1000 grams: the mass of bacteria currently living in your gut. You will defecate your own weight in bacteria every year and play host to some 500 different species in your intestine.
300 million: the number of cells in your skin, your largest organ. The outer layer, the epidermis, is constantly being renewed and replaces itself about once every 75 days. There are an average of 10 hairs, 100 sweat glands and 1 metre of blood vessels in each 3 square centimetres.
20鈥�20,000 hertz: the hearing range of a young adult. This shrinks dramatically to 50 to 8000 hertz in old age. Sounds become painful at about 130 decibels (near a plane taking off), but prolonged exposure to anything above 90 decibels (a tractor engine) will permanently damage hearing.
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1 litre: the average amount of sweat lost in a day.
300 million: tiny air sacs or alveoli in each adult lung. If these were spread out, their total surface area could cover a tennis court. The major airways in each lung are tilted at different angles. This means that if you ever inhale a peanut, it will nearly always lodge in your right lung.
2.5 billion: the average number of heartbeats in a lifetime. At rest your heart pumps about 5 litres per minute, during exercise its output can reach 30 litres. Most people鈥檚 hearts lie to the left side of the chest, but in people with a condition called situs inversus totalis (1 in 8500) it lies to the right, and the orientation of all their other organs is switched too.
31 million: the neurons you lose from the main part of your cerebral cortex every year 鈥� about one per second.
25 billion: the average number of red blood cells in your body (about 5 billion per litre of blood). 25 million (or 1 per cent) of them die every day, which works out at roughly 300,000 a second. A red blood cell can circumnavigate the entire body in less than 20 seconds.
100 million: the number of bacteria per square centimetre of a sweaty armpit by the end of the day.
1015 the estimated number of synapses linking the 100 billion or so neurons in your brain. There are about 200 million neurons more in the left side of the brain than the right.
70 per cent: thanks to the liver鈥檚 amazing powers of regeneration, it can lose this much tissue and still grow back to its original size in a few months. The liver is the largest gland in the body, producing about half a litre of bile per day. It weighs some 1.8 kilograms in men and 1.3 kilograms in women. It holds about 13 per cent of the total blood supply at any given moment. Each liver cell (hepatocyte) has an average lifespan of 150 days.
206 the number of bones in the human body 鈥� more than half of them in the hands and feet. The knee is the most complex joint of the human body and also the most easily injured.