1The heart — the organ located in the chest that pumps blood throughout the body.
全力で走ったら心臓がドキドキした。
After running at full speed, my heart was pounding.
祖父は心臓の手術を無事に終えた。
My grandfather safely finished his heart surgery.
医師は聴診器で心臓の音を確認した。
The doctor checked the sound of the heart with a stethoscope.
2Courage, nerve, or guts — a figurative use of 'heart' to describe boldness in the face of fear.
満員の会場で堂々と話せるとは、彼女は心臓が強い。
Being able to speak confidently in front of a packed venue — she really has nerves of steel.
そんな嘘を平気でつけるなんて、心臓に毛が生えているとしか思えない。
Telling a lie like that without batting an eye — the guy must have nerves of steel.
3The central, most vital part of something — the 'heart' of an organization or system.
このエンジンは自動車の心臓部と言える。
This engine could be called the heart of the car.
サーバー室はこの会社のシステムの心臓部にあたる。
The server room is, so to speak, the heart of this company's system.