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Human body

Bones, organs, blood and cells: 40 facts about how the human body is built and how it works, from the skeleton to DNA.

40 cards · Science

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Each Zap teaches up to 5 new cards, so you learn the deck a few at a time.

Checked by the Brain Zap team · reviewed 10 Oct 2026

Sources: Human body (Wikipedia), MedlinePlus, Britannica Kids

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  • Number of bones in the adult human body

    Show answer: Number of bones in the adult human body

    206

    Learn more: Many of a newborn's bones fuse as it grows, such as the plates of the skull. More than half are in the hands and feet.

  • Largest organ of the human body

    Show answer: Largest organ of the human body

    The skin

    Learn more: In an adult it covers about 1.8 square metres and weighs around 4 kg. It keeps germs out and water in.

  • Largest bone in the human body

    Show answer: Largest bone in the human body

    The femur (thigh bone)

    Learn more: The femur runs from hip to knee and is about a quarter of your height. It is also the strongest bone.

  • Smallest bone in the human body

    Show answer: Smallest bone in the human body

    The stapes (in the middle ear)

    Learn more: The stapes is about 3 mm long. With the malleus and incus, it carries sound from the eardrum inward.

  • Number of chambers in the human heart

    Show answer: Number of chambers in the human heart

    4

    Learn more: The atria receive blood and the ventricles pump it out: the right side to the lungs, the left to the body.

About this topic

The human body is the one machine everyone lives in and few of us were ever shown round. Its systems work together without a thought from us: the heart pumps, the lungs swap gases, the kidneys clean the blood and the brain keeps it all in step.

Knowing the main parts and what they do makes health news, a doctor's explanation or a first-aid course far easier to follow, and it is simply good to know what is going on inside you.

Medical names look hard but mostly come from Latin and Greek words that describe what you see. The patella is a 'little dish', the clavicle a 'little key', the cochlea a 'snail' and the cerebellum a 'little brain'. Learn the picture behind the name and the word stops being a label to memorise.

All 40 cards

  1. Number of bones in the adult human body

    206

    Learn more: Many of a newborn's bones fuse as it grows, such as the plates of the skull. More than half are in the hands and feet.

  2. Largest organ of the human body

    The skin

    Learn more: In an adult it covers about 1.8 square metres and weighs around 4 kg. It keeps germs out and water in.

  3. Largest bone in the human body

    The femur (thigh bone)

    Learn more: The femur runs from hip to knee and is about a quarter of your height. It is also the strongest bone.

  4. Smallest bone in the human body

    The stapes (in the middle ear)

    Learn more: The stapes is about 3 mm long. With the malleus and incus, it carries sound from the eardrum inward.

  5. Number of chambers in the human heart

    4

    Learn more: The atria receive blood and the ventricles pump it out: the right side to the lungs, the left to the body.

  6. Blood cells that carry oxygen

    Red blood cells

    Learn more: Also called erythrocytes, they live about 120 days. They have no nucleus, which leaves room for haemoglobin.

  7. Protein in red blood cells that carries oxygen

    Haemoglobin

    Learn more: It picks up oxygen in the lungs and drops it in the tissues; each molecule carries up to four oxygen molecules.

  8. Blood cells that help fight infection

    White blood cells

    Learn more: Leukocytes, such as neutrophils and lymphocytes, find and fight germs. Their count rises during an infection.

  9. Cell fragments that help blood to clot

    Platelets

    Learn more: Also called thrombocytes, they are fragments of megakaryocytes. At a cut they clump to plug the leak.

  10. Organ that makes insulin

    The pancreas

    Learn more: Its islets of Langerhans make insulin, which lowers blood sugar. In type 1 diabetes those cells are destroyed.

  11. Organ that produces bile

    The liver

    Learn more: It makes about half a litre to a litre of bile a day. Bile helps break down fats in the gut.

  12. Small organ that stores bile

    The gallbladder

    Learn more: It stores and concentrates bile, then squeezes it into the gut after a fatty meal. People can live without it.

  13. Organs that filter blood to make urine

    The kidneys

    Learn more: Together they filter about 180 litres of blood plasma a day, yet make only 1 to 2 litres of urine.

  14. Tiny air sacs in the lungs where gas exchange happens

    Alveoli

    Learn more: Adult lungs hold around 480 million of them. Their thin walls let oxygen in and carbon dioxide out.

