99 Fun Facts About Human Anatomy

How many bones are typically found in the adult human body?

The adult skeleton has 206 bones, from the femur, the longest, down to the stapes in the middle ear, the smallest.

What is the folded outer layer of grey matter covering the brain called?

The cerebral cortex is only a few millimetres thick, so its folds, the ridges and grooves, are what let so much of it fit inside the skull; its neocortex is built from six layers.

Weighing around 16% of your body mass, which organ is the largest in the human body?

Skin outweighs every other organ at roughly 16% of body mass, and you shed and replace its outer layer about once a month.

What is the primary function of the larynx?

Producing sound is why it is called the voice box; it also closes over the windpipe when you swallow so food goes down the right way.

Which blood type is the 'universal recipient' for red cell transfusions?

AB+ red cells already carry the A, B and Rh antigens, so there is nothing new in a donor's red cells for the immune system to attack; O negative is the universal donor.

Which gland is often referred to as the 'master gland' of the endocrine system?

The pituitary gland is called the 'master gland' because it produces hormones that regulate many vital body functions and controls the functions of many other endocrine glands.

What is the collective name for the dura, arachnoid and pia mater?

The meninges are what meningitis inflames, which is why a stiff neck is a warning sign; the toughest layer, the dura mater, means 'hard mother' in Latin.

Which artery carries oxygen-poor blood away from the heart?

The pulmonary artery runs from the right ventricle to the lungs for a refill; the real rule is that arteries carry blood away from the heart, not that they carry oxygen.

What is the largest gland in the human body?

The liver is both the largest gland and the heaviest internal organ, and the only one that can regrow from as little as a quarter of its mass.

How many chambers does the human heart possess?

The human heart is a double pump with four chambers: two upper atria (right and left) and two lower ventricles (right and left).

Roughly how many bones does a human have at birth, before many fuse together?

A newborn has around 270 bones; the count falls to about 206 in adulthood as bones such as those of the skull and sacrum fuse.

The skull, spine and rib cage together make up which part of the skeleton?

The axial division, the skeleton's central column of skull, vertebrae, ribs and sternum, has 80 bones; the limbs and girdles form the appendicular division.

How many bones make up the appendicular skeleton (limbs plus the shoulder and pelvic girdles)?

The appendicular skeleton has 126 bones: 64 in the shoulders and arms and 62 in the pelvis and legs, which with the 80 axial bones makes the adult total of 206.

In adults, where does blood cell production (haematopoiesis) mainly take place?

In adults red marrow is largely confined to the pelvis, cranium, vertebrae and sternum, plus the upper ends of the femur and humerus; in children the long-bone marrow is also active.

Vesalius corrected Galen's claim that which body part was made of two bones rather than one?

Galen, who dissected animals, described a two-part lower jaw; Vesalius showed in De humani corporis fabrica (1543) that the human mandible is a single bone.

The stapes, the smallest bone in the body, is named after its resemblance to what?

Classical Latin had no word for it, since stirrups did not exist in the ancient world.

About how long is the stapes bone in the middle ear?

The stapes measures about 2 to 3 mm and weighs a few milligrams; it is the innermost of the three ossicles that pass vibrations to the inner ear.

On average, the femur accounts for roughly what fraction of a person's height?

The femur is about 27% of stature, so a 1.8 m person has a femur near 48 cm, which lets forensic anthropologists estimate height from the bone alone.

Which skull bone houses the inner ear within its dense petrous part?

The temporal bone's petrous part, the densest bone in the body, encloses the cochlea and semicircular canals of the inner ear.

Roughly how long is the human small intestine?

The small intestine runs about 6.5 metres from the stomach to the large intestine, and its villi and microvilli give it an absorptive surface of roughly 30 square metres.

Which is the shortest of the three regions of the small intestine?

The duodenum is only about 25 cm long, against roughly 2.5 m for the jejunum and 3 m for the ileum.

The intestinal projections called villi take their name from the Latin for what?

