cavity n.腔，洞;蛀牙，龋洞

A dental cavity is a hole in the tooth. A cavity is caused by bacteria in the mouth. Brushing your teeth frequently and regularly can help prevent cavities.

  cracked    adj.有裂缝的;声音沙哑的;精神失常的

v.破裂;崩溃(crack的过去分词);发沙哑声

A cracked tooth can be caused by biting down on some hard. Dentists the treat cracked tooth by covering it with a crown(牙冠).

  crown    n.王冠;花冠（有时象征胜利）;王权;（某物的）顶部  vi.露顶

vt.（尤指通过增添成就、成功等）使圆满;为…加冕;使圆满完成;给…以荣誉

A dental X-ray is a picture of your teeth and gums. An X-ray shows what can’t be seen on the surface of teeth and gums.

Braces are used by dentists to straighten teeth over a periods of years. Those braces are usually for children. they can also be used for adults with crooked teeth.

Brace  vt.支撑;系紧;准备，预备;振作起来  vi.准备好;支持;打起精神

n.支持物;铁钳，夹子;[语]大括号;绷紧（身体部位的）肌肉

Straighten  vt.& vi.变直，把…弄直;整理，整顿;使…改正;使…好转

vi.变好起来

crook    n.骗子;弯曲;钩;诡计  vt.使成钩状;使弯曲

vi.弯曲      adj.不正当的;骗人的

crooked    adj.弯曲的;不正当的;歪扭的;用不正当手段得来的

To extract a tooth means to remove it from the mouth. Dentists have to pull hard to extract a tooth.

The heart pumps blood to all parts of bodies through a system of arteries and vein.  It’s located in the chest, between the lungs and it beats about once per second.

Artery  n.[解剖]动脉;干线，要道

toxin    n.毒素;毒质

The kidneys remove waste and toxins from the blood. They have been shaped  and about 10 cm long, and 6 cm wide.

The lungs exchange oxygen from the air with CO2 from the blood. They expand when inhaling air and contract when exhaling air.

Inhale    vt.& vi.吸气;吸入;〈非〉狼吞虎咽

Exhale    vt.发散出;放出，放射;使蒸发;[医]渗出

vi.呼气;发散;放出

The stomach is below the heart and it’s part of the digest system. It uses acids to digest food before passing it to the small intestine.

Small intestine n.小肠

The bladder is a balloon-shaped organ that collects urine from the kidneys. In an audit, the bladder can hold about half a liter of urine.

urine    n.尿;小便;下泉

a liter of 公升