  15. Main muscle used for breathing

    The diaphragm

    Learn more: It contracts and flattens to pull air into the lungs. A hiccup is a sudden spasm of this muscle.

  16. Coloured part of the eye

    The iris

    Learn more: Its muscles widen or narrow the pupil to control light. Its patterns are unique enough to identify people.

  17. Opening in the eye that lets light in

    The pupil

    Learn more: It looks black because the light that enters is absorbed inside. It widens in the dark and narrows in bright light.

  18. Light-sensitive layer at the back of the eye

    The retina

    Learn more: It turns light into nerve signals sent along the optic nerve. Rods work in dim light; cones see colour.

  19. Spiral, snail-shaped part of the inner ear

    The cochlea

    Learn more: Fluid in its spiral moves tiny hair cells, which turn sound into nerve signals. It makes about 2.5 turns.

  20. Bones that protect the brain

    The skull

    Learn more: An adult's has 22 bones, and all but the lower jaw are locked together. A baby's has soft spots, fontanelles.

  21. Number of pairs of ribs

    12 pairs

    Learn more: The top seven join the breastbone directly; the last two, the floating ribs, attach only at the back.

  22. Bones that make up the spine

    Vertebrae

    Learn more: In adults some fuse, leaving about 26 bones in the spine. The neck has seven, just like a giraffe's.

  23. Medical name for the kneecap

    Patella

    Learn more: The patella sits inside a tendon, a sesamoid bone. It helps the thigh muscles straighten the knee.

  24. Medical name for the collarbone

    Clavicle

    Learn more: The clavicle links the shoulder to the breastbone. It is one of the most often broken bones, usually in a fall.

  25. Largest muscle in the human body

    Gluteus maximus

    Learn more: It extends the hip, so it powers climbing stairs, running and standing up from a chair.

  26. Thickest tendon in the human body

    Achilles tendon

    Learn more: It joins the calf muscles to the heel bone. In the myth, the hero could be wounded only at his heel.

  27. Largest part of the human brain

    The cerebrum

    Learn more: It handles thought, language, memory and voluntary movement. Its wrinkled outer layer is the cortex.

  28. Brain part that coordinates fine movement

    The cerebellum

    Learn more: It sits at the back, under the cerebrum, and holds more than half of the brain's neurons.

  29. Nerve cells that carry electrical signals

    Neurons

    Learn more: The brain has about 86 billion. They pass signals as electrical pulses and chemicals called neurotransmitters.

  30. Hardest substance in the human body

    Tooth enamel

    Learn more: It is about 96% mineral, mostly hydroxyapatite. It cannot grow back, which is why cavities need fillings.

  31. Number of teeth in a full adult set

    32

    Learn more: Eight incisors, four canines, eight premolars and twelve molars, including the four wisdom teeth.

  32. Number of baby (primary) teeth

    20

    Learn more: Eight incisors, four canines and eight molars, with no premolars. Most children have them all by age three.

  33. Largest artery in the human body

    The aorta

    Learn more: It carries oxygen-rich blood from the heart to the rest of the body, and is about 2.5 cm wide in adults.

  34. Where most nutrients from food are absorbed

    The small intestine

    Learn more: Its folds and finger-like villi give it about 30 square metres of surface for taking in food.

  35. Acid made by the stomach

    Hydrochloric acid

    Learn more: Its pH is about 1.5 to 3.5. A layer of mucus keeps the stomach from digesting itself.

  36. Molecule that carries hereditary information

    DNA

    Learn more: Deoxyribonucleic acid stores genes in four bases, A, T, C and G. Its structure was described in 1953.

  37. Number of chromosomes in a typical human cell

    46

    Learn more: One set of 23 comes from each parent. The 23rd pair, XX or XY, usually sets biological sex.

  38. Butterfly-shaped gland in the neck

    The thyroid

    Learn more: It needs iodine to make its hormones, which is why salt is often iodised. Its name is Greek for 'shield-shaped'.

  39. Vitamin the skin makes in sunlight

    Vitamin D

    Learn more: UVB light lets the skin make it. It helps the body absorb calcium, so a lack of it can weaken bones.

  40. Main pigment that gives skin its colour

    Melanin

    Learn more: Cells called melanocytes make it. It absorbs UV light, which helps protect skin cells from sun damage.

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