I mentioned before that arteries, arterioles, small arteries,

我之前讲过像动脉细动脉小动脉
all arteries have muscular walls.

所有的动脉都有肌肉壁
They have a special kind of muscle called smooth muscle

这种特殊的肌肉叫做平滑肌
which is arranged circumferentially,

沿着圆周方向
sort of wrapped around the vessel,

包裹在血管外侧
and that muscle can contract to constrict the vessel

这些肌肉收缩就能压缩血管
or relax to dilate the vessel.

放松的话血管就扩张
One feature of arteries that makes them important in physiology

在生理学上动脉的一个特征十分重要
is that they can change their diameter.

就是它能改变自身的直径
When they change their diameter they change their resistance,

动脉直径改变时阻力也改变
and they change the flow that goes through them

在固定的压差下
at a fixed pressure drop.

动脉能够改变血液的流速