阻力血管
小动脉和微动脉的管径小,对血液的阻力大,它们的管壁富含平滑肌,其舍缩活动可引起血管口径的明显变化,从而改变对血流的阻力,又称毛细血管前阻力。
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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