It is thought that high cholesterol levels in the blood can harden the arteries (= make them thicker and stiffer, causing disease).据信血液中的胆固醇含量高会导致动脉硬化。
(SEVERE)
（使）变严厉，（使）变坚强，（使）变冷酷
to become more severe, determined, or unpleasant
Living in the desert hardened the recruits (= made them more strong and determined).沙漠的艰苦生活把新兵锻炼得更加坚强。
As the war progressed, attitudes on both sides hardened (= became more severe and determined).随着战争的持续，双方的态度都变得更加强硬。
Related word: hardening
Idiom
harden your heartidiom硬起心肠
to make yourself stop feeling kind or friendly towards someone
You've just got to harden your heart and tell him to leave.你只有硬起心肠让他离开。