Science · Cells & organisms
Organ systems
Heart, lungs, kidneys, brain.
What are organ systems?
Your body contains different organs that work together in teams called organ systems. Each system has a special job. The heart pumps blood around your body. The lungs take in oxygen from air. The kidneys clean waste from your blood. The brain controls everything you think and do. These organs cannot work alone—they depend on each other to keep you alive and healthy.
The heart and circulation
The heart is a powerful muscle that beats about 70 times every minute. It pumps blood through tubes called blood vessels (arteries, veins and capillaries). Blood carries oxygen and nutrients to every cell in your body. It also collects carbon dioxide and other waste products. The heart has four chambers that fill and squeeze in a regular rhythm.
The lungs and breathing
Your lungs are two spongy organs in your chest. When you breathe in, air travels down your windpipe into millions of tiny air sacs called alveoli. Oxygen passes from the alveoli into your blood. Carbon dioxide moves the opposite way—from blood into the lungs—so you can breathe it out. Your diaphragm (a muscle below your lungs) helps you breathe by moving up and down.
The kidneys and waste removal
You have two kidneys shaped like beans near the middle of your back. They filter your blood to remove waste and extra water. This forms urine, which travels to your bladder and leaves your body. Kidneys also balance salts and minerals in your blood. They clean about 180 litres of blood every day!
The brain and control
Your brain sits inside your skull and controls your whole body. It receives messages from your senses (eyes, ears, skin, nose and tongue). It sends signals through nerves to your muscles so you can move. Different parts of the brain do different jobs. The brain also stores memories and lets you think, learn and feel emotions.
Worked examples
1. Explain how the heart and lungs work together to get oxygen to your muscles.
- You breathe in air and oxygen enters the lungs through tiny alveoli.
- Oxygen passes from the alveoli into the blood flowing through capillaries around them.
- The heart pumps this oxygen-rich blood through arteries to all parts of the body.
- Muscles take oxygen from the blood and use it to release energy from food.
- The blood picks up carbon dioxide (a waste product) and carries it back to the heart.
- The heart pumps this blood to the lungs, where carbon dioxide is breathed out.
2. A person drinks a large bottle of water. Describe what happens in the kidneys.
- Water is absorbed from the digestive system into the bloodstream.
- Blood containing extra water flows through the kidneys.
- Kidneys filter the blood and remove the extra water along with waste products.
- This forms urine, which travels down tubes called ureters to the bladder.
- When the bladder is full, the person feels the need to urinate.
- Urine leaves the body, keeping the right water balance in the blood.
3. Why does your breathing rate increase during exercise?
- Your muscles work harder during exercise and need more energy.
- Muscles use oxygen to release energy from glucose (a type of sugar).
- This produces carbon dioxide as a waste product.
- Your brain detects more carbon dioxide in your blood.
- The brain signals your lungs to breathe faster and deeper.
- Faster breathing brings in more oxygen and removes carbon dioxide more quickly.
Try it yourself
A runner's heart beats 150 times per minute during a race but only 65 times per minute when resting. Explain why the heart rate increases during exercise.
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