We tend to regard hair only as a reflection of our personal style, but as a living, growing part of our body, it's also tied in to our physiology. "Hair reflects internal changes -- things like sickness, nutrition and stress," says dermatologist Paradi Mirmirani, M.D., a faculty member of the American Academy of Dermatology and staff dermatologist with Kaiser Permanente. There are three parts to your hair: the hair shaft (the fiber), the hair bulb (the root) and the hair cycle (the period of rest and regeneration). Most problems are related to one of these three things, says Mirmirani.

Issue: Excessive hair shedding

Each individual strand of hair has its own cycle, growing for six to eight years, resting for a few months, and then shedding. "About 10 percent of your hair is shedding at any one given time," Mirmirani says, pointing out that many things can interrupt the cycle and cause more hairs to shed earlier, including stress, fever, weight loss (even 10 pounds) or low iron levels. If you're noticing more shedding than usual, examine your lifestyle to see what might have changed or consult your dermatologist for tests.

Issue: Hair that is dry and has lost its luster

Oh, the things we do to our hair in the name of beauty, like daily blow-drying and using the flat iron. "Hair becomes dry when the cuticle starts to break down and the inner fiber is exposed," Dr. Mirmirani says. Excessive heat can cause this -- especially heat applied to wet hair. She recommends air-drying your hair partially before blow drying, only using the flat iron on dry hair, and using a daily conditioner. Remember that hair fiber with color is more prone to damage, so be extra cautious if you color-treat your hair.

Issue: Brittle or easily breaking hair

Some hairstyles can actually affect the health of your hair's bulb, Dr. Mirmirani says. She sees this a lot with hair extensions or ponytails pulled too tight, day after day. "Think of it this way, if you keep pulling on the bulb of a plant, eventually, you can damage it," she says. Take it easy when you're pulling your hair back, and don't brush your hair vigorously when wet. Use a wide-tooth comb and comb through gently. As for Marsha Brady's 100 strokes a day for shine? Not so much, Mirmirani says. The less brushing, the better.

Issue: Split ends

Split, or "dead" ends are the result of damaged or weathered hair. "The hair splays apart because the outer layer (or cuticle) of the hair shaft has worn away," Dr. Mirmirani says. But the good news is, you can snip them right off, and immediately your hair will look healthier. Another trick is to keep your ends well-conditioned. Remember there is a difference between the function of cleansing and conditioning: Shampoo is for your scalp, and conditioner is for the actual hair fibers.

Issue: Gray hair

The unavoidable truth is that gray hair is genetic, and aside from coloring, you're stuck with it. Most people only have a handful of hair cycles (remember, a cycle is 6 - 8 years) where the pigment-producing cells that surround the follicle and transfer pigment are working at their best. The process just gets . . . tired. Dr. Mirmirani can't confirm the old wives' tales that tell you if you yank one gray hair, five more will sprout up. However, once a fiber is gray -- even if you pluck it -- it will more than likely grow back in gray again.

Issue: Dandruff

Dandruff can be hereditary, but it's still a mystery what causes the flaky stuff. Daily shampooing is your first line of defense, but if the itchiness and flakes persist, try an anti-dandruff product. Dr. Mirmirani cautions sparing use of these products, however, because they are very harsh to your hair fiber. Try alternating the anti-dandruff product with regular shampoo until the dandruff clears up, and then switch back to your regular shampoo.

Issue: An unexpected bald patch of hair

You may have Alopecia Areata, which occurs when your immune system starts attacking the hair bulbs. It doesn't destroy it, but the bulb won't start to grow again until the attack is called off. According to the National Alopecia Areata Foundation, it affects five million people in the U.S. "It could be stress or it could be environmental factors -- we really don't know," Dr. Mirmirani says. It can happen one time or recur with some frequency. The hair can grow back though -- it's not permanent damage. If you find a sudden bald spot, see your dermatologist.

Issue: Thicker hair than you've ever had before.

Either you're just super lucky, or you're about to have a baby. Sure, the hormones of pregnancy can make you lose your lunch, but they are also the cause behind the thick, lustrous hair you'll probably notice if you're expecting. When you're pregnant (and for a few months postpartum), your hair stays in a high state of hair growth, with only minimal hair loss. Enjoy it while it lasts!

