130 Fun Facts About The Human Body

How many bones are in the adult human body?

Babies start with dozens more, and many of those fuse together during growth.

What is the largest organ in the human body?

It makes up roughly 16% of body mass; among internal organs the title goes to the one that makes bile.

Which type of blood cell is responsible for fighting infections?

White blood cells (leukocytes) are the body's primary defense against infections and disease. They identify and destroy pathogens.

What is the hardest substance in the human body?

It is about 96% mineral, yet unlike bone it cannot repair itself once damaged.

Which chamber of the heart pumps blood to the entire body?

Its walls are much thicker than the right ventricle's because it has to push blood around the whole body rather than just to the lungs.

How many chromosomes do humans typically have?

They come in 23 pairs, one of each pair inherited from each parent.

What is the smallest bone in the human body?

The stapes (stirrup bone) in the middle ear is the smallest bone in the human body, measuring just 2-3 millimeters long.

Which organ is primarily responsible for filtering blood and producing urine?

They filter the entire blood volume dozens of times a day, reabsorbing almost all of the filtrate and leaving only a litre or two of urine.

Which small brain region controls body temperature, hunger, thirst and circadian rhythms?

It also links the nervous system to the endocrine system by directing the 'master gland' hanging just beneath it.

What pigment gives human blood its red color?

This iron-containing protein binds oxygen; when iron runs short the cells turn pale, the hallmark of iron-deficiency anaemia.

What is the longest bone in the human body?

The femur (thighbone) is the longest, heaviest, and strongest bone in the human body, making up about a quarter of a person's total height.

Approximately how many taste buds does the average human tongue have?

They are replaced roughly every 10 to 14 days, though the turnover slows with age.

What is the primary job of platelets?

They gather at a break in a vessel wall and plug it, then trigger the clotting cascade that lays down fibrin to seal it.

Which vitamin does the body make for itself when sunlight falls on the body's outer surface?

Ultraviolet B rays convert a form of cholesterol in the outer layers into the vitamin, which is why it is only conditionally essential in the diet.

How many liters of blood does an average adult human body contain?

The average adult body contains about 5 liters (roughly 1.3 gallons) of blood, which makes up about 7% of total body weight.

What is the tough fibrous protein that forms hair, nails and the outer layer of skin?

The same protein family builds feathers, horns, claws and hooves in other vertebrates.

Which part of the digestive system, about six metres long, absorbs roughly 90% of the nutrients from food?

The small intestine's villi and microvilli give it a surface area of tens of square metres; the large intestine mainly reclaims water.

What is the average resting heart rate for a healthy adult?

Ultra-trained endurance athletes can sit as low as 37–38 beats per minute at rest.

Which organ, the largest solid organ inside the body, is also the one that produces bile?

It weighs about 1.5 kilograms and handles hundreds of jobs, from detoxification to storing glycogen.

How many pairs of ribs does a typical human have?

Vesalius counting 24 in 1543 caused a stir, since tradition held that men had one fewer than women.

What is the name of the thin membrane that covers the outside of bones?

The periosteum is a dense layer of connective tissue that covers the outer surface of most bones. It contains nerves and blood vessels that nourish the bone.

Which hormone is responsible for regulating blood sugar levels?

Insulin's opposite is glucagon, which raises blood sugar; both come from the islets of Langerhans in the pancreas.

Approximately how many neurons are in the human brain?

There are roughly as many non-neuronal cells again, and each neuron can form thousands of connections.

What type of joint is the knee?

The knee is classified as a modified hinge joint (ginglymus). While it primarily allows flexion and extension, it also permits slight rotation when the knee is bent.

What percentage of the human body is water?

The adult human body is approximately 60% water by weight. The brain and heart are about 73% water, while the lungs are about 83% water.

At about one millimetre long, which muscle of the middle ear is the smallest skeletal muscle in the body?

It steadies the stirrup bone and dampens loud sounds before they reach the inner ear.

What is the name of the structure that connects muscle to bone?

Tendons are tough bands of fibrous connective tissue that connect muscles to bones. Ligaments, in contrast, connect bone to bone.

Where in the body are new oxygen-carrying cells of the circulation made?

In adults it happens mainly in flat bones such as the pelvis, sternum and skull, at a rate of about 2 million cells per second.

In a living adult, the part of the gut where most nutrients are absorbed is typically how long?

Measurements at autopsy run longer, around 6.5 metres, because the muscle relaxes after death; the organ is called "small" for its diameter, not its length.

What is the colored part of the eye called?

It also acts as an adjustable aperture, changing the size of the pupil to control how much light gets in.

What is the name of the valve between the two chambers on the left side of the heart?

