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Why are there long hairs on my head, but only short hairs on my arms and legs? Hair follicles produce hair according to a cycle of growth and rest. Cells are produced in the follicles that continuously push the hair outwards, adding to its length during the active period or growth phase.
This is followed by a period of rest when growth stops, the hair shaft breaks and the hair falls out.
After a while, the cycle will start again and a new hair shaft will grow in the follicle.
The maximum length of the hair depends on the length of the growth phase. The hair on our arms have a growth cycle of a couple of months.
After two months' growth, the hair on our arms will stop growing and take a rest before falling out.
The hairs on our heads, however, have growth cycles lasting years. Therefore, these hairs have a longer time to reach greater lengths. It can take about 10 years for hair to reach waist length.
What would happen to a man thrown into space without any equipment? SHIRLEY Tai Po District Secondary School He would make a nasty mess.
Interplanetary space is almost like a vacuum, and the pressure is very low.
The cells in the human body are under a much higher pressure, since they have evolved to cope with the atmospheric pressure on earth.
As a result, a person who left a spacecraft without a protective pressurised suit would explode like a human bomb.
He would not be around long enough for the lack of oxygen, water, food, the dangers of cosmic radiation or the possibility of being hit by a piece of orbiting satellite debris to bother him.
How does uranium cause radiation sickness? Uranium is the heaviest metal found in nature. It is used in nuclear power plants to produce electricity.
It is an unstable element which means its atomic structure breaks down spontaneously.
As uranium decays on an atomic level, three types of emissions, or radiation, are produced - gamma rays, beta particles and alpha particles.
Gamma rays are very high energy waves that can even penetrate metals. They past through soft body tissue like light through a window.
Beta and alpha particles can also penetrate cells to varying degrees.
All three types of radiation are harmful and break chemical bonds in living cells at random.
Radiation can disrupt the normal functions of cells, cause changes to the enzymes and hormones cells produce, stop cells from dividing to replace themselves and cause damage to DNA.
Massive exposure to radiation can cause death within a few days or weeks. Smaller doses cause burns, loss of hair, nausea and changes in the blood.
Low levels of radiation can cause cancer, leukaemia, abnormalities in children born afterwards and possible genetic defects in future generations.