Villus is Latin for 'shaggy hair' or a tuft of hair; the lining looks like velvet, a word that shares the same root.

Roughly how many neurons does the human brain contain?

There are roughly as many non-neuronal cells again.

The brain is only about 2% of body weight, yet it uses roughly what share of the body's oxygen?

It also takes 15% of cardiac output and a quarter of the body's glucose.

What is the largest of the nerve tracts connecting the brain's two hemispheres?

The corpus callosum bundles some 200 million axons between the hemispheres; severing it to treat epilepsy produced the famous 'split-brain' patients.

How much does an average adult brain weigh?

An adult brain weighs about 1.2–1.4 kg, roughly 2% of body weight, with men's averaging a little heavier than women's.

Which layer of the body's outer covering contains no blood vessels at all?

The epidermis has no blood vessels; its cells are fed by diffusion from capillaries in the dermis below, which is why a shallow scrape does not bleed.

The stratum lucidum layer is found only where?

The stratum lucidum appears only in the thick skin of the palms and soles, as a thin clear band between the stratum corneum and stratum granulosum.

Which protein in the dermis lets skin stretch and then snap back into shape?

Elastin fibres in the dermis give skin its recoil; their breakdown with age and sun exposure is a main cause of sagging and wrinkles, while collagen provides strength.

Which part of the eye provides about two-thirds of its refractive power?

The cornea provides roughly 43 of the eye's 60 dioptres of refractive power; the lens adds the adjustable remainder for near focus.

Which retinal cells handle dim light and give you grey night vision, outnumbering their colour-sensing partners about twenty to one?

Rods number about 120 million per eye and are useless for colour; cones handle bright light and colour, which is why everything looks grey at night.

How many primary ('milk') teeth do humans normally have?

They are replaced by 32 permanent teeth, 16 in each jaw.

Wisdom teeth are formally known as what?

Wisdom teeth are the third molars, the last teeth at the back of each jaw, usually erupting between ages 17 and 25 if at all.

What is the hardest substance in the human body, and one it can never regrow once damaged?

Tooth enamel is about 96% mineral and has no living cells, which is why a cavity never heals on its own; women's teeth have proportionately more enamel, men's more dentin.

On a liver ultrasound, the portal vein, hepatic artery and bile duct form a pattern named after whom?

Radiologists call it the 'Mickey Mouse sign': on a transverse view of the porta hepatis the portal vein forms the head and the hepatic artery and bile duct the ears.

About how many nephrons does each adult human kidney contain?

Each human kidney has around 1 million nephrons, though individuals range from a few hundred thousand to over 2 million; a mouse kidney gets by with about 12,500.

Which kidney typically sits slightly lower and is slightly smaller?

The right kidney sits about 1 to 2 cm lower and a little smaller than the left because the liver occupies the space above it.

How many lobes does the right lung have?

The right lung has three lobes (upper, middle and lower); the left has only two because it shares chest space with the heart.

Together the lungs contain roughly how many alveoli?

The lungs hold around 300 to 500 million alveoli, which together spread a gas-exchange surface of about 70 square metres served by some 2,400 km of airways.

What is the mass of a typical adult human heart?

An adult heart weighs about 250–350 grams, roughly the size of a fist, and at rest beats close to 72 times a minute.

Which hormone released by bone cells helps regulate blood sugar and fat deposition?

Osteocalcin, secreted by osteoblasts, boosts insulin secretion and sensitivity, a hormonal role for the skeleton first shown in Gerard Karsenty's mouse studies in 2007.

Which type of bone cell resorbs (breaks down) bone tissue?

Osteoclasts dissolve bone mineral and matrix while osteoblasts lay down new bone; the balance between them keeps the skeleton constantly remodelling.

What is the name for the study of tissues, usually done under a microscope?

Histology is the microscopic study of tissues; the body has four main tissue types: epithelial, connective, nervous and muscle.

Where does the heart's electrical impulse begin with each beat?

