Fun Facts About the Human Body

A newborn arrives with roughly 300 bones, while a typical adult skeleton has about 206. The difference is not loss but fusion: separate pieces of cartilage and bone knit together as the body matures, most obviously in the skull and the sacrum.

Source: National Institutes of Health

More than half the bones in the human body are in the hands and feet. Each hand and wrist accounts for 27 and each foot and ankle for 26, which is 106 of the adult total of roughly 206.

The stapes in the middle ear is the smallest bone in the body, at around three millimetres long. It transmits vibration from the eardrum to the fluid of the inner ear, and it is fully grown at birth.

Source: Encyclopaedia Britannica

The femur is the longest and heaviest bone in the body, typically about a quarter of a person's height. It also carries the largest loads, routinely handling several times body weight during running.

Bone is not inert scaffolding. Adults replace roughly ten percent of their skeleton each year through remodelling, with osteoclasts dissolving old bone and osteoblasts laying down new material.

Source: National Institutes of Health

The gluteus maximus is the largest muscle in the body by mass, and it is unusually well developed in humans compared with other primates. Anatomists link its size to the mechanics of upright running.

Source: Nature

The masseter, the jaw muscle running from cheekbone to lower jaw, is among the strongest muscles relative to its size. Recorded human bite forces at the molars run into the hundreds of newtons.

Skeletal muscle contains a mix of slow-twitch fibres, which resist fatigue, and fast-twitch fibres, which contract quickly but tire fast. The proportions vary between individuals and between muscles in the same person.

Source: Cleveland Clinic

Shivering is muscle work repurposed as a heater. Rapid involuntary contractions produce almost no useful movement, so nearly all the energy comes out as heat.

A resting adult heart beats roughly 60 to 100 times a minute and pumps about five litres of blood in that time, which is close to the body's entire blood volume. Under hard exercise that output can rise several times over.

Source: American Heart Association

The heart has its own dedicated blood supply, the coronary arteries, because the blood rushing through its chambers cannot nourish the muscle walls directly.

Source: Cleveland Clinic

A red blood cell survives about 120 days in circulation, then is broken down mainly in the spleen and liver. Mature red cells carry no nucleus, which leaves more room for haemoglobin and gives them their flexible biconcave shape.

Source: National Institutes of Health

Blood is roughly 55 percent plasma by volume, and plasma is about 90 percent water. The straw colour of plasma is the actual colour of blood minus its cells.

Source: American Red Cross

The two lungs are not mirror images. The right lung has three lobes and the left has two, because the left one gives up space to the heart in a notch called the cardiac impression.

Human lungs contain on the order of 300 to 500 million alveoli, the tiny sacs where gas exchange happens. Spread flat, that gas-exchange surface covers something close to the area of a tennis court.

Source: National Institutes of Health

Breathing out is usually the lazy half of the cycle. At rest the diaphragm and rib muscles do the work of inhaling, and exhalation happens largely by elastic recoil of the lungs and chest wall.

A yawn is one of the few reliably contagious behaviours in humans, and simply reading about yawning can trigger one. The physiological purpose of yawning itself remains unsettled.

The adult brain accounts for roughly two percent of body weight but consumes around twenty percent of the body's energy at rest. It is the most metabolically expensive organ per gram in ordinary daily life.

Source: National Institutes of Health

Current estimates put the number of neurons in the adult human brain at about 86 billion, alongside a comparable number of non-neuronal glial cells. The older textbook figure of 100 billion was an estimate that never had a careful count behind it.

Source: Journal of Comparative Neurology

Nerve signals travel at very different speeds depending on the fibre. Thick myelinated motor fibres can conduct at well over 50 metres per second, while thin unmyelinated fibres carrying dull aching pain crawl along at around one metre per second.

Source: Cleveland Clinic

Brain tissue itself has no pain receptors, which is why neurosurgery can be performed on an awake patient. Headaches come from surrounding structures such as blood vessels, membranes and muscles.

Source: Johns Hopkins Medicine

The gut has its own nervous system, the enteric nervous system, containing hundreds of millions of neurons embedded in the wall of the digestive tract. It can coordinate digestion largely on its own.

Source: National Institutes of Health

Skin is the body's largest organ, covering roughly 1.5 to 2 square metres in an adult and accounting for about 15 percent of body weight. Its thickness ranges from well under a millimetre on the eyelids to several millimetres on the soles.

Source: Cleveland Clinic

The outer layer of skin is continuously replaced, with cells taking roughly a month to travel from the base of the epidermis to the surface and flake away. A large share of household dust is shed human skin.

Source: American Academy of Dermatology

Fingerprints form in the womb, driven partly by genetics and partly by the chance conditions of fetal development. Identical twins share DNA but not fingerprint patterns.

Source: National Institute of Justice

Goosebumps are a leftover reflex. A tiny arrector pili muscle at the base of each hair contracts and lifts it, which would fluff up an insulating coat in a furrier animal and does very little for us.

The cornea has no blood vessels, since vessels would scatter light and blur vision. It takes oxygen directly from the air and nutrients from tears and the fluid behind it.

Source: National Eye Institute

Each eye has a blind spot where the optic nerve leaves the retina and there are no photoreceptors. The brain fills the gap so seamlessly that most people never notice it without a deliberate test.

Source: National Eye Institute

The inner ear does double duty. Alongside the cochlea for hearing sit the semicircular canals and otolith organs, which sense rotation and acceleration and supply much of your sense of balance.

Source: National Institutes of Health

Healthy young humans hear roughly from 20 to 20,000 hertz, and the upper limit falls steadily with age. Sensitivity is sharpest in the range that carries speech.

Source: National Institute on Deafness and Other Communication Disorders

Adults have about 10,000 taste buds, and every bud can detect all the basic tastes rather than specialising by region of the tongue. Taste buds are also found on the palate and in the throat.

Source: National Institutes of Health

Much of what people call flavour is actually smell. Odour molecules reach the nose from the back of the mouth while chewing, which is why food tastes flat when nasal congestion blocks that route.

Source: Monell Chemical Senses Center

The small intestine is around six to seven metres long in a living adult and is where most nutrient absorption occurs. Folds, finger-like villi and microscopic microvilli multiply its absorbing surface enormously.

Source: National Institutes of Health

Stomach acid is strongly acidic, typically around pH 1.5 to 3.5, and the stomach survives it by secreting a thick mucus layer with bicarbonate underneath. The lining cells are also replaced rapidly.

Source: National Institutes of Health

The liver performs hundreds of distinct jobs, from filtering blood and processing nutrients to making bile and clotting proteins. It is also the only major internal human organ capable of substantial regeneration after part of it is removed.

Source: Mayo Clinic

The immune system keeps records. After an infection, memory B and T cells persist for years or decades, which is why a second encounter with the same pathogen is usually met far faster than the first.

Source: National Institutes of Health

Fever is not the infection but the response. The hypothalamus raises the body's temperature set point in reaction to immune signalling, and the chills people feel are the body working to reach the new target.

Source: Cleveland Clinic

A typical adult carries roughly as many bacterial cells as human cells, at a ratio near one to one rather than the ten to one figure long repeated. Most of them live in the large intestine.

Source: PLOS Biology

Any two unrelated humans are about 99.9 percent identical in their DNA sequence. The remaining fraction, spread over roughly three billion base pairs, still amounts to millions of individual differences.

Source: National Human Genome Research Institute

Mitochondrial DNA is inherited essentially from the mother alone, a separate small genome of about 16,500 base pairs sitting outside the cell nucleus. Genealogists use it to trace maternal lines across many generations.

Source: National Human Genome Research Institute