When was the last time you thought about breathing？ Most people usually do not. When we are healthy, breathing happens naturally and easily. Without much effect on our part, the human respiratory system（呼吸系统）works hard. Experts say we take about 20，000 breaths every day.
One such expert is James Hoyt, a doctor at the University of Colorado's Health Pulmonology Clinic. He notes our "respiratory muscles are working every minute of the day, every day of our lives." A muscle called the diaphragm（横膈膜）separates the chest and abdominal cavities（腹腔）. As we breathe in, the diaphragm tightens. The chest cavity opens, and the lungs expand. When we breathe out, the diaphragm relaxes and moves upward, pushing air out.
When you breathe deeply, the air coming in through your nose fully fills your lungs, and the lower belly（腹部）rises. On its website,Harvard Medical School notes that deep breathing may slow the heartbeat,lower blood pressure and lower stress. Deep breaths help your body fully exchange incoming oxygen with outgoing carbon dioxide. However, many people do the opposite of deep breathing. They take short breaths and have shallow breathing. Shallow breathing makes you feel short of breath and worried, or anxious.
The American Lung Association notes that shallow breathing, over time, leaves old, stale air in the lungs. This leaves less room for the diaphragm to bring in fresh oxygen. And that means lower oxygen levels and less oxygen for exercise and activity.
Several health websites explain an easy deep breathing exercise. Find a place to sit or lie down. Place one hand just below your ribs（肋骨）. Take a slow, deep breath-or inhale-through your nose. Feel your hand go up. Your stomach should rise and expand. Now breathe out slowly through your mouth. Make sure to breathe out-or exhale-all the way. Feel your hand and stomach go down.



How can we breathe air out？ ______

A. By the diaphragm's tightening.
B. By the chest cavity's opening.
C. By the lungs' expanding.
D. By the diaphragm's moving upward.

What does the author think of deep breathing？ ______

A. It's harmful.
B. It can cause anxiety.
C. It can reduce stress.
D. It's bad for the heart.

What's the probable meaning of the underlined word "stale" in paragraph 4？ ______

A. No longer new.
B. Not beneficial.
C. Poisonous.
D. Available.

Which column is the text taken from？ ______

A. Special News.
B. Health Report.
C. The Making of a Nation.
D. People in America.