The sinoatrial node, a patch of pacemaker cells in the wall of the right atrium, fires about 70 times a minute and sets the heart's rhythm.

Which structure do mature red blood cells lose, leaving them without DNA?

Mammalian red blood cells eject their nucleus as they mature in the bone marrow, freeing room for haemoglobin and giving them their biconcave shape.

Roughly how much blood do the kidneys filter each day?

The kidneys produce about 170 litres of filtrate a day, a glomerular filtration rate near 120 ml per minute; almost all is reabsorbed and only one to two litres leaves as urine.

Roughly how many cells does a mature human body contain?

A 2016 estimate put the body at about 30 trillion human cells, most of them red blood cells, narrowly outnumbered by roughly 38 trillion resident bacteria.

Which structure deliberately slows the heart's electrical signal so the ventricles can fill?

The atrioventricular node delays the impulse by about a tenth of a second, letting the atria finish emptying into the ventricles before the bundle of His carries the signal on.

Which vessel supplies the gallbladder with blood in most people?

The cystic artery, usually a branch of the right hepatic artery, supplies the gallbladder, which fills and drains through the cystic duct.

Hydroxyapatite makes up roughly what proportion of bone?

Hydroxyapatite, a crystalline calcium phosphate, makes up about 70% of bone by dry weight and is nearly 40% calcium; most of the rest is type I collagen.

Which is the longest and widest single nerve in the human body?

The sciatic nerve, formed from spinal roots L4 to S3, runs from the lower back down the back of each leg; irritating it produces the shooting pain of sciatica.

Which muscle, just over a millimetre long, is the smallest skeletal muscle in the body?

The stapedius, about 1 mm long, sits in the middle ear and damps the stapes; when the facial nerve paralyses it, as in Bell's palsy, ordinary sounds can seem painfully loud.

Which is the single largest muscle in the human body?

The gluteus maximus is the largest single muscle; its size is tied to walking upright, holding the trunk erect, a job other primates with flatter hips cannot sustain.

Which horseshoe-shaped bone in the neck is the only one not connected to any other bone?

The hyoid is anchored only by muscles and ligaments and helps with tongue movement and swallowing; its name is Greek for 'shaped like the letter upsilon'.

How many cervical (neck) vertebrae does a human have, numbered C1 downwards?

Humans have seven cervical vertebrae, a count shared by almost all mammals, giraffe included; the exceptions are manatees, with six, and sloths, with five to nine.

How many pairs of ribs does a typical human have?

12 pairs, regardless of sex, which upset readers who took the Adam's-rib story to mean men should have one fewer; about 1% of people have an extra 'cervical rib'.

Which cranial nerve is numbered I, the first of the twelve pairs?

The olfactory nerve carries smell; the numbering follows the order in which the nerves emerge from the brain, front to back, a scheme settled by Samuel Soemmerring in 1778.

How many pairs of spinal nerves branch from the human spinal cord?

There are 31 pairs: eight cervical, twelve thoracic, five lumbar, five sacral and one coccygeal; oddly there are eight cervical nerve pairs but only seven cervical vertebrae.

The kneecap is the body's largest example of which kind of bone?

Sesamoid bones form inside tendons, and the patella, embedded in the quadriceps tendon, is the largest; mice, cats, birds and dogs have kneecaps too.

Not counting sesamoids, how many bones does a human hand contain?

The hand has 27 bones: eight carpals in the wrist, five metacarpals and fourteen phalanges, the thumb having only two phalanges.

Which part of the adrenal gland produces the body's steroid hormones?

The adrenal cortex makes the steroid hormones, including cortisol from its middle zone, the zona fasciculata; the inner medulla makes adrenaline instead.

The insulin-producing islets of the pancreas are named after which anatomist?

Paul Langerhans described the islets in his 1869 Berlin doctoral thesis as a medical student; they make up only 1 to 2% of the pancreas by volume yet take 10 to 15% of its blood flow.

