On the smooth muscle cells controlling some blood vessels, there are receptors called alpha receptors — when norepinephrine, or epinephrine, bind to those receptors, they make those smooth muscle cells contract, thereby restricting blood flow.

在控制一些血管的平滑肌细胞上，有一种叫做α受体的受体——当去甲肾上腺素或肾上腺素与这些受体结合时，它们使这些平滑肌细胞收缩，从而限制血液流动。