How people measured things in the early days
In early times various body parts were used as tools and guides, hence the word "feet," or "the rule of thumb."
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Body Part Facts
Body part facts and facts about body parts.
121 facts on this shelf
How many neurons are in the human brain?
The human brain contains about 86 billion neurons.
How much energy does the brain use?
The brain makes up only about 2% of body weight but uses about 20% of the body's energy.
Can the brain feel pain?
The brain itself has no pain receptors, which is why some brain surgeries can be done while the patient is awake.
Do we only use 10% of our brains?
The idea that people use only 10% of their brains is a myth. Brain scans show activity across nearly all regions.
How much of the brain is fat?
The brain is nearly 60% fat, making it one of the fattiest organs in the body.
When does the brain finish developing?
The brain isn't fully developed until about the mid-20s, and the prefrontal cortex, which handles planning and impulse control, is one of the last parts to mature.
What happened to Albert Einstein's brain?
After Albert Einstein died in 1955, pathologist Thomas Harvey removed and kept his brain for study, reportedly without the family's permission.
How fast do brain signals travel?
Signals in the fastest nerve fibers can travel at about 268 miles per hour.
What does the brain do while you sleep?
During sleep, the brain's glymphatic system flushes out waste that builds up while you're awake.
What does a living brain feel like?
A living brain has a soft texture often compared to firm tofu.
How many times does the heart beat in a lifetime?
The heart beats about 100,000 times a day and more than 2.5 billion times over an average lifetime.
How much blood does the heart pump each day?
The heart pumps about 2,000 gallons of blood a day.
Can a heart beat outside the body?
The heart has its own electrical system, so it can keep beating briefly outside the body as long as it has oxygen.
How long are all your blood vessels?
Laid end to end, the blood vessels in one human body would stretch about 60,000 miles.
How big is the human heart?
An adult human heart is about the size of a fist.
Is broken heart syndrome real?
Broken heart syndrome is a real condition in which intense stress temporarily weakens the heart muscle.
Are heart attack symptoms different in women?
Women are more likely than men to have heart attack symptoms besides chest pain, like nausea, shortness of breath, or back or jaw pain.
When was the first human heart transplant?
Surgeon Christiaan Barnard performed the first human heart transplant in South Africa in 1967.
Why is heart cancer so rare?
Cancer that starts in the heart is very rare, partly because heart muscle cells rarely divide.
Is your heart on the left side of your chest?
The heart sits near the center of the chest, tilted slightly to the left.
What is the eye's blind spot?
Every eye has a blind spot where the optic nerve connects, but your brain fills in the missing part, so you don't notice it.
Do we see the world upside down?
The lens of the eye projects images onto the retina upside down, and the brain flips them right side up.
How many colors can the human eye see?
The human eye can tell apart about a million different colors.
How often do we blink?
People blink about 15 to 20 times a minute.
Why are iris scans used for security?
The patterns in the iris are unique to each person, so they can be used for identification.
Can some people see more colors than others?
Some people, nearly all of them women, have tetrachromacy and may be able to see more colors than most people.
How common is color blindness?
About 1 in 12 men are colorblind.
Where does the cornea get its oxygen?
The cornea has no blood vessels and gets oxygen directly from the air.
What are the most active muscles in the body?
The eye muscles are among the most active muscles in the body.
Do all blue-eyed people share a common ancestor?
A 2008 study suggested that blue-eyed people share a common ancestor who lived about 6,000 to 10,000 years ago.
What is the largest organ in the body?
Skin is the body's largest organ, covering about 20 square feet in adults.
How many skin cells do we shed?
People shed about 30,000 to 40,000 dead skin cells every minute.
Do identical twins have the same fingerprints?
Fingerprints form before birth, and even identical twins have different ones.
Why do we get goosebumps?
Goosebumps are a leftover reflex from furrier ancestors, whose hair stood up to trap warmth or look bigger.
What gives skin its color?