Most cerebrospinal fluid is produced by which structure inside the brain's ventricles?

The choroid plexus, a frond of capillaries and ependymal cells in each ventricle, secretes about 500 ml of cerebrospinal fluid a day; the arachnoid villi drain it back into the blood.

Which gland, about the size of a grain of rice, produces melatonin?

The pineal gland is named for its pine-cone shape and sits unpaired on the brain's midline, releasing melatonin in darkness to signal night.

The hippocampus takes its name from its resemblance to which animal?

The Venetian anatomist Aranzi wavered between silkworm and seahorse in 1587; a rival name, 'horn of Ammon', survives in the label CA for its subfields.

Which brain structure's Latin name translates as 'little brain'?

The cerebellum, Latin for 'little brain', does not start movements but fine-tunes their timing and precision; in mormyrid electric fishes it rivals the rest of the brain in size.

Breathing, heart-rate and vomiting centres sit in which part of the brainstem?

The medulla oblongata, Latin for 'elongated marrow', is the lowest part of the brainstem; coughing, sneezing and swallowing reflexes live here too.

Roughly how long does a red blood cell circulate before its parts are recycled?

A red cell lasts about 120 days before the spleen and liver recycle its iron; adults make about 2.4 million new ones every second.

Platelets are cytoplasm fragments shed by which giant bone-marrow cells?

Megakaryocytes, huge polyploid marrow cells, each shed thousands of platelets; platelets have no nucleus and are unique to mammals, as birds and amphibians clot with nucleated thrombocytes.

Plasma makes up roughly what share of total blood volume?

Plasma is about 55% of blood volume and roughly 92% water, carrying dissolved proteins, glucose, hormones and clotting factors; red cells make up most of the remaining 45%.

The phrenic nerve, which drives the diaphragm, arises from which spinal segments?

Medical students remember it as 'C3, 4, 5 keeps the diaphragm alive'. A spinal injury above this level removes the ability to breathe unaided.

The sacrum forms as five vertebrae fuse, typically between which ages?

The five sacral vertebrae fuse between about 18 and 30, forming a triangular bone wedged between the hip bones; horses also fuse five, dogs only three.

Which valve lets blood pass from the right atrium into the right ventricle?

The tricuspid valve's three flaps close during ventricular contraction to stop backflow; it and the mitral valve are the only valves anchored by chordae tendineae.

The heart's mitral valve is named after the ceremonial headdress worn by whom?

Its two leaflets resemble the mitre worn by bishops; the mitral, or bicuspid, valve sits between the left atrium and left ventricle.

The ring of tonsils around the throat is named after which anatomist?

Waldeyer's ring, named for the German anatomist Wilhelm von Waldeyer-Hartz, includes the adenoid, two tubal tonsils, the palatine tonsils people usually mean by 'tonsils', and the lingual tonsils.

The windpipe is held open by 16 to 20 incomplete cartilage rings shaped like which letter?

The open side faces the back, where the trachealis muscle bridges the gap. That soft wall lets the oesophagus bulge forward as food passes.

Which leaf-shaped flap folds over the airway during swallowing?

The epiglottis stays upright while you breathe and folds over the larynx during swallowing to steer food down the oesophagus, which is why talking while eating can send things 'down the wrong way'.

Roughly how long is the adult oesophagus?

The adult oesophagus is about 25 cm long, running from the throat to the stomach, with only the last couple of centimetres lying below the diaphragm.

Hydrochloric acid in the stomach is secreted by which type of lining cell?

Parietal cells pump hydrogen ions against a gradient of a million to one or more, among the steepest in any mammalian tissue, and also make the intrinsic factor needed to absorb vitamin B12.

The appendix dangles from which pouch at the start of the large intestine, just past where the small intestine ends?

The cecum, Latin for 'blind', is the pouch; the appendix is now thought to shelter helpful gut bacteria rather than being a useless leftover.

Roughly how long is the human large intestine?

