How long does it take the body to...

Part two Also in this week's column:

• How long does it take the body to...(Part one)
• Do our ears grow longer with age?
• Do we still remove the appendix as often as we used to?

Part Two: How long does it take the body to...

• It takes time for everything, including what happens in the human body.
• On average, nerve regeneration takes four to six weeks.
• Gums are renewed every one to two weeks.
• Eyelashes, which are more plentiful on the upper eyelid than on the lower eyelid, are shed continuously. Each of the more than 200 hairs per eye lasts from three to five months.
• The vibrations in the air constitute sound waves. The higher the pitch, the greater the frequency or cycles per second. Adults can detect sound waves that have a frequency between about 16 and about 20,000 cycles per second. Yet they hear best at frequencies ranging from about 1,000 to 2,000 cycles per second. Children hear higher-pitched notes better than adults. After puberty, this sensitivity declines at the same time as the voice deepens. Thus, our ears are best adapted to the pitch or sound frequencies of human conversation.
• The ability of the brain to detect the location of a sound depends on the differences in the time of the arrival of the sound to the two ears. We can detect the source of a sound even if it arrives in one ear a hundredth of a second before it gets to the other.
• The body loses water through the skin (from simple diffusion) at the rate of half a litre per day. The body loses about the same amount of water each day from the lungs - in breath. A breath lasts about five seconds. Inhaling takes about two seconds, exhaling about three.
• Nicotine, a component of tobacco smoke, gets into the body very rapidly. With one puff on a cigarette, nicotine reaches the brain in seven seconds. This is several seconds faster than it takes alcohol to get into the brain. Even heroin, when it is injected into the arm below the elbow, takes twice as long as it does for nicotine to reach the brain.
• Statistically, one's life is shortened 14 minutes for every cigarette smoked.
• On average, a single cancer cell divides only once every hundred days. At that rate, owing to exponential growth (one cell dividing into two, two into four, four into eight, and so on), one cancer cell may take eight years to form a pea-sized tumor. But two years later, the tumor will be about the size of a cantaloupe melon and weigh more than a pound (half a kilo).
• Human adult males have four or five erections per night while sleeping. Each lasts about 10 minutes, if uninterrupted
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Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to s.juan@edfac.usyd.edu.au