Human Body Systems

The human body is one of the most complex machines in the known universe. It contains around 37 trillion cells, organised into about 200 different cell types, building 78 organs, which together form 11 organ systems that all work in close partnership. Your heart beats around 100,000 times every day, your brain holds about 86 billion neurons, and your skeleton is the most flexible engineering wonder on the planet. Biology calls the science of how the human body is built anatomy, and how it works physiology.

  • Organ systems11Each made of several organs
  • Bones in an adult206Babies are born with about 270 (some fuse)
  • Musclesapprox. 640Plus three special heart muscles
  • Length of blood vesselsapprox. 100,000 km2.5 times around the Earth
  • Heartbeats per dayapprox. 100,000About 3 billion in a lifetime
  • Neurons in the brainapprox. 86 billionPlus approx. 85 billion other brain cells

The 11 organ systems

Every cell, organ and bit of plumbing in your body falls under one or more of these systems.

  • Skeletal system: 206 bones that hold you up and protect your insides.
  • Muscular system: around 640 muscles that move your bones and your insides.
  • Nervous system: your brain, spinal cord and nerves, your body's electrical network.
  • Circulatory system: your heart and blood vessels, delivering oxygen and nutrients everywhere.
  • Respiratory system: your lungs and airways, bringing oxygen in and pushing carbon dioxide out.
  • Digestive system: your gut, breaking down food and absorbing nutrients.
  • Endocrine system: glands that release hormones, the body's slow chemical messengers.
  • Immune system: the army of cells that fights infections.
  • Excretory system: your kidneys and bladder, removing waste and balancing water.
  • Reproductive system: makes the cells and structures needed to have babies.
  • Integumentary system: your skin, hair and nails, the body's outer wrapping.

How the systems work together

The systems are not separate units; they constantly work together. When you run for a bus, your muscles need extra oxygen, so your respiratory system breathes faster, your circulatory system pumps blood harder, your nervous system coordinates the muscle movements, your endocrine system releases adrenaline, your skin sweats to keep you cool, and your digestive system slows down to free up energy. All in seconds, all without you having to think about it.

Fact If you laid out all the blood vessels in an adult human body end to end, they would stretch about 100,000 km, more than twice the way round the Earth. The thinnest vessels, called capillaries, are so fine that red blood cells have to squeeze through them one at a time. They are also where almost all the exchange of oxygen and nutrients happens, between your blood and your cells.

Pick a topic below to explore individual body systems in more depth.

The Skeletal SystemThe 206 bones that hold your body up, protect your organs, and let you move. Babies are born with about 300 bones that fuse as they grow.
The Muscular SystemOver 600 muscles that pull on your bones to make you move, and keep your heart, lungs and gut working without you even thinking.
The Circulatory SystemYour heart and blood vessels delivering oxygen, food, and warmth to every cell in your body, around the clock.
The Respiratory SystemYour lungs and airways, pulling oxygen out of the air and sending it round your body in your blood.
The Digestive SystemThe tube from your mouth to your bum that breaks food down so your body can use it. About 9 metres long when stretched out.
The Nervous SystemYour brain, spinal cord, and nerves. The control system that lets you think, feel, move, and sense the world.
The Immune SystemThe army of cells and chemicals that defend you against germs and disease. The reason most colds eventually go away.
The Endocrine SystemThe glands that make hormones, the chemical messengers that tell your body when to grow, sleep, get hungry, and a hundred other things.
The Reproductive SystemThe parts of the body that allow humans to make babies. Different in male and female bodies.
Skin and the Integumentary SystemYour skin, hair and nails. Skin is your largest organ and your first line of defence against the outside world.