The Systems of the Human Body

A team of systems

Your body is like a busy city. Just as a city needs roads, power and water working together, your body needs organ systems working together. Each system is a group of organs that does a particular job. On their own they can do a lot — but the real magic is how they cooperate. Let's meet the main ones.

The circulatory system: the delivery network

At the centre is your heart, a muscle about the size of your fist. It pumps blood through a huge network of tubes called blood vessels: arteries, veins and tiny capillaries.

Blood is a delivery service. It carries:

• Oxygen to every cell,
• Nutrients from your food,
• and it carries away waste, such as carbon dioxide.

An adult heart beats around 60–100 times a minute, every minute, for your whole life. That is over 2 billion beats in a lifetime.

The respiratory system: bringing in oxygen

When you breathe in, air travels down your windpipe into your two lungs. Inside the lungs are millions of tiny air sacs called alveoli. Oxygen passes from these sacs into the blood, while carbon dioxide (a waste gas) passes out and you breathe it away.

This is why the respiratory and circulatory systems are best friends: one collects the oxygen, the other delivers it.

The digestive system: turning food into fuel

Your cells need energy, and that energy comes from food. The digestive system breaks food into small pieces your body can absorb.

1. Mouth — teeth chew and saliva starts breaking down the food.
2. Stomach — acids and muscles churn the food into a soupy mush.
3. Small intestine — nutrients pass into the blood here.
4. Large intestine — water is absorbed and waste is removed.

The nutrients then travel in the blood to feed every cell — another example of systems working as a team.

The nervous system: the control centre

The nervous system is made of your brain, spinal cord and a web of nerves. It works like the body's electrical wiring and command centre.

Nerves carry messages as fast electrical signals. When you touch something hot, sensors in your skin send a signal to your spinal cord and brain, and instantly a signal comes back telling your muscles to pull away. All of this happens in a fraction of a second.

The skeletal and muscular systems: support and movement

Your skeleton has 206 bones in an adult. It does three big jobs: it gives your body shape, it protects soft organs (your skull protects your brain, your ribs protect your heart and lungs), and it gives muscles something to pull against.

Your muscles are attached to bones. When a muscle tightens, it pulls a bone and you move. Because they work so closely, scientists often call them together the musculoskeletal system.

Everything connects

Think about running for a bus. Your nervous system decides to run. Your muscles pull your bones. Your respiratory system breathes faster to grab more oxygen. Your circulatory system races that oxygen to your leg muscles. Your digestive system supplied the energy earlier from your breakfast. Five systems, one simple action.

This teamwork is similar to how living things depend on each other in nature — see our lesson on food chains and ecosystems.

Try it yourself: measure your heart at work

Your circulatory and respiratory systems change when you exercise. Let's measure it.

1. Sit still for two minutes. Find your pulse on your wrist or neck.
2. Count the beats for 15 seconds, then multiply by 4. That is your resting heart rate in beats per minute.
3. Now do 30 star jumps.
4. Immediately measure your heart rate again the same way.
5. Compare the two numbers.

Your heart beats faster after exercise because your muscles need more oxygen, so the circulatory and respiratory systems work harder together. Record your results and try it after different activities — walking, jogging, sprinting.