我们往往仅仅把头发当作个人风格的一种反映，但是作为我们身体的一个不断生长的部分，它也和我们的生理系统有联系。皮肤科医生Mirmirani，医学博士，美国皮肤学会教员和凯色永久皮肤科的工作人员说：“头发内部反映了变化-像疾病，营养和压力。”你的头发包括了三部分：毛干(纤维)，毛球(根部)和毛发周期(休息和再生的周期)。Mirmirani说大多数问题都与这三部分的一部分有关系。

问题：过多的脱发

每一根头发都有它自己的周期，生长6-8年，停止生长几个月，然后脱落。Mirmirani说：“百分之十左右的头发会在任何一个特定的时间脱落。”她指出，压力，发烧，体重减轻(甚至10磅)或缺铁等等将打断头发的周期，使更多头发脱落的更早。如果您发现头发脱落的比平时更多，检查一下你的生活方式看看什么发生了改变或者咨询您的皮肤科医生并做些检查。

问题：头发干燥并失去光泽

哦，我们以美发名义做的事情，像每天的吹干燥和使用扁铁。Mirmirani医生说：“头发变得干燥，角质层开始破坏和内部纤维暴露，”过度加热将导致这种情况发生，特别是对湿发加热。她建议在用电吹风吹干头发之前，先采用自然风干，在干燥的头发上仅仅使用扁铁并且使用日常护发素。请记住，毛纤维的颜色更容易损坏，所以如果你染发的话要格外小心。

问题：头发变脆容易断裂

Mirmirani医生说有些发型实际上会影响到你发球的健康。她看到了大量的这种情况，日复一日的拉伸头发或者把马尾辫拉的太紧。她说：“这样想想看，如果你一直拉植物的根部，最终，你将破坏它。”当你向后拉头发的时候放松，当头发湿的时候不要用力地梳头发，应使用宽齿梳子轻轻地梳。像玛莎布雷迪为了光泽，一天梳个100次？Mirmirani医生说并不是这样。梳的越少越好。

问题：头发末梢分叉

分叉或死的头发末梢是头发顺坏或风干造成的。Mirmirani医生说：“头发分叉是因为毛干的表层(或角质层)已经损坏。”不过，好消息是，你可以立即剪掉它，马上你的头发看起来就更健康。另一个窍门是妥善调理你的发梢。记住清洗和调理的功能是不同的：洗发水是针对你的头皮，而护法素实际是针对头发纤维。

问题：白发

不可避免的事实是，除了染色外，白发是遗传的，你应该坚持这种观点。大多数人只有一小段头发周期(请记住，一个周期是6 -8岁)，此时色素细胞包围毛囊而且产生色素工作处于最佳状态的。然后过程变得...疲劳。Mirmirani医生不能确认老妇人讲的如果你拔一根白发，五根将会生长出来的传言。然而，一旦头发纤维变白，即使你拔掉头发，很可能再次长回白头发。

问题：头皮屑

头皮屑是可遗传的，但是是什么原因导致了片状的东西产生还是一个谜。每天洗头是你的第一道防线，但是如果痒而且片状一直存在的话，试试去屑的产品。然而Mirmirani告诫大家保守地使用这些产品，因为他们对你的头发纤维刺激很大。使用去屑产品代替常规洗发水知道头皮屑被清除，然后换回常规洗发水。

问题：意想不到的秃顶

当你的免疫系统开始攻击毛球，您可能已经秃发，免疫系统并没有摧毁它，但是毛球在攻击停止之前不会再生长了。根据全国秃发基金会，它影响到在美国的500万人。Mirmirani医生说：“这可能是压力或者环境因素造成的--我们也不知道确切原因。”秃发可能发生一次也可能以一定的频率发生。头发还能变黑因为这不是永久性的损坏。如果您发现突然的秃发，去找您的皮肤科医生看一下。

问题：头发比以前更密

要么你只是超级幸运，或者你即将有一个孩子。当然，怀孕的激素可以使你对午餐失去胃口，但是如果你怀孕的话你可能注意到它们也是你头发变密变光泽的背后原因。当你怀孕(好几个月)，你的头发保持高增长的状态，只有很少的头发会脱落。享受这持续高增长的时候吧！