Also called the bicuspid valve, it has two flaps, unlike the tricuspid valve on the right side.

What is the approximate speed at which nerve impulses travel through the body?

That is the top speed along thick myelinated fibres; the slowest unmyelinated fibres crawl along at about 0.5 metres per second.

Which gland is often called the "master gland" because it controls other endocrine glands?

The pituitary gland, about the size of a pea at the base of the brain, produces hormones that regulate many other glands including the thyroid, adrenals, and gonads.

What is the medical term for the collarbone?

This S-shaped strut between the shoulder blade and breastbone is the most commonly broken bone in the body.

Which structure connecting the cerebrum to the spinal cord controls breathing and heart rate?

Its medulla houses the cardiac, respiratory and vasomotor centres, and it connects the rest of the brain to the spinal cord.

What is the name of the protein responsible for muscle contraction?

Actin and myosin are the two main proteins involved in muscle contraction. They slide past each other in a mechanism called the sliding filament theory.

Into how many stages does the American Academy of Sleep Medicine divide non-REM sleep?

N3 is the deep slow-wave stage; add REM and a full cycle takes about 90 minutes.

What is the function of the epiglottis?

This leaf-shaped flap of cartilage folds down over the larynx during swallowing so food and drink are routed down the esophagus instead.

Which part of the eye is responsible for detecting light and sending signals to the brain?

The retina is a thin layer of tissue at the back of the eye containing photoreceptor cells (rods and cones) that detect light and convert it into electrical signals sent to the brain via the optic nerve.

What is the largest muscle in the human body?

The gluteus maximus is the largest muscle in the human body. It makes up the bulk of the buttocks and is essential for maintaining an upright posture and climbing stairs.

What is the name of the process by which cells divide to produce two identical daughter cells?

Meiosis, by contrast, halves the chromosome number and produces genetically different cells for reproduction.

Which small organ stores and concentrates bile until fat arrives in the gut?

It receives the bile through the common hepatic duct and squeezes it out when a fatty meal needs emulsifying.

What is the technical name for the kneecap?

It is the body's largest sesamoid bone, embedded in the quadriceps' attachment to protect the joint and improve the muscle's leverage.

What is the Broca's area of the brain primarily responsible for?

Broca's area, located in the left frontal lobe, is crucial for speech production and language processing. Damage to it causes Broca's aphasia, where a person understands language but struggles to speak fluently.

Approximately how many cells are in the human body?

The human body contains approximately 37.2 trillion cells, according to recent estimates. This number does not include the roughly equal number of bacteria living in and on the body.

What is the name of the connective tissue that connects bone to bone?

Ligaments are bands of tough, elastic connective tissue that connect bones to other bones at joints, providing stability and limiting excessive movement.

What is the medical term for low blood sugar?

Hypoglycemia means low blood sugar (glucose). The prefix 'hypo' means low/under, and 'glycemia' refers to sugar in the blood. Hyperglycemia is the opposite — high blood sugar.

Which spiral of the inner ear turns sound vibrations into nerve signals?

The spiral is fluid-filled; damage to it or the auditory nerve causes sensorineural hearing loss, while the semicircular canals handle balance.

Which type of blood vessel carries blood away from the heart?

Arteries carry blood away from the heart (remember: A for Away). Veins carry blood back to the heart. Capillaries are tiny vessels connecting arteries to veins.

Which salivary enzyme starts breaking down starch before you've even swallowed?

Salivary amylase (also called ptyalin) begins breaking down starches into simpler sugars as soon as food enters the mouth. This is why bread tastes slightly sweet if chewed for a long time.

What is the outermost of the body's main integumentary layers called, sitting above the dermis?

It contains no blood vessels; its deepest cells are fed almost entirely by oxygen diffusing in from the air.

Roughly how many bones does a newborn baby have, before many of them fuse?

By adulthood the count drops by more than sixty as bones in the skull, spine and pelvis fuse together.

Which blood type is considered the 'universal donor' for whole-blood transfusions?

Its cells carry none of the A, B or Rh antigens, so they can be given to almost anyone in an emergency.

Which part of the brain, tucked beneath the back of the cerebrum, fine-tunes movement and balance?

The cerebellum holds more than half of all the brain's neurons despite its size, and alcohol hits it early, hence the stagger.

Which dome-shaped muscle does most of the work of breathing?

The diaphragm flattens as it contracts, pulling air in; a hiccup is a spasm of it followed by the vocal cords snapping shut.

Which part of the digestive system mainly absorbs water and turns what is left of food into stool?

The large intestine is also home to most of your gut bacteria, which ferment whatever you could not digest.

Which gland behind the stomach produces the hormones that regulate blood sugar?

It is also a digestive organ, pouring enzymes into the gut, and its role in blood sugar was pinned down in 1921.

