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.

It can be inferred from the passage that which of the following presents the greatest danger to a diver?

Câu hỏi liên quan

• The word "they" refers to ................

• The word “they” refers to___________.

• How do theorists believe a person can remember more information in a short time?

ZUNIA12

• What is the main purpose of the passage?

• Why was painting inside the Lascaux complex a difficult task?

• The main idea of this passage is that_________.

• The passage suggests that industrialization in the West led to all of the following EXCEPT .............

• According to the passage, writing ..............

• The word “it” in paragraph 1 refers to ..............

• Which of the following helps to create the Earth’s magnetic fields?

• In compared with low-achieving students, successful students use ...........

• According to this passage, what is a signal?

• From the expression “on record” in line 19, it can be inferred that the tsunami that accompanied the Krakatoa volcano ___________ .

• The passage is organized by .....

• The word “eliminate” in paragraph 5 could be best replaced by which of the following words?

• The word “emblems” in the final paragraph is closest in meaning to ______.

• According to the passage, the doers of education are  .....................

• According the passage, who receives the Pulitzer Prize?

• Which of the following could be considered a cause of the distress signals of trees?

• The writer seems to imply that Europeans travel mostly for the reason that ...............