Skin color comes mainly from melanin.
How often does skin renew itself?
The outer layer of skin renews itself about once a month in young adults.
Why do fingers wrinkle in water?
Fingers wrinkle in water because of a nervous system response, which may help us grip wet objects.
Do koalas have fingerprints?
Koalas have fingerprints that look very similar to human ones.
Where is the thickest skin on the body?
The thickest skin is on the soles of the feet, and the thinnest is on the eyelids.
Why are tattoos permanent?
Tattoos stay because immune cells in the skin absorb the ink and hold onto it.
Can the liver regrow?
The liver can regenerate. When part of it is donated, both the donor's liver and the transplanted portion can grow back to nearly full size.
What is the largest internal organ?
The liver is the largest internal organ, weighing about three pounds.
How many jobs does the liver do?
The liver performs more than 500 functions.
What does the liver store?
The liver stores vitamins and glycogen.
What Greek myth involves a regenerating liver?
In Greek mythology, Prometheus was punished by having an eagle eat his liver every day, only for it to grow back each night.
What does bile do?
The liver makes bile, which helps digest fat.
How does the body break down alcohol?
The liver breaks down alcohol.
Why is polar bear liver toxic?
Polar bear liver contains so much vitamin A that eating it can poison humans.
How much blood flows through the liver?
The liver receives blood from two sources, the hepatic artery and the portal vein, and about a quarter of the blood the heart pumps passes through it every minute.
Why does jaundice make skin yellow?
Jaundice turns skin yellow because of a buildup of bilirubin.
How many bones are in the human body?
Adults have 206 bones, while babies are born with around 300 that later fuse together.
What is the smallest bone in the body?
The smallest bone in the body is the stapes, in the middle ear.
What is the longest bone in the body?
The femur, or thigh bone, is the longest and strongest bone in the body.
Where are most of the body's bones?
More than half of the body's bones are in the hands and feet.
How many blood cells does bone marrow make?
Bone marrow makes hundreds of billions of new blood cells every day.
How often is the skeleton replaced?
The adult skeleton is gradually replaced about every 10 years.
Which bone isn't connected to any other bone?
The hyoid bone in the neck is the only bone not connected to another bone.
Is the funny bone a bone?
The "funny bone" isn't a bone. It's the ulnar nerve.
Does cracking your knuckles cause arthritis?
Cracking your knuckles doesn't cause arthritis. One doctor cracked the knuckles of only one hand for 60 years to prove it.
Do astronauts lose bone in space?
Astronauts can lose about 1% of their bone mass per month in space.
Is the tongue map real?
The idea that different parts of the tongue taste only certain flavors is a myth.
Who discovered umami?
Umami, the fifth basic taste, was identified by Japanese chemist Kikunae Ikeda in 1908.
Are tongue prints unique?
Tongue prints are unique to each person.
How often are taste buds replaced?
Taste buds are replaced about every one to two weeks.
How many muscles are in the tongue?
The tongue is made up of eight muscles.
Is tongue rolling genetic?
Tongue rolling isn't controlled by a single simple gene.
What is a supertaster?
About 25% of people are "supertasters" who taste some flavors more intensely.
Is fat a taste?
Some scientists think fat may be a sixth basic taste.
Who has the longest tongue?
Nick Stoeberl holds the Guinness World Record for the longest tongue, at about 10.1 centimeters.
How much of taste is smell?
Much of what people think of as taste is actually smell.
Do ears keep growing?
Ears get longer as people age.
How do ears help with balance?
The inner ear helps control balance.
Why do some people have dry earwax?
Whether earwax is wet or dry is determined by genetics.
What range of sounds can humans hear?
Humans can hear sounds from about 20 to 20,000 hertz, but we lose the highest frequencies as we age.
Are ears self-cleaning?
Ears are self-cleaning.
Do ears work while you sleep?
Your ears keep working while you sleep.
What shape is the cochlea?
The cochlea, the hearing organ in the inner ear, is shaped like a snail shell.
