50 Fun Facts About Hard Human Body

Roughly how many bones does a newborn baby have before many fuse together?

Adults end up with around 206, not counting the little accessory bones some people have. Fusion in the skull, spine and pelvis accounts for the drop.

Which is the only bone in the human body that does not connect to any other bone?

It sits in the neck, held in place by muscles and ligaments, and anchors the tongue. Its fracture is a classic forensic sign of strangulation.

The kneecap is the largest example of what type of bone?

Sesamoid bones grow inside tendons. The patella glides over the femur and protects the front of the knee joint.

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

It steadies the stapes in the middle ear and contracts to dampen loud sounds. Its nerve is a branch of the facial nerve.

Which muscle, running diagonally across the thigh, is the longest in the human body?

Its old French name was couturier, the tailor, because sitting cross-legged as tailors once did uses every one of its actions.

Roughly what fraction of a healthy young adult's body weight is skeletal muscle?

There are more than 600 skeletal muscles. Muscle also has endocrine functions that researchers are still mapping.

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

It is the largest branch of the sacral plexus and runs from the back of the pelvis down to the foot.

Which cranial nerve, numbered X, wanders down from the brainstem to control the heart, lungs and gut?

One count found about 105,000 fibres in the right vagus and 87,000 in the left. Its name is Latin for 'wandering'.

Between which two vertebrae does the adult spinal cord end, tapering into the cauda equina?

It is about 45 cm long in men and 43 cm in women, which is why lumbar punctures are done lower down, below the cord.

How many vertebrae does the human spinal column normally have, counting the fused sacrum and coccyx?

Seven cervical, twelve thoracic, five lumbar, then the fused sacral and coccygeal segments. Giraffes also have seven neck vertebrae.

Which ribs are the 'floating' ribs, attached to no part of the sternum?

Ribs one to seven are 'true' ribs joining the sternum directly; eight to twelve are 'false', and the last two float free at the front.

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

A mouse kidney manages with about 12,500. One-fifth of the blood entering the kidneys is filtered at the glomerulus.

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

Together they give a gas-exchange surface of 70 to 80 square metres, and each alveolus is wrapped in capillaries covering about 70 percent of it.

Why is the left lung smaller than the right?

Together the lungs hold about 2,400 kilometres of airways.

Roughly how many new red blood cells does an adult produce every second?

Each one lives 100 to 120 days. Red cells are about 84 percent of all the cells in your body.

Why do mature human red blood cells have no nucleus?

The nucleus is expelled during development, so a red cell is essentially a flexible sack of haemoglobin with a membrane around it.

How many oxygen molecules can one haemoglobin molecule carry?

Each of its four subunits binds one oxygen molecule. In tissues it also mops up excess oxygen as an antioxidant.

Plasma makes up what share of blood by volume?

Plasma itself is 92 percent water, carrying proteins, glucose, salts and hormones. Blood is classed as a connective tissue.

Karl Landsteiner's 1901 blood-group labels were A, B and which letter, later changed to O?

He was working at the University of Vienna. There are now 48 recognised blood group systems, but ABO and Rh matter most for transfusion.

Which physician published De Motu Cordis in 1628, first fully describing blood circulation?

Earlier writers such as Servetus and Colombo had described parts of it. He published the book in Frankfurt.

Andreas Vesalius's landmark 1543 anatomy text is known by what short title?

'On the fabric of the human body in seven books' overturned much of Galen. Vesalius taught at Padua and became physician to Emperor Charles V.

Who illustrated the first edition of Gray's Anatomy in 1858?

Gray was only 31 when it was published and died of smallpox three years later. The book is still in print.

Insulin was first isolated from dog pancreas in 1921 in whose laboratory at the University of Toronto?

Banting and Best did the bench work; Banting and Macleod shared the 1923 Nobel Prize, and each split his share with a colleague.

Which cells of the pancreatic islets secrete insulin?

Their alpha-cell neighbours release glucagon when glucose is low. Only about one percent of the pancreas is endocrine tissue.

What proportion of the pancreas is exocrine, digestive tissue rather than hormone-producing?

The endocrine one percent secretes insulin, glucagon, somatostatin and pancreatic polypeptide.

Which three hormones does the thyroid gland secrete?

The most common cause of an underactive thyroid worldwide is iodine deficiency. The gland sits below the Adam's apple.

The hypothalamus links the nervous system to the endocrine system via which gland?

It releases 'releasing hormones' that tell that gland what to secrete or hold back.

Which steroids do the adrenal glands produce alongside adrenaline?

They sit on top of the kidneys, hence their other name, suprarenal glands.

What is the brain's largest white-matter structure, linking the two hemispheres?

It is about 10 centimetres long. Cutting it was once a treatment for severe epilepsy, producing the famous split-brain patients.

What does the word 'cerebellum' mean in Latin?

It does not start movements but fine-tunes their coordination, precision and timing.

The adult human brain weighs 1.2 to 1.4 kilograms. Roughly what share of body weight is that?

It contains more than 86 billion neurons and about as many glial cells.

Which cell type in the retina was identified around 2002, alongside rods and cones?

They respond to overall light levels and help set the body clock, which is why bright screens at night disturb sleep.

The cornea has no blood vessels. How does it get its oxygen?

Its transparency, avascularity and immune privilege are why corneal grafts rarely need tissue matching.

Which structures in the inner ear handle balance while you are moving, as opposed to standing still?

The utricle and saccule cover balance when stationary, and the cochlea handles hearing.

Per a 2016 estimate, roughly how many bacterial cells live in and on the human body?

That is roughly the same as the 30 to 40 trillion human cells, not the ten-to-one ratio once quoted. Their genes outnumber ours about 100 to one.

Who discovered cells in 1665 and named them after monks' rooms?

A typical adult body holds around 30 trillion of them, about 36 trillion in a man and 28 trillion in a woman.

Which cells resorb bone during remodelling?

Osteoblasts build and mineralise bone and osteocytes maintain it. The organic matrix is mostly ossein, a form of collagen.

Tooth enamel is the hardest substance in the body. What percentage of it is mineral?

The mineral is hydroxyapatite, a crystalline calcium phosphate. Acids from food and drink are what wear it away.

What is the average lifespan of a taste bud?

A tongue carries 2,000 to 8,000 of them, detecting salt, sour, bitter, sweet and umami.

After a 2007 reassessment, the appendix was proposed to do what?

It is no longer classed as vestigial. It averages 9 millimetres in diameter and is surrounded by immune tissue.

Which metal does the spleen recycle from the worn-out red blood cells it removes?

It also holds a reserve of blood that can be released in haemorrhagic shock, and is built like a giant lymph node.

Roughly how long is the human small intestine?

It folds many times to fit. The duodenum is the shortest of its three regions, followed by the jejunum and ileum.

What saddle-shaped depression in the sphenoid bone cradles the 'master gland'?

The name means 'Turkish saddle'. A fold of dura called the diaphragma sellae roofs it over.

Which layer of the epidermis exists only on the palms and the soles of the feet?

The epidermis has no blood vessels; its deepest cells are fed almost entirely by oxygen diffusing from the air.

Viewed from below, the liver shows four lobes: left, right, quadrate and which other?

From above the liver has just two lobes; from below the caudate and quadrate lobes appear between them.

As little as what fraction of a liver can regrow into a whole liver, the only internal organ able to do so?

Just 25% of a liver can regenerate into a full one — technically compensatory growth, since removed lobes do not regrow.

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

Femur length averages 26.74% of height, a ratio consistent across populations, which is why it is used to estimate stature.

The skin is the largest organ, but which body surface is 15 to 20 times larger in area?

Skin covers 1.5–2 square metres and about 16% of body mass; the intestinal lining is far larger in surface area.

Roughly what percentage of body mass does the skin make up?

Skin accounts for roughly 16% of body mass and has a surface area of 1.5–2.0 square metres in an adult.

The human heart sits in the mediastinum at the level of which thoracic vertebrae?

The heart lies at T5–T8, wrapped in the double-membraned pericardium, with its back surface near the spine.