air into the throat, hitting the voice box, creating the “hic!” sound. The respiratory system takes oxygen from the air we breathe and expels unwanted carbon dioxide. Our respiratory system consists of our nose, windpipe, and lungs. Sometimes we breathe through our mouths as well. 


The gases that make up air are:

  • Nitrogen (78%) 
  • Oxygen (21%)
  • Carbon dioxide, water vapour, and other gases (such as Argon) (1%)

In the lungs, there are air sacs which facilitate gaseous exchange. These air sacs are surrounded by many blood vessels. (They have a peculiar grape-like shape as this increases surface area in contact with blood vessels to absorb oxygen into the bloodstream faster).


In the lungs at the air sacs, the oxygen in the air enters the bloodstream. Oxygen-rich (oxygenated) blood travels to the heart. At the same time, carbon dioxide is exchanged at the air sacs to be exhaled out of the body. The respiratory system and circulatory system work together in this complicated process. 


And now, onwards to the Respiratory System

The circulatory system consists of the heart, blood vessels, and blood.


Our red blood cells carry important substances! Think of them as delivery menThey transport important substances such as digested food, oxygen, water and even waste products like carbon dioxideOur blood vessels are like “roads” or tunnels for these “delivery men” transporting substances to flow through. Think of the heart as a pump, directing where the blood flows to.



But how is a higher percentage of carbon dioxide exhaled out? 

The circulatory system and respiratory system work together in the following steps:  

  2. Air sacs in lungs fill with air.
  3. Blood in the blood vessels surrounding the air sacs absorbs oxygen.
  4. Oxygenated (oxygen-rich) blood is transported to the heart.
  5. Heart pumps oxygenated blood to all parts of the body.
  6. The cells of the body utilise the oxygen during respiration (to get energy), and release carbon dioxide.
  7. The blood absorbs carbon dioxide from our cells.
  8. The deoxygenated blood (blood richer in carbon dioxide) is transported back to the heart.
  9. Deoxygenated blood is pumped by the heart back to the lungs.
  10. Deoxygenated blood in the air sacs remove carbon dioxide, exchanging with oxygen.
  11. EXHALE

However, do note that we do not exhale 100% carbon dioxide! There is still oxygen in the air we breathe out, or else we would not be able to perform CPR (Cardiopulmonary resuscitation).

This is the composition of air/ percentage of gases in inhaled and exhaled air:

inhaled-exhaled-air-compositionLet’s practise what we’ve learnt on a real-life application question!

The amount of oxygen in the atmosphere changes at different heights above sea levels.

Ahmad went on an overseas mountain climbing trip.

(a) State the relationship between the amount of oxygen and the height above sea level. Explain how Ahmad’s heart rate would change as he climbed up the mountain. (2 marks)

As the height above sea level increases, the amount of oxygen decreases. 

Ahmad’s heart rate would be faster as he climbed up the mountain as he needs more energy. His heart needs to pump blood faster to transport more oxygen and digested food to all body parts and respire to release energy faster.

Take note: do not state pump “more” blood! The blood only flows faster, the amount of blood stays the same. 

(b) Mason is a mountain guide and lives in the mountains.


Explain why Mason has bigger lungs and heart when compared to those who live below the mountains. (2 marks)

The amount of oxygen is lower up in the mountain than below the mountains. Mason has bigger lungs to take in more oxygen with each breath and a bigger heart to pump blood faster to supply more oxygen to all parts of the body.

Fun fact: As altitude (height between an object and the Earth’s surface) increases, less oxygen is available for breathing (due to lower air pressure). One normal effect of altitude is shortness of breath, since the lungs have to work harder to deliver oxygen to the bloodstream.

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