Attention deficit (不足) hyperactivity (活动过度) disorder is known by the letters A. D. H. D. with A. D. H. D. might have trouble paying attention. They might not seem to listen and might forget things. They might not be able to stay seated or play quietly. They might talk too much and act and speak without thinking about the results of their behavior. These are among the signs named on the web site of America's Centers for Disease Control and Prevention.

Doctors usually identify the disorder in children. But experts say the behaviors often last into adulthood. Researchers have been looking for the cause or causes of attention deficit hyperactivity disorder. Now, a brain-imaging study offers more evidence that could lead to new ways to treat it. Researchers say they observed shortages in the brain's reward system in patients with A. D. H. D. The study found that levels of some proteins were lower than normal. Nora Volkow, director of the National Institute on Drug Abuse, says the lack of attention and self-control that mark A. D. H. D. could be caused by problem in the flow of dopamine (多巴胺) . She says people might have difficulty completing an activity if they cannot expect some kind of reward in return. Researchers studied the pathways on which dopamine travels in 53 adults with A. D. H. D. Doctor Volkow says the researchers then compared the pathways to those of 44 adults without the disorder. "There was a lower concentration of dopamine markers in the brain of individuals with A. D. H. D., specifically in the areas of the brain that are involved with reward and motivation. And the dopamine levels were directly linked to the severity, of the patient's inattention" she says.

The study used brain images taken at Brookhaven National Laboratory in New York State. Gene -Jack Wing is chairman of the Brookhaven medical department. He says the finding might also help explain why people with A. D. H. D. are mere likely to abuse drugs or overeat. He says they might be attempting to increase their dopamine levels to make up for the deficits in their reward system.

During the teenage years, hormonal (荷尔蒙的) changes lead to physical changes like rapid muscle growth, burst in height, and weight gain in both boys and girls. Take a look at the information and height and weight charts for teens presented in this article if the question “How much should I weigh for my age and height” is bothering you.

Average Height and weight Charts for TeensFaster growth in kids usually can be noticed at age 8. Genetic factors also determine the physical growth of a child. So, it is perfectly normal for two kids of the same age to have different weights and heights. The following figures should be used as guidelines only.

Ideal Body Fat Percentage

Fit men have 14% -17% body fat. Fit women have 21% - 24% body fat.

Male athletes have 6% - 13% body fat. Female athletes have 14% - 20% body fat.

The ideal body fat percentage varies mainly according to age, sex and the physical activity of the individual. But it is a fact that some female athletes have body fat as low as 10% and some male athletes had lower than 4% body fat. So, it can be observed that other than the above mentioned factors, genetic factors also influence the body-fat ratio (比率)．

Body Mass Index (BMI)

You may use the following formula to calculate our BMI:

As per SI Units (国际单位)

BMI= mass (kg)/[height(meters)]²

Following conclusions can be derived from (得到) calculating the BMI:

While faking and fierce looks are among animals great defenses, many species know that everyone runs from a big stink(臭气)too.

Vulture(秃鸳)

Vultures, are street sweepers that feast on the rotting flesh of dead animals, which benefits us by ridding our highways and landscapes of animal bodies and the bacteria they might carry. When vultures feel threatened they vomit, and the smell of vomited-on dead bodies puts of most predators. Throwing up allows the vulture to fly away more quickly-and the vomit can hurt the aggressor's eyes and face.

Opossum(负鼠)

In some ways opossums have it easy. In order to become “dead", they don’t have to fax anyone a death certificate. They just lie there with their tongues hanging out with the smell of dead flesh, sometimes for hours, effectively convincing potential predators they can find a much fresher meal elsewhere. Even if they keep getting attacked, they won't move any more than a human statue until the threat has passed.

Hoatzin(磨雉)

Hold your nose and meet the hoatzin, a bird of distinctions, not the least of which is that it smells like fresh cow shit. The animal mostly eats leaves and it's the only bird known to digest by fermentation, like a cow. This process is what causes its smell and has earned it the nickname the “stink bird

Millipedes (千足虫)

Millipedes are tricky. For starters they look wormy. Their name is deceptive, too: Their legs number about 750. Their major defense is to curl up into a ball. They, though, also release a harm eyes, and leave a horrible smell on their attackers.

Sea Hare(海兔)

The graceful sea hare is plain in taste in the first place, so it's not the most popular dish in the seafood chain. Nonetheless this type of sea creature has a pretty creative smell-related defense that is almost the opposite of its smelly companions on this list. The sea hare gives out a slimy, purple ink, the substance which makes food less palatable to predators.

Researchers using lobsters(2-f)as model predators found that the sea blocks the lobsters receptive mechanism. In other words, the sea hare gives its attacker the equivalent of a stuffy nose so they don't know how appealing it is.