[填空题]Nicotine dilates the blood vessels that carry it around the body.

[填空题]Nicotine dilates the blood vessels that carry it around the body.

[单项选择]Blood vessels running all through the lungs carry blood to each air sac (囊), or alveolus (肺泡), and then back again to the heart. Only the thin wall of the air sac and the ’thin wall of a capillary (毛细血管) are between the air and the blood. So oxygen easily diffuses from the air sacs through the walls into the blood, while carbon dioxide easily diffuses from the blood through the walls into the air sacs.
When blood is sent to the lungs by the heart, it has come back from the cells in the rest of the body. So the blood that goes into the wall of an air sac contains much dissolved carbon dioxide but very little oxygen. At the same time, the air that goes into the air sac contains much oxygen but very little carbon dioxide. You have learned that dissolved materials always diffuse from where there is more of them to where there is less. Oxygen from the air dissolves in the moisture on the lining of the air sac and diffuses through the lining into the blood. Meanwhile, carbon dioxide dif
A. produce energy for breathing
B. diffuse immediately in the blood
C. penetrate slowly into the air sacs
D. travel in opposite ways in the lungs

[单项选择]Passage OneA. It keeps blood vessels from being blocked.
B. It speeds up their recovery after surgery.
C. It increases the blood flow to the heart.
D. It adjusts their blood pressure.