Skin is the largest organ of the body and covers a surprisingly large surface.

The longer version

The integumentary system is counted as a single organ even though it covers the whole outside of the body, because its layers work as one continuous structure. The outer layer is thin but tough, made of cells packed with a waterproofing protein, while the layer beneath holds blood vessels, nerves, glands and hair roots.

Together they handle several jobs at once: keeping water in, keeping microbes out, sensing pressure and temperature, and helping regulate heat. Spread flat, the surface is remarkably large for something that seems like a simple covering, and it weighs a substantial fraction of total body mass.

Where this entry sits

These facts look at how the human body works in ordinary moments, from breathing and blinking to balance and sleep. They focus on anatomy, senses, and small reactions that happen constantly without any conscious effort. The full list sits on the Human Body page, where every entry is listed in order.