The hammer, anvil and stirrup, the smallest bones in the body, are known collectively as what?

The ossicles amplify vibrations from the eardrum to the inner ear; the stirrup, or stapes, is the smallest bone you have.

Which organelle, the cell's 'powerhouse', generates most of its ATP?

Mitochondria are membrane-bound organelles that generate most of the chemical energy needed to power the cell's biochemical reactions, stored in adenosine triphosphate (ATP).

Which jaw muscle, used for chewing, is often called the strongest in the body for its size?

The masseter can clamp the molars together with well over 100 kilograms of force, and it is the most superficial of the chewing muscles.

Which protein lets artery walls and lungs snap back after stretching, and whose loss makes faces sag with age?

Elastin is one of the two parts of elastic fibres; collagen, its partner, gives strength rather than stretch.

About how many liters of filtrate do the body's two bean-shaped filters generate each day?

Almost all of it is reabsorbed, leaving only a litre or two of urine.

What is the largest artery in the body, carrying blood out of the heart's left ventricle?

The aorta arches up from the heart, runs down the abdomen and splits in two at the pelvis; it is about the width of a garden hose.

Which fatty sheath insulates nerve fibres and speeds their electrical impulses?

Think of the axon as a wire and this lipid-rich wrapping as its insulation; losing it is what causes the symptoms of multiple sclerosis.

What is the second-largest bone in the human body, better known as the shinbone?

The tibia takes most of the body's weight below the knee; the slender fibula beside it mostly anchors muscles.

Which acid does the stomach lining secrete, strong enough to kill most of the bacteria you swallow?

Hydrochloric acid gives stomach juice a pH of around 1 to 2; a layer of mucus stops it from digesting the stomach itself.

Which hormone is most closely associated with the 'fight or flight' response?

Also called epinephrine, it is released by the adrenal glands and boosts heart output, dilates the pupils and raises blood sugar.

Which small pit at the centre of the retina gives the sharp central vision needed for reading and driving?

The fovea is packed with cones and has no rods at all, which is why a faint star vanishes when you look straight at it.

Which cells in the circulation are primarily responsible for transporting oxygen throughout the body?

Better known as red blood cells, they are packed with an iron-rich pigment that grabs oxygen in the lungs and hands it over to the tissues.

How many chambers does the human heart have?

The two atria receive blood and the two ventricles pump it out, with the left side feeding the whole body and the right side only the lungs.

Which long bone of the upper arm ends at the elbow?

Its 'surgical neck' is so called because it fractures so often.

Which is the only bone in the human body not connected to any other bone?

It sits in the neck and anchors the tongue and larynx muscles.

How many vertebrae are there normally in the human spinal column?

The lower nine are fused in adults: five in the sacrum and four in the coccyx.

The human skull is generally considered to consist of how many bones?

That is eight cranial bones and fourteen facial bones.

How many bones are in each human hand, not counting sesamoids?

Fourteen of them are the phalanges of the fingers and thumb; eight are wrist (carpal) bones.

How many bones does the human foot contain?

It also has 33 joints and more than a hundred muscles, tendons and ligaments.

Which is the longest muscle in the human body?

It runs across the thigh and was historically called 'couturier', French for tailor.

Cardiac muscle is described as which two things?

Its striped look under a microscope comes from myofibrils organised into sarcomeres.

Which organ acts primarily as a blood filter and resembles a large lymph node?

It plays important roles for red blood cells and the immune system.

Which type of immune cell matures within the thymus?

T cells are central to the adaptive immune system's targeted responses.

Which butterfly-shaped gland in the neck secretes thyroxine and calcitonin?

It sits just below the Adam's apple.

Where in the body are the adrenal glands found?

They produce adrenaline and the steroids aldosterone and cortisol.

Which gland produces melatonin, the hormone that modulates sleep patterns?

In some amphibians and reptiles it is linked to a light-sensing 'third eye'.

Roughly how many alveoli does a typical pair of human lungs contain?

Together they provide 70 to 80 square metres of surface for gas exchange.

How many lobes does the right lung have?

The left lung has only two, leaving room for the heart.

The trachea is reinforced by 16–20 rings of what?

The rings are incomplete and C-shaped, about 4 mm high in adults.

Blood plasma makes up roughly what share of total blood volume?

Red blood cells account for about 45% and white cells under 1%.

For roughly how long do red blood cells circulate before being recycled?

They lack a nucleus, leaving maximum room for hemoglobin.

Who distinguished the main ABO blood groups in 1901?

He later co-identified the Rhesus factor, making transfusions far safer.

Which region of the right atrium acts as the heart's natural pacemaker?

