A heart that never takes a day off
Your HEART beats around 100,000 times a day, which adds up to about three billion beats in a long life. Each beat pushes BLOOD out through an ARTERY, and it returns through a VEIN. Laid end to end, all the blood vessels in an adult body would stretch for tens of thousands of kilometres, enough to go around the Earth more than twice, most of it made up of tiny capillaries thinner than a hair.
For centuries doctors believed that blood was made fresh in the liver and used up by the body. In 1628 the English doctor William Harvey showed that the heart pumps the same blood round and round the body in a circuit, and that there was simply too much of it to be made new every day.
A brain that uses a fifth of your energy
Your BRAIN weighs about 1.4 kilograms, around two per cent of your body weight, but it uses about a fifth of the energy you burn, even when you are asleep. It contains about 86 billion nerve cells, each linked to thousands of others. Messages travel along a NERVE as electrical signals, the fastest at over 100 metres a second, down the SPINE and out to every part of you.
Your LUNGS are not hollow bags but spongy trees of tubes ending in about 480 million tiny air sacs. Spread out flat, their surface would cover roughly a badminton court, all to pass oxygen into the blood and carbon dioxide out. Your body makes around two million new red blood cells every second, and each lasts about four months. A hiccup is a sudden spasm of the muscle under the lungs; an American farmer, Charles Osborne, hiccupped almost non-stop for sixty-eight years.
Organs that clean and rebuild
The LIVER has hundreds of jobs, from storing energy to breaking down poisons, and it is the only ORGAN that can grow back. A person can donate part of their liver, and both pieces will regrow close to full size within months. Each KIDNEY filters the blood constantly; together they clean around 180 litres of fluid a day, returning almost all of it and sending the waste out as urine.
The STOMACH is full of acid strong enough to dissolve metal, so it protects itself with a thick layer of slime, and the cells of its lining are replaced every few days. The SKIN is the body's largest organ, and you shed tens of thousands of dead skin cells every minute; some of the dust in a house is old skin.
Trillions of cells
Every part of you is built from a CELL, and an adult body contains around thirty trillion of them, grouped into TISSUE such as MUSCLE or skin, which make up organs. You have more than 600 muscles, and a TENDON joins each one to a bone. The bones themselves, from the SKULL and RIBS to the PELVIS, each JOINT and the whole SKELETON, have their own page, and so does your sense of taste, which starts on the TONGUE. Our medical jobs page covers the people who look after it all.