Structure and Function

In Grade 6 you learned that living things are organised: cells → tissues → organs → organ systems → organism. This year we zoom in on that fourth level — the organ systems — and look at four of them in real detail.

An organ system is a group of organs that work together on one big job. Your body has several systems running at the same time, every second of the day, without you having to think about it.

• The digestive system breaks food down so the body can use it.
• The circulatory system transports materials around the body.
• The breathing system brings in oxygen and removes carbon dioxide.
• The skeleton and muscles support the body and let it move.

Notice that no system works alone. The breathing system puts oxygen into the blood; the circulatory system then carries that oxygen everywhere. The digestive system absorbs nutrients; the blood delivers them. Keep asking the question that runs through all of biology: how is each part's structure suited to its function?

Food cannot be used by your cells in the lumps you swallow. The digestive system breaks it down into pieces small enough to pass into the blood. It is one long tube with several organs along the way.

• Mouth — teeth cut and crush food into smaller pieces; saliva begins to break it down.
• Oesophagus — a muscular tube that pushes food down to the stomach.
• Stomach — a muscular bag that churns food and mixes it with digestive juices.
• Small intestine — where digestion finishes and the small, soluble food molecules are absorbed into the blood.
• Large intestine — absorbs water from the leftover material before waste leaves the body.

The small intestine is very long and has a folded, hairy inner lining. That gives it a huge surface area — perfect for absorbing as much food as possible.

Once food and oxygen are inside the body, something has to deliver them to every cell. That is the job of the circulatory system — a network of tubes with a powerful pump at its centre.

• The heart is an organ made mostly of muscle. It contracts to pump blood, and its thick muscular walls give it the strength to push blood all around the body.
• Blood is the transport fluid. Red blood cells carry oxygen, the liquid plasma carries dissolved food and carbon dioxide, and white blood cells fight disease.
• Blood vessels are the tubes blood travels through. Arteries carry blood away from the heart and have thick walls to cope with high pressure. Veins carry blood back to the heart. Capillaries are tiny vessels with very thin walls, so oxygen and food can pass easily into the cells.

Your circulatory system can only deliver oxygen if oxygen gets into the blood in the first place. That is the job of the breathing system (also called the respiratory system).

When you breathe in, air travels down the trachea (windpipe), through two bronchi, and into the lungs. The diaphragm, a sheet of muscle below the lungs, helps pull air in and push it out.

Deep inside the lungs the air reaches millions of tiny air sacs. Their walls are extremely thin and there are millions of them, giving a vast surface area. This is where gas exchange happens: oxygen passes from the air into the blood, and carbon dioxide passes from the blood into the air to be breathed out.

Without a frame inside it, your body would collapse into a soft heap. The skeleton is that frame — around 200 bones doing three key jobs.

• Support — the skeleton holds the body up and gives it a shape.
• Protection — bones shield delicate organs. The skull protects the brain; the ribcage protects the heart and lungs.
• Movement — bones act as a stiff structure that muscles can pull on.

Bones cannot move themselves. Muscles do that, and they work in a clever way: a muscle can only pull, never push. So muscles are arranged in antagonistic pairs — one muscle pulls a bone one way, and its partner pulls it back.

In your arm, the biceps contracts to bend the elbow while the triceps relaxes. To straighten the arm again, the triceps contracts and the biceps relaxes.

Knowing how organ systems are built makes the next topics far clearer:

• Life processes — respiration, gas exchange and nutrition all happen inside the systems you have just met.
• Health and disease — understanding healthy systems helps you see what happens when one stops working well.
• Surface area — the gut lining and the lungs both rely on it; this idea returns again and again in biology.
• Energy and forces — muscles, bones and pumps connect biology to ideas about forces in physics.

Whenever a later topic feels tricky, picture the organ system involved and ask how its structure suits its job. Take your time — really understanding these systems is worth far more than rushing through them.