Human Body Layers: Skin, Muscles, Bones and Organs

Skin, the largest organ

The picture opens on the outside of the body, the part everyone sees. Skin is an organ, and in an adult it is the largest one, covering roughly 1.5 to 2 square metres. It has three layers: a thin waterproof epidermis on top, a thicker dermis with nerve endings, hair roots and blood vessels, and a fatty layer underneath that cushions and insulates. Skin keeps water in and germs out, helps to control body temperature through sweat, and makes vitamin D when sunlight falls on it. Turn the model with the sandbox and notice how smoothly this one layer wraps around every curve.

Muscles that only pull

Now the skin lifts away and the muscles appear. An adult has about 600 skeletal muscles, the ones you can move on purpose. Each muscle is a bundle of long fibres that can do one thing only: shorten and pull. They cannot push. That is why muscles work in pairs. When the biceps in your upper arm pulls, your elbow bends; when the triceps on the other side pulls, it straightens again. Muscles attach to bones by strong cords called tendons. Two other kinds of muscle work without your orders: the heart muscle and the smooth muscle in the gut and in blood vessels.

A frame of 206 bones

Next the muscles fade and the skeleton stands alone. A grown person has 206 bones. A newborn has more, roughly 270 to 300, because many small bones are still separate and fuse together as the child grows. The skeleton does four jobs: it holds the body up, protects soft organs such as the brain and the lungs, gives the muscles something to pull on, and stores minerals. Inside many bones sits marrow, where blood cells are made. The skeleton gets a whole chapter of its own, so here just enjoy the shape.

Organs and organ systems

The skeleton opens like a cage and the organs glow inside it. An organ is a group of tissues that does one job together: the heart pumps, the lungs swap gases, the stomach and intestines digest, the kidneys filter the blood, and the liver, at about 1.5 kilograms the largest organ inside the body, has hundreds of tasks. Organs team up in organ systems, and textbooks usually count eleven of them, from the nervous system to the digestive system. Try the take-apart slider in the sandbox: the systems drift away from each other, and you can see how closely packed they really are.

The wiring and the pipes

The last layer is the one that connects all the others. Blood vessels reach every part of the body, from thick arteries as wide as a garden hose to capillaries so narrow that red blood cells pass through in single file. Laid end to end, all the vessels would stretch for tens of thousands of kilometres. Beside them run the nerves, branching from the brain and the spinal cord to the skin, the muscles and the organs. Press assemble in the sandbox and watch every layer return to its place, with nerves and vessels reaching into all of them.

How the layers cooperate

Think about picking up a cup. Your eyes find it, and nerves carry that message to the brain. The brain sends orders along other nerves, muscles pull on bones, and joints bend. Meanwhile the heart sends extra blood to the working muscles, and the lungs bring in the oxygen they burn. No layer works alone. This is the big idea of the whole book: the body is a team of systems, and each chapter that follows zooms into one member of the team, from bones and the brain to the cells that all of them are made of.

The body, layer by layer