Under certain circumstances, the human body must cope with gases at greater-than-normal atmospheric pressure. For example, gas pressures increase rapidly during a drive made with scuba gear because the breathing equipment allows divers to stay underwater longer and dive deeper. The pressure exerted on the human body increases by 1 atmosphere for every 10 meters of depth in seawater, so that at 39 meters in seawater a diver is exposed to pressure of about 4 atmospheres. The pressure of the gases being breathed must equal the external pressure applied to the body, otherwise breathing is very difficult. Therefore all of the gases in the air breathed by a scuba diver at 40 meter are present at five times their usual pressure. Nitrogen, which composes 80 percent of the air we breathe, usually causes a balmy feeling of well-being at this pressure. At a depth of 5 atmospheres, nitrogen causes symptoms resembling alcohol intoxication, known as nitrogen narcosis. Nitrogen narcosis apparently results from a direct effect on the brain of the large amounts of nitrogen dissolved in the blood. Deep dives are less dangerous if helium is substituted for nitrogen, because under these pressures helium does not exert a similar narcotic effect.

As a scuba diver descends, the pressure of nitrogen on the lungs increases. Nitrogen then diffuses from the lungs to the blood, and from the blood to body tissues. Nitrogen then diffuses from the lungs to the blood, and from the blood to body tissues The reverse occurs when the diver surfaces, the nitrogen pressure in the lungs falls and the nitrogen diffuses from the tissues into the blood, and from the blood into the lungs. If the return to the surface is too rapid, nitrogen in the tissues and blood cannot diffuse out rapidly enough and nitrogen bubbles are formed. They can cause severe pains, particularly around the joints.

Another complication may result if the breath is held during ascent. During ascent from a depth of 10 meters, the volume of air in the lungs will double because the air pressure at the surface is only half of what it was at 10 meters. This change in volume may cause the lungs to distend and even rupture. This condition is called air embolism.

To avoid this event, a diver must ascend slowly, never at a rate exceeding the rise of the exhaled air bubbles, and must exhale during ascent.

Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions

 The word “exert” in bold in paragraph 1 is closest in meaning to ................

Câu hỏi liên quan

• According to the passage, when James Monroe came to the White House, it had been .......

• As used in paragraph 2, the word "seeking" could best be replaced by which of the following?

• In the author’s judgment, ...............

ZUNIA12

• According to the passage, nowadays we can find the Asiatic lion ........

• In paragraph 2, the word “those” refers to ...............

• What can be said about the mother and son in the passage?

• How did John feel about his fears in the end?

• Which piece of equipment is used at the end of the recording process?

• According to the writer, failure in life and less support from parents will ______.

• Which of the following is NOT true about the tea production process?

• According to the passage, some Americans choose to live in the country because ___.

• Which word is similar in meaning to the word “glue” ?

• What can be inferred about machines that are not calculators or computers?

• Why did the author write the passage?

• In the United States, Thanksgiving is ______.

• Which of the following would be the most appropriate title for this text?

• According to the passage, all of the following contributed to the spread of English around the world except ..............

• Which aspect of colonial printing does the passage mainly discuss?

• In the second paragraph, the pronoun "them" refers to which of the following?

• According to the passage, a regional university rector in Germany is elected by _____.