The large intestine is about 1.5 m long, only about a fifth of the whole gut, and it absorbs water and salts from the liquid residue the small intestine delivers.

Roughly how much bile does the gallbladder hold at any one time?

The pear-shaped gallbladder holds about 30 to 60 ml of bile, concentrating what the liver makes and squirting it into the duodenum to help digest fats.

Which purple organ in the upper left abdomen is the largest collection of lymphoid tissue?

The spleen, the body's largest lymphoid organ, filters blood and recycles worn-out red cells; it can be removed after injury, though that leaves patients more prone to certain bacterial infections.

T cells mature inside which organ behind the breastbone, which shrinks after puberty?

The thymus is largest and most active in infancy; from puberty its tissue is gradually replaced by fat, though it keeps producing some T cells into adulthood.

At roughly what volume does a full bladder produce a strong urge to urinate?

At about 300–400 ml stretch receptors in the bladder wall fire strongly; folds called rugae flatten as it fills, so pressure barely rises until then.

The cochlea of the inner ear takes its name from the Latin word for what?

Cochlea is Latin for 'snail shell'; in humans it makes about 2.75 turns around its axis and is coiled in all mammals except the egg-laying monotremes.

The Eustachian tube links the middle ear to which space?

The nasopharynx, the space behind the nose above the throat, is the other end; the tube is about 35 mm long in adults, and it is why a cold can give you earache.

The eye's blind spot is where which structure leaves the retina?

The optic nerve exits through the optic disc, which has no photoreceptors, so the brain fills the gap from the other eye; octopus eyes, wired from behind, have no blind spot.

Which tiny retinal pit, made entirely of cones, gives the sharpest vision?

The fovea is about 1.5 mm wide and its centre has no blood vessels at all, so light reaches its tightly packed cones without scattering.

Which cranial nerve's Latin name means 'wandering', fitting its route down to the gut?

The vagus, Latin for 'wandering', is the longest nerve of the autonomic nervous system and was once called the pneumogastric nerve for serving both lungs and stomach.

Roughly how many taste buds does the average human tongue carry?

The tongue carries roughly 2,000–8,000 taste buds, each renewed about every ten days; they sit on papillae, and three of the four papilla types carry them.

On average, how much do human fingernails grow per month?

About 3.5 mm a month, so a lost fingernail takes three to six months to regrow while a toenail can take a year and a half.

Hair, nails and the outer layer of skin are built mainly from which protein?

Keratin is the tough structural protein of hair, nails and the epidermis; the same family forms feathers, horns, hooves and the baleen plates of filter-feeding whales.

Which cells in the bottom layer of the epidermis produce the pigment that colours skin?

Melanocytes make melanin; darker and lighter skin have similar numbers of them, and the difference lies in how many pigment packets, melanosomes, they hand to neighbouring keratinocytes.

How many separate or fused vertebrae make up the human tailbone, the coccyx?

The coccyx is made of three to five rudimentary vertebrae, usually four, often fused in adults; it is the remnant of a vestigial tail and anchors pelvic-floor muscles.

Which long, thin muscle running diagonally across the thigh is the body's longest?

The sartorius, running from the hip to the inner knee, is the longest muscle; its name comes from the Latin for tailor, supposedly because it helps cross the legs as tailors sat to work.

Which is the only long bone that lies horizontally in the human body?

The clavicle, the only long bone that lies horizontally, has a Latin name meaning 'little key' because it rotates like one when you raise your arm; it is among the most often broken bones.

Hitting your 'funny bone' actually strikes which nerve against the elbow?

The ulnar nerve runs exposed behind the inner elbow, trapped between bone and skin, so a knock sends a jolt to the little finger; 'funny bone' is probably a pun on 'humerus'.

Carpal tunnel syndrome comes from pressure on which structure at the wrist?

The median nerve shares the carpal tunnel with nine flexor tendons; pressure on it there numbs the thumb, index and middle fingers, the classic pattern of carpal tunnel syndrome.