How the Human Body Is Organised

Your body isn't a mystery. It's built from a few simple parts, stacked up in a clear order. Once you can see that order, the rest of medicine starts to make sense.

Before we talk about hearts or hormones or what goes wrong when you're ill, we need a map. Think of it like learning a city. You could memorise a thousand individual streets and still feel lost. Or you could first learn that the city is made of districts, the districts are made of neighbourhoods, and the neighbourhoods are made of streets. Suddenly the whole place makes sense and you can find your way anywhere. Your body works the same way. It's built in levels, each one made of the level below it. Learn those levels once and you'll have a map you can use for everything that follows. Here's the whole idea in a single sentence. Honestly, if you remember only this from today, you've got the foundation: cells build tissues, tissues build organs, organs build systems, and systems build you. Cells: the bricks A cell is the smallest living unit of your body. Everything you are is made of them, somewhere around 30 trillion of them, give or take. Here's a useful way to picture one: a tiny bag of salty water with a job to do. And it really is alive in its own right. It takes in fuel, gets on with its work, and produces waste, much as you do. Many cells eventually wear out and are replaced, although some specialised cells last much longer and are difficult for the body to replace. So when you look in the mirror, what you're actually seeing is a community of trillions of tiny living things, all cooperating to be you. Cells aren't all the same, though. A muscle cell looks and behaves nothing like a brain cell or a skin cell, because each is shaped for its particular job. That's a theme you'll meet again and again: in the body, structure follows function. The way something is built tells you what it's for. Tissues: bricks of the same kind When you gather lots of similar cells together to do one job, you get a tissue. Muscle tissue is just millions of muscle cells working as a team to do one thing: contract. The lining of your gut is a tissue too, and so is the fat under your skin. Many tissues are made mainly from similar cells working together, sometimes alongside other supporting cells and material. They usually carry out a group of closely related jobs. Organs: different tissues teaming up Now it gets interesting. An organ is what you get when different tissues combine to do something more complex than any one of them could manage alone. Take the heart. It has muscle tissue to do the squeezing, a built in cluster of specialised cells that set its own rhythm, and a smooth inner lining so blood flows cleanly through it. Several different tissues, assembled into one structure, with one overall job: pump blood. Your stomach is an organ. So are your lungs, your skin, your kidneys. Each one a team of different tissues, pulling in the same direction. Systems: organs working as a department An organ rarely works alone. Usually it's part of a system, a group of organs that together handle one big function for the body. Your digestive system is a good example. Most people think "stomach," but the stomach is only one stop on a long production line. Food travels from your mouth, down the gullet, into the stomach, then on through the intestines, with the liver and pancreas pitching in along the way. A whole chain of organs, each handing the job to the next, all to turn a sandwich into something your cells can burn. You have around eleven of these systems, and later lessons meet each of them properly. The organism: you Stack all the systems together, get them cooperating, and you have the whole living organism: you. And here's the quietly remarkable part. All these levels are working right now, without you thinking about it, keeping you alive while you read this sentence. One useful rule ties the levels together: a change at one level can affect the levels around it. Think about a heart attack. It usually begins when blood flow through a blood vessel that feeds the heart (coronary artery) is suddenly reduced or blocked. Heart muscle cells downstream are starved of oxygen and may become injured or die. As an area of heart muscle is affected, the heart's pumping or electrical rhythm may be disrupted, which can threaten circulation and the whole person. One event can therefore affect cells, tissue, an organ, a body system and the whole organism. This hierarchy is one useful map for understanding illness, although clinicians combine it with many other kinds of information.

The body is built in five levels: cells, tissues, organs, systems, and finally you. A change at one level ripples to the others.