Why do ears pop?
Ears pop when the Eustachian tubes equalize air pressure.
How common is tinnitus?
About 15% of people experience tinnitus, a ringing in the ears.
Why does your recorded voice sound different?
Your voice sounds different on a recording because you normally hear it partly through the bones in your head.
Why is the left lung smaller?
The left lung is smaller than the right to make room for the heart.
How big is the surface area of the lungs?
The inner surface area of the lungs is roughly the size of a tennis court.
How many air sacs are in the lungs?
The lungs contain about 480 million tiny air sacs called alveoli.
How many breaths do we take each day?
People breathe about 20,000 times a day.
Which organ can float in water?
The lungs are the only organs that can float in water.
What muscle helps us breathe?
The lungs have no muscles of their own. The diaphragm does most of the work of breathing.
What causes hiccups?
Hiccups are caused by sudden spasms of the diaphragm.
Can you live with one lung?
A person can live with one lung.
Why do premature babies need surfactant?
Premature babies may need surfactant because their lungs don't yet make enough of it.
What is the record for holding your breath?
The Guinness record for holding your breath, set after breathing pure oxygen, is more than 24 minutes.
How long is the small intestine?
The small intestine is about 20 feet long.
How many bacteria live in the gut?
The gut is home to trillions of bacteria, known as the gut microbiome.
Where is most serotonin made?
Most of the body's serotonin is made in the gut.
Why is the gut called the second brain?
The gut has its own nervous system with about 500 million neurons, which is why it's called the "second brain."
Why doesn't stomach acid destroy the stomach?
The stomach lining is constantly replaced to protect it from its own acid.
How acidic is stomach acid?
Stomach acid has a pH of about 1 to 2.
What is a growling stomach called?
The scientific term for stomach growling is "borborygmi."
Does the appendix have a purpose?
The appendix may store helpful gut bacteria.
How long does food take to pass through the body?
Food takes about 24 to 72 hours to move through the digestive tract.
Who studied digestion through a hole in a man's stomach?
Doctor William Beaumont studied digestion through a permanent hole in patient Alexis St. Martin's stomach in the 1820s.
What is the hardest substance in the body?
Tooth enamel is the hardest substance in the human body.
Can teeth heal themselves?
Teeth can't repair enamel damage on their own.
Are teeth unique?
Every person's teeth are unique, which is why dental records are used to identify people.
How many teeth do people have?
Children have 20 baby teeth, and adults have 32 permanent teeth.
Does everyone get wisdom teeth?
Many people never develop some or all of their wisdom teeth.
Were George Washington's dentures made of wood?
George Washington's dentures weren't made of wood. They included ivory and human teeth, some likely from enslaved people.
Where was the first bristle toothbrush made?
The first bristle toothbrush was made in China in 1498.
When do teeth start forming?
Teeth begin forming before birth.
How much saliva do we make?
People make about half a liter to a liter and a half of saliva every day.
Are teeth bones?
Teeth aren't bones.
How many scents can the human nose detect?
A 2014 study estimated that humans can tell apart about a trillion scents, though other scientists dispute the number.
What is the nasal cycle?
The nose switches which nostril does most of the breathing every few hours, in a pattern called the nasal cycle.
Why do smells trigger memories?
Smell is closely linked to memory because the brain's smell center is connected to areas involved in memory and emotion.
Can you hum with your nose plugged?
You can't hum with your nose plugged.
How much mucus does the nose make?
The nose and sinuses produce about a quart of mucus a day.
What is the photic sneeze reflex?
Some people sneeze when they see bright light, a trait called the photic sneeze reflex.
What is anosmia?
Anosmia is the loss of the sense of smell.
What do nose hairs do?
Nose hairs filter dust and particles out of the air.
Does the sense of smell change with age?
The sense of smell declines with age.
Can humans track scents like dogs?
A 2007 UC Berkeley study found that people can follow a scent trail like dogs.