（拿回你的499）懂你英語level4 unit1 part3

The heart pumps blood to all parts of the body through a system of arteries and veins.

It's located in the chest（胸腔）, between the lungs（肺）, and beats about once per second.

The kidneys remove waste and toxins（有毒物質） from the blood.

They are bean shaped organs and are about 10 centimeters long and 6 centimeters wide.

The lungs exchange oxygen from the air with carbon dioxide from the blood.

They expand when inhaling （吸入）air, and contract when exhaling （呼出）air.

The stomach*（胃） is below the heart, and it's part of the digestive （消化）system.

It uses acids（酸） to digest food before passing it to the small intestine.

The bladder（膀胱） is a balloon（氣球） shaped organ（器官） that collects urine（尿） from the kidneys（腎臟）.

In an adult, the bladder（膀胱） can hold about half a liter （升）of urine（尿）