Its pacemaker cells set the sinus rhythm.

What is the double-walled, fluid-filled sac that contains the heart called?

It has a tough fibrous outer layer and a serous inner layer.

How many pairs of cranial nerves do humans conventionally have?

They are numbered with Roman numerals I to XII; the olfactory nerve is I.

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

It runs from the top of the leg down to the foot.

Which structure connects the left and right cerebral hemispheres?

It is the brain's largest white-matter structure, with 200–300 million axons.

Which brain region consolidates short-term memories into long-term ones?

It is one of the first regions damaged in Alzheimer's disease.

Which almond-shaped brain structure plays a key role in processing fear?

It is also involved in memory, decision-making and aggression.

Which neurotransmitter is central to the brain's reward-motivated behaviour?

Many addictive drugs increase its release or block its reuptake.

Which cranial nerve, the tenth, is the longest of the autonomic nervous system?

Its name means 'wandering'; it regulates involuntary functions of the heart, lungs and gut.

Which retinal cells respond to low-intensity light and give grayscale vision?

Rods number around 120 million to the cones' 6 million, which is why you see no colour by starlight.

Roughly how many olfactory receptor genes do humans have?

Mice have around 1,400, the largest multigene family in vertebrates.

What is the medical name for the eardrum?

This thin cone-shaped membrane separates the outer ear from the middle ear.

How many semicircular canals are there in each inner ear?

The lateral, anterior and posterior canals sense rotation and help with balance.

Which specialised cells produce the pigment melanin?

The name comes from the Greek 'melas', meaning black or dark.

Which type of sweat gland is largely limited to the armpits and perineal area?

Eccrine glands cover almost the whole body, densest on palms and soles.

What is the wave of muscle contractions that pushes food along the gut, even if you eat upside down?

Peristalsis is why swallowing works in any position: smooth muscle squeezes in sequence behind each ball of food.

Bile, produced by the liver, aids the digestion of which nutrients?

It is a yellow-green fluid stored and concentrated in the gallbladder.

Roughly how many nephrons does a healthy adult have in each kidney?

The nephron is the kidney's microscopic functional unit.

Cortisol belongs to which class of steroid hormones?

It rises in response to stress and low blood glucose.

Which hormone is released during childbirth and is linked to social bonding?

It is released into the bloodstream in response to sexual activity as well.

Antibodies are also known by what name?

They identify and neutralise antigens on bacteria and viruses.

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

20 milk teeth are replaced by 32 adult teeth, including four wisdom teeth that many people never get at all.

Wisdom teeth are formally known as what?

They are the most posterior teeth in each quadrant and erupt late, hence the name.

Which German physician set the 'normal' body temperature of 37 °C in his 1868 book?

He put temperature charts into widespread clinical use.

Approximately how many base pairs does the human genome contain?

Most cells are diploid, so they hold about 6.2 billion base pairs.

Which two joints are the classic examples of ball-and-socket joints?

In the hip, the femur's head sits in the pelvis's acetabulum.

What are the specialised cells that make up cartilage called?

They get their nutrition by diffusion, since cartilage lacks blood vessels.

Which organ is commonly called the voice box?

It houses the vocal cords and protects the trachea against food.

The Adam's apple is formed by the angle of which cartilage?

It becomes more prominent in most men during puberty.

Hitting your 'funny bone' actually strikes which nerve?

It is the largest nerve in the body unprotected by muscle or bone.

What thick cord at the back of the lower leg attaches the calf muscles to the heel?

It attaches the calf muscles to the calcaneus, or heel bone.

Which nerve provides exclusive motor control of the diaphragm?

It originates from the C3–C5 spinal nerves in the neck.

Who discovered the hormone-producing pancreatic islets in 1869?

Their beta cells produce insulin and alpha cells produce glucagon.

Which hormone has the opposite effect to insulin, raising blood glucose?

It is considered the body's main catabolic hormone.

From mouth to anus, the human digestive tract covers roughly what distance?

That is roughly 30 feet.

How many phalanges are there in the human body altogether?

Fourteen on each hand and foot; thumbs and big toes have two each, other digits three.

Which two pairs of ribs are the 'floating ribs', not attached to the sternum at all?

Pairs one to seven are 'true ribs' attached directly to the sternum.

Which are the most abundant type of white blood cell in humans?

They make up 40% to 70% of all white cells and roughly 100 billion are produced daily.

Mitochondrial genes are normally inherited from which parent?

This is why mitochondrial DNA is used to trace maternal ancestry.

Which are the smallest blood vessels in the body?

Their walls are a single layer of squamous endothelial cells.

The knee-jerk (patellar) reflex tests which segments of the spinal cord?

The signal synapses in the spinal cord without involving the brain.