OUR ASTONISHING BODY

• Doubling a child's height on their second birthday gives a close estimate of their final adult height. A boy of two is 49.5 per cent of his adult height; a girl of two is 52.8 per cent of her adult height.
• Each finger and toenail takes about 6 months to grow from its base to the tip.
• During pregnancy a woman's blood volume can increase by up to 50 per cent to a total of 6.75 litres (12 pints) as a reserve against possible loss of blood during delivery.
• The brain accounts for about 3 per cent of body weight. But it uses 20 per cent of all the oxygen we breathe, 20 per cent of the calories in the food we eat, and about 15 per cent of the body's blood supply.
• The adult human body contains approximately 650 muscles, over 100 joints, 100,000km (60,000 miles) of blood vessels and 13,000 nerve cells. An adult has 206 bones - nearly half of them in the hands and feet. A baby has 300 bones at birth, but 94 of them fuse together during childhood.
• Human bone is as strong as granite in supporting weight. A block the size of a matchbox can support 9 tonnes (8.86 tons) - four times as much as concrete.
• A man's testicles manufacture 10 million new sperm cells a day - enough in six months to populate the entire world.
• The heart beats more than 2000 million times during the average human life span, and in that time will pump around 500 million litres (110 million gallons) of blood. Even during sleep, the fist-sized heart of an adult pumps about 340 litres (75 gallons) an hour - enough to fill an average car's petrol tank every 7 minutes. It generates enough muscle power every day to lift an average-sized car about 15m (50ft).
• The average pulse rate is 70 - 72 beats per minute at rest for adult males and 78 - 82 beats per minute for adult females. The rate can increase to as much as 200 beats per minute during violent exercise .
• The lungs contain a total of 300,000 million capilliaries - tiny blood vessels - which would stretch 2,400km (1,500 miles) if laid end to end.
• The body of an average adult contains 45 litres (79 pints) of water - about 65 per cent of their weight.
• The stomach's digestive acids are strong enough to dissolve zinc. But the cells in the stomach lining are renewed so quickly - 500,000 cells are replaced every minute, and the entire lining every three days - that the acids do not have time to dissolve the lining.

• Each kidney contains some 1 million individual filters, and between them the two kidneys filter an average of 1.3 litres (2.2 pints) of blood in a minute. The waste products are expelled as urine at the rate of about 1.4 litres (2.5 pints) a day.
• The body's entire blood supply - some 4.5 litres (8 pints) - washes through the lungs about once a minute. Human red blood corpuscles are created by bone marrow at the rate of about 1.2 million corpuscles per second. Each lives for 100 - 120 days. In a lifetime the marrow creates about half a tonne (1,102lbs) of red corpuscles.
• The body's largest organ is the skin. In an adult man it covers about 1.9m² (20 sq.ft); a woman has about 1.6m² (17 sq.ft). The skin is constantly flaking away and being completely replaced by new tissue about once every 50 days. On average, each person sheds about 18kg (40 lb) of skin during their lifetime.
• The smallest human muscle is in the ear; it is a little over 1mm (.04 in) long. The ear also contains one of the few parts of the body which has no blood vessels. Cells in part of the inner ear, where sound vibrations are converted to nerve impulses, are fed by a constant bath of fluid instead of blood - otherwise the sensitive nerves would be deafened by the sound of the body's own pulse.
• You grow by about 8mm (0.31 in) every night when you are asleep, but shrink to your former height the following day. During the day, the cartilage discs in the spine are squeezed like sponges by gravity while you stand or sit. But at night, when you lie down to sleep, the pressure is relieved and the discs swell again. For the same reason, astronauts can be temporarily 50mm (2 in) taller after a long spaceflight.
• Besides water, the body contains an assortment of other substances. On average, it has enough lime to whitewash a small shed, the equivalent in carbon of a 12.7kg (28 lb) bag of coke, enough phosphorus to make 2,200 match heads, about a spoonful of sulphur, enough iron to make a 25mm (1 in) nail - and about 30g (1 oz) of other metals.
• The focusing muscles of the eye move about 100,000 times a day. To give the leg muscles the equivalent amount of exercise would involve walking 80km (50 miles) a day.
• The retina inside the eye covers about 650mm² (1 sq in) and contains 137 million light-sensitive cells: 130 million rod cells for black and white vision; and 7 million cone cells for colour vision.
• The average person in the West eats 50 tonnes (49.2 tons) of food and drinks 50,000 litres (11,000 gallons) of liquid during their lifetime.