The human body

15 entries on organs, bones, teeth, blood, senses and the body in numbers. Structural reference rather than health advice — what a thing is, where it is, and what it does.

Things everyone knows that are wrong

• The heart is not on the left. It sits near the centre, angled so the apex points left, which is why the beat is felt there.
• You do not use only ten per cent of your brain. Imaging shows essentially all of it active over a day, and there is no dormant reserve — damage to almost any region produces deficits.
• Lungs do not "pull" air in. Air is pushed in by atmospheric pressure once the diaphragm lowers and the chest volume increases — which is why a punctured chest cavity stops the lung inflating even though the lung is undamaged.
• The count of 206 is a convention, not a constant. A minority of people have extra sesamoid bones or an additional rib, and the figure depends on whether such variants are counted.
• The blood type diet has no evidence behind it. A controlled study found the diets produced effects unrelated to the participants’ actual blood groups.
• Deoxygenated blood is not blue. It is dark red. Veins look blue because of how skin scatters light, not because of the blood inside them.
• The five-senses framing comes from Aristotle and does not reflect the anatomy. Touch alone covers at least four separate receptor systems for pressure, vibration, temperature and pain.
• The tongue map is wrong. The idea that sweet is tasted at the tip and bitter at the back comes from a mistranslation of a 1901 German paper and was disproved in 1974. Every basic taste is detectable across the whole tongue.
• The claim that bacteria outnumber human cells ten to one is obsolete. It traced to a 1972 estimate never intended as precise; careful counting puts the ratio close to 1:1.

Organs

• Heart A four-chambered pump, roughly fist-sized, beating about 100,000 times a day.
• Brain About 1.4 kg, using roughly 20% of the body’s energy at 2% of its mass.
• Liver The largest internal organ, around 1.5 kg, and the only one that regenerates.
• Lungs Two unequal lungs with a gas-exchange surface of about 70 square metres.
• Skin The largest organ — about 2 m² and 4 kg in an adult.
• Kidneys Two fist-sized filters processing about 180 litres of blood a day.

Bones

• Bones 206 in the adult skeleton, down from about 270 at birth.
• Femur The longest and strongest bone — about a quarter of adult height.

Teeth

• Teeth 32 adult teeth including wisdom teeth; 20 in the primary set.

Blood

• Blood types Eight common groups from the ABO and Rh systems — O+ is the most frequent.
• Blood About 5 litres in an adult, roughly 7% of body weight.

Senses

• The senses Far more than five — at least nine, and by some counts over twenty.
• Taste Five basic tastes, detected across the whole tongue rather than in zones.

Body in numbers

• Body water About 60% of an adult male by mass, 55% of an adult female.
• Cells in the body About 36 trillion, of which roughly 80% are red blood cells.

None of this is medical advice. Individual variation in every figure is substantial, and anything about your own